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Category > [48]6/2017 Journal

The Sound of 1-bit: Technical Constraint and Musical Creativity on the 48k
Sinclair ZX Spectrum

Kenneth B. McAlpine (Abertay University)

[49]pf-pdf-icon [50]PDF Download

Sinclair 48K ZX Spectrum motherboard, Issue 3B. 1983, Manufactured 1984.
[51]CC BY-SA 3.0 – Bill Bertram.

Abstract:

This article explores constraint as a driver of creativity and
innovation in early video game soundtracks. Using what was, perhaps,
the most constrained platform of all, the 48k Sinclair ZX Spectrum, as
a prism through which to examine the development of an early branch of
video game music, the paper explores the creative approaches adopted by
programmers to circumvent the Spectrum’s technical limitations so as to
coax the hardware into performing feats of musicality that it had never
been designed to achieve. These solutions were not without
computational or aural cost, however, and their application often
imparted a unique characteristic to the sound, which over time came to
define the aesthetic of the 8-bit computer soundtrack, a sound which
has been developed since as part of the emerging chiptune scene. By
discussing pivotal moments in the development of ZX Spectrum music,
this article will show how the application of binary impulse trains,
granular synthesis, and pulse-width modulation came to shape the sound
of 1-bit music.

Keywords: 1-bit game music; ZX Spectrum; technical constraint

Introduction

For those who grew up gaming on the video game consoles and home
computers of the early 1980s, the bleeps of the in-game music were as
much a soundtrack to life as were Iron Maiden or Depeche Mode. Indeed,
many teen gamers, myself included, spent much more time playing games
and absorbing the sights and sounds of those games than we did spinning
vinyl. Certainly, the familiar chirp of Rob Hubbard’s theme from Monty
On the Run (Harrap, 1985) on the Commodore 64 (C64) or Tim Follin’s ZX
Spectrum soundtrack for Agent X (Tatlock et al., 1986) has a definite
nostalgic appeal, but the game music of that period is of more than
just sentimental value, with a legacy that extends into the
contemporary musical mainstream.

The early days of video game music are replete with tales of ingenuity
and creativity [52]^1, which were driven largely by the constraints of
the sound hardware. Microcomputers like the C64, whose specifications
offered a degree of audio hardware support used Programmable Sound
Generators (PSGs), dedicated sound chips that provided their voice by
synthesizing simple waveforms. Other machines, like the ZX Spectrum
(Christie 2016), whose computer architecture was constrained by cost,
offered no dedicated hardware support at all, and its
motherboard-mounted speaker was controlled using a single-bit value on
one of the processor’s addressable memory ports.

Regardless of whether the sounds were generated by dedicated PSGs or
directly by the Central Processing Unit (CPU), the computer hardware
offered little in the way of musical expression. At most, PSGs offered
only a few channels of polyphony and a prescriptive palette of simple
waveforms, while the monophonic 1-bit Spectrum beeper was more
restrictive still, providing just a single-channel square wave with no
level control. In response, however, there arose from this digital
frontier an explosive period of technical creativity as game
programmers and musicians (they were often one and the same) coaxed the
hardware into performing feats of musicality that it had never been
designed to achieve. The methods that were adopted to broaden and
expand the musical capabilities of the PSGs were not without cost,
however, and their application often imparted a unique characteristic
to the sound, which, over time, came to define the aesthetic, if not
the style, of the 8-bit computer soundtrack. Here, 8-bit refers to the
generation of microcomputers, of which the C64 and ZX Spectrum were
part, which used 8-bit microprocessors at their core. This is distinct
from the notion of 1-bit music, which uses only a single bit of
information to encode volume level or speaker displacement.

The characteristic 8-bit sound that accompanied the video game
soundtracks of the early- and mid-1980s has currency through a number
of related contemporary subcultures, including the retrocomputing
scene, a distributed community of enthusiasts who continue to drive
development on obsolete computing platforms (Takhteyev & DuPont, 2013),
the demoscene, a distributed technoculture focused on real-time
computer art (Carlsson, 2009), and the chipscene, a vibrant lo-fi
musical subculture that repurposes obsolete gaming hardware to make
music (Paul, 2015). Appearances of that 8-bit style of music in movie
soundtracks (see, for example Brian LeBarton’s C64 arrangement of Sex
Bob-Omb’s Threshold, which features in the end credits of Edgar
Wright’s Scott Pilgrim vs. the World), television advertisements
(Jonathan Dunn’s hypnotic theme from the Gameboy version of Ocean’s
Robocop (1990) was used as the basis for Ariston’s And on… and on…’
campaign in the early 1990s), and major exhibitions, such as that at
the Smithsonian in 2014 (Melissinos, 2014), suggest a growing
acceptance of chip music, alongside 8-bit video game art and animation,
as a legitimate form of artistic expression, while the adoption of
elements of chiptune by major artists like Mark Ronson (Knowles, 2010)
suggests the style is more than a niche crossover. Even Iron Maiden,
those stalwarts of the 80s new wave of British heavy metal, have
embraced the sound, launching their 2015 album Book of Souls with a
NES-style game, which features an 8-bit arrangement of the band’s
“Speed of Light” (Dickenson & Smith, 2015) as the background track.

To understand and fully appreciate the evolution of that sound it is
necessary to approach it from a number of different angles. Of course,
we can examine the music itself to understand the stylistic influences
that helped to shape the sound; however, stylistic analysis alone does
not tell us very much about video game music as a media form.
Sometimes, stylistic choices were driven by the narrative of the game,
so that the music might provide context to game levels. However, as
emerging from interviews carried out by the author with video game
coders and composers from the early bit through to the -bit era young
coders, many of them were keen to turn around their games quickly and
with little appreciation or regard for copyrights and intellectual
property. Often, composers just reached for the nearest sheet music to
hand or arranged whatever vinyl was spinning in the background.

To really understand how video game music functions as media music we
must also delve into the source code and the hardware, employing a
platform approach to learn more about how the computer architectures
and the games that were written for them shaped both the structure of
video game music and how it was realized. It is by examining this
broader structural context to video game music that we begin to
appreciate the challenges facing early game designers, and see how
those constraints functioned as a spur for creativity. This, in turn,
can shed light on how the aesthetics of early video game music evolved.

The physical design of musical instruments creates affordances and
constraints that, to a great extent, shape the music that is written
for them; this is as much the case for electronic instruments
(including PSGs) as it is for more traditional acoustic instruments.
Video game hardware shaped the sound of early video game music by
way of the affordances they offered and the constraints that they
imposed.

The aesthetics of constraint?

Constraint has long been recognized as a powerful driver for musical
creativity. Many cultures express ideas and expectations about how
music ought to be performed, and arguably, it is the role of the
professional musician both to satisfy and to challenge these
expectations by exploring imaginative departures from the norm. Boden
explores this idea (1995, p. 95), noting that, “(c)onstraints map out a
territory of structural possibilities which can then be explored, and
perhaps transformed”. Such common understanding of what music is and
how it should sound emerges primarily from the musical structures: the
form, timbre, harmony, melody, and rhythm of performance, the grammar
and vocabulary of which define the conventions of musical style, and
the conventions of interpretation and performance that communicate
these from composer through performer to listener (see, for example,
Ball 2010, for a detailed yet accessible discussion on this topic). It
is from these conventions that there arises one of the most delightful
aspects of music, an implicit guessing game between composers and their
listeners. Composers provide sufficient structure and familiarity for
their audiences to anticipate what is coming next at least some of the
time, while providing enough novelty to maintain engagement with the
listener (Huron, 2006, p 141). Without the shared notion of musical
and performative norms that arises from the constraints of musical
structures in particular audiences, this guessing game would often not
be possible (there is little point or reward in guessing what is likely
to follow if all eventualities are possible and equally likely) and it
would often be difficult, in this light, to distinguish creative
innovation from chance variation.

The constraints of musical form and grammar can be understood as
cultural constructs, emerging by consensus and evolving as composers
and performers experiment at the boundaries of style as public tastes
and fashions change, but other externalities can impose constraints on
musical expression, both implicitly and explicitly. For example, while
it is more than just indulgent for a musician to write a sixty-four bar
intro for the radio mix of a song, it is commercially reckless, since
it limits the number of stations prepared to broadcast the track and
the amount of airplay the song will receive. In particular, a
commercially-aware musician will implicitly impose self-constraint to
ensure that their compositions suit their chosen medium.

Perhaps more significantly, the physical design of musical instruments
creates affordances and constraints that, to a great extent, shape the
music that is written for them; this is as much the case for electronic
instruments (including PSGs) as it is for more traditional acoustic
instruments. In short, video game hardware shaped the sound of early
video game music by way of the affordances they offered and the
constraints that they imposed. Of these affordances, of importance was
the complete top-down control provided to videogame composers by the
sound hardware and its hosting computing platform (R. Hubbard, personal
communication, June 9, 2017). This not only allowed detailed control
over each and every aspect of the music and its performance (similar to
that of Stockhausen’s principle of ‘total control’––White, 1968, p,
319), but it was also enabled the means by which the territory of
structural possibilities could be explored, mapped out by the
hardware’s affordances, and transgressively pushed against and stepped
beyond the boundaries imposed by its constraints.

The ZX Spectrum: A model of technical constraint

A strong hobbyist community exists in the UK (see, for example, Kline,
Dyer-Witheford & de Peuter 2003, pp. 84-108). Therefore it may be
little surprise that the first home computers were sold in the UK as
component kits that required considerable time and technical dexterity
to assemble. It was against this backdrop that Science of Cambridge
(later to become Sinclair Research Ltd.) launched the Microcomputer Kit
14 (MK14) in February 1978 as a ‘minimum cost computer’ (Science of
Cambridge 1978). Science of Cambridge launched the MK14 at a price
point of £39.95, something Practical Electronics described as “a
landmark of […] unassailable proportions” (Berk, 1979). While it was
relatively cheap and accessible, the MK14 looked positively primitive
alongside its contemporaries, the Commodore PET and the Apple II.
Nevertheless, the MK14 sold well enough to justify a successor, named
the ZX80 for its 3.25MHz Zilog Z80 processor, with an added X to denote
a magical X-factor (Tomkins, 2011).

[53]Figure 1: The Science of Cambridge MK14 has a form factor and
interface that shows a clear lineage from the company’s handheld LED
calculators, and was an important first step in the development of mass
home computing.

Figure 1: The Science of Cambridge MK14 has a form factor and interface
that shows a clear lineage from the company’s handheld LED calculators,
and was an important first step in the development of mass home
computing.

Following the broadcast of the Mighty Micro (1979), a groundbreaking
documentary series about the developing computer revolution, the
British Broadcasting Corporation’s (BBC) Further Education Department
began to take an interest in the burgeoning home computer market, and
established the BBC Computer Literacy Project, a series of television
and radio programs that would be based around a BBC-branded
microcomputer. The project was initially scheduled for launch in the
autumn of 1981, which left little time for the BBC to develop its
microcomputer in-house. Instead, they collaborated with the
Cambridge-based firm, Newbury Labs, to draw up a specification for the
machine. This spec matched very closely that of Newbury’s NewBrain, the
intention being, presumably, that Newbury Labs would pick up the BBC
contract. As the project developed, however, Newbury Labs pulled out of
the agreement and did not tender a design. The BBC was forced to
postpone the Computer Literacy Project and broaden their search for a
partner. Sinclair pitched its new machine, the ZX81.

Sinclair lost out on the BBC contract to rivals Acorn, but the ZX81 was
picked up and aggressively promoted by the national newsagent chain,
WHSmith, which had an exclusive contract to supply the machine for six
months. It sold by the thousand. Growing support from the popular press
and a thriving mail-order games network grew the market for the
machine, so that when the ZX Spectrum launched the following year,
Sinclair had an established user base and many developers selling
through a national network of retail outlets.

Free to specify its own components and price point, Sinclair, designed
the most compact and powerful computer that they could to a price,
undercutting the Acorn-designed 32K BBC Model B by over £200 at launch
(Smith, 2011). With the Computer Literacy project giving the machine
free marketing by pushing the idea of the home computer as a tool for
learning, thousands of parents bought into the idea, giving the cheaper
ZX Spectrum a home.

[54]Figure 2: The Sinclair ZX Spectrum, released in April 1982, was
massively successful in the UK, bringing home computing to the masses.

Figure 2: The Sinclair ZX Spectrum, released in April 1982, was
massively successful in the UK, bringing home computing to the masses.

As a consequence of being designed to a low price point, the ZX
Spectrum was a very simple machine. Available in two guises, both
models had 16K of ROM and either 16 or 48K of RAM. It was also, if one
discounts the analogue cassette interface of the ZX81, which could be
co-opted to output simple melodies by POKE-ing certain memory registers
from BASIC [55]^2, Sinclair’s first machine to feature any kind of
onboard sound interface, a motherboard-mounted 22mm, 40 Ohm “beeper”
speaker, which provided just a single channel of 1-bit playback across
a 10-octave range.

[56]Figure 3: The 22mm speaker from a 1983 Issue 4B Sinclair ZX
Spectrum, which provided a single channel of 1-bit sound.

Figure 3: The 22mm speaker from a 1983 Issue 4B Sinclair ZX Spectrum,
which provided a single channel of 1-bit sound.

To compound matters, the sound commands were managed directly by the
main CPU (a Zilog Z80A processor running at 3.5MHz) and a custom
Ferranti Uncommitted Logic Array (ULA) chip. Without the availability
of dedicated sound hardware, calls to the speaker occupied the
processor; therefore, while the Spectrum was beeping it was unable to
do anything else.

Implied polyphony: a channel for free

It is perhaps not surprising then that few of the early Spectrum titles
featured very much in the way of sound or music. Typically, games would
feature a single-channel melody as a title tune, and only limited
in-game sound effects to punctuate key elements of the gameplay.
Chuckie Egg (Alderton, 1983) is typical of this model, featuring the
melody from “Birdie Song (Birdie Dance)” by the Tweets (Rendall &
Thomas, 1981), itself a cover of Werner Thomas’s accordion tune, as its
title music. Such repurposing of existing musical themes was not
uncommon in the early days of gaming. This was as true of graphics and
gameplay as it was of music: Hungry Horace (Tang, 1982), for example,
one of Sinclair’s launch titles, was essentially PacMan (Iwatani, 1980)
in disguise, and Artic Computing’s ZX Galaxians (Wray, 1982a) and
Invaders (Wray, 1982b) unofficially recreated those arcade classics as
closely as was possible on the Spectrum’s hardware. Ben Daglish, a
celebrated C64 musician, recalls:

“I had no idea that copyright existed. Quite seriously […] I really
didn’t. When we wrote all the Jarre stuff and all that […] we had no
real idea as a 14- or 15-year-old kid that you couldn’t just take some
music that you liked, whether it was Beethoven or whether it was Jean
Michel Jarre. We’d just write it down and put it in a computer game”
(Burton & Bowness, 2015).

It was one such act of creative appropriation that lay behind
Perfection Software’s Fahrenheit 3000 (Jones & Williams, 1984), a
64-screen platform game. Perfection wanted a title theme that would
make an impact as soon as the game loaded, and Peter Jones suggested
using Johann Sebastian Bach’s 18^th-century composition Toccata and
Fugue in D minor which he had heard opening the movie Rollerball
(Jewison, 1975). Working from the sheet music of Sky’s 1980 cover
(rearranged by Kevin Peek), Jones coded a five-minute beeper
arrangement in Sinclair BASIC, before Tim Williams converted it to
machine code for the final game.

What makes the music in Fahrenheit 3000 significant is not so much the
arrangement, which doesn’t quite stick faithfully to either the Bach or
the Sky sources but, rather, the choice of musical material itself. The
opening statement of Bach’s fugue is a sequence of semiquavers, which
alternate between the melody and an implied pedal point on A. The
effect, particularly when played at speed, is to create a sense of
two-voice polyphony by using the pedal note to continually reinforce
the sense of the tonal center against the melody.

[57]Figure 4: The opening section of J. S. Bach’s Fugue in D minor
creates a sense of two-voice polyphony by contrasting a repeated pedal
note against a melody line.

Figure 4: The opening section of J. S. Bach’s Fugue in D minor creates
a sense of two-voice polyphony by contrasting a repeated pedal note
against a melody line.

Jet Set Willy (Smith, 1984) features a similar technique in its
arrangement of Beethoven’s Piano Sonata no. 14, Moonlight. Using a
pattern of broken octaves, similar to the left-hand bass patterns of
Boogie Woogie or Stride piano, the arrangement creates a sense of
continuous movement between melody and accompaniment. The effect is
striking, and it is easy to forget that there is nothing more complex
here than a sequence of single-channel square wave tones.

Ben Daglish took the idea to its logical extreme with his soundtrack
for Gremlin Graphics’ Arkanoid clone, Krakout (Toone et al., 1987),
providing an implied bass, accompaniment and melody, all played at
breakneck speed. Importantly, he recalls that part of the joy of
working on a Spectrum was the sense of challenge that it gave. It
forced composers to look for ways to circumvent its limitations and
find novel ways to introduce dynamic movement and musical interest, and
often that involved harnessing the power of the computer itself:

“Half the point of writing some of the music that I did, writing it on
a computer, was that it meant that I could use notes that were never
actually meant to be played by human beings. I could do really fast
runs, scales and arpeggios” (Burton & Bowness, 2015).

His earlier port of Thing Bounces Back (Kerry et al, 1987) for the
Spectrum used a similar approach, but alternates between a bluesy bass
vamp in broken octaves and a bright blues melody. The effect works in
much the same way a blues harpist will alternate between vamping and
soloing to self-accompany, making use of the listener’s aural memory
and a strong sense of harmonic familiarity with the I-IV-V chord
progression.

Granular synthesis: Players can’t help acting on impulse

Artic’s Invaders was an unofficial clone of Taito’s Space Invaders
(Nishikado, 1978) and features near-identical graphics and field of
play to the original coin-op. The soundtrack also mimics the original,
which uses a descending, four-note Dorian scale pattern that repeats
and gradually speeds up, as the invaders are picked-off by the player.

[58]Figure 5: The descending Dorian scale pattern from Artic’s 1982
game, Invaders.

Figure 5: The descending Dorian scale pattern from Artic’s 1982 game,
Invaders.

The above descending scale sequence plays continuously throughout the
game, marking the first use of a continuous non-diegetic soundtrack on
the Spectrum, being released around a year earlier than Bug Byte’s
Manic Miner (Smith, 1983b), whose rendition of Grieg’s In the Hall of
the Mountain King is often credited with this accolade. So how did
Invaders, and indeed Manic Miner, achieve this feat? The solution was
to think small.

Granular synthesis is an approach to sound synthesis and manipulation
that was posed initially by the Greek composer Iannis Xenakis (1971),
who created the composition Analogique B from hundreds of splices of
tiny fragments of magnetic tape (Robindoré & Xenakis, 1996 pp. 11-12).
Conceptually, the idea of treating sounds at times as continuous waves
and at others as though they were composed of tiny sound quanta, or
grains, opens up many interesting and creative ways of working. For
example, two effects that have become commonplace in recent years are
the time-stretch and the pitch-shift, which allow for the independent
manipulation of tempo and pitch in recorded audio. Usually, these two
parameters are inextricably linked: slow down the playback of a sound
recording, and the pitch will drop proportionally. Granular synthesis
enables the pitch and speed to be processed independently by applying
the processing individually to sound grains, before recombining them to
construct the final sound output.

Invaders uses granular synthesis as a technical strategy to create an
in-game soundtrack that addresses the key limitations of the Spectrum’s
hardware. Recall that its speaker was controlled directly by the
computer’s main CPU and ULA, meaning that it was not normally possible
to combine both gameplay and sound. Also, because the speaker was
1-bit, controlled via a single pin of the ULA, the speaker was either
fully driven or at rest. No intermediate states were addressable, and
consequently, there was no level control over the signal, which was a
square wave by default. However, a 1-bit device can produce more than a
square waveform. It was by recognizing this, and working directly
within the software to manipulate the state of the ULA at a low-level,
that author William Wray was able to create multiple independent
channels of sound within the game.

A single cycle of a digital square wave is little more than a sequence
of ones followed by an equal number of zeroes. Repeating this pattern
over and over creates a continuous tone whose period, and therefore
frequency, is determined by the number of ones and zeroes in each
cycle. Increasing the number of ones and zeroes increases the period,
and so lowers the pitch, and vice versa.

A Fourier analysis (Roads 1996, pp. 1084-1112) of the square wave
reveals a well-defined and characteristic spectral signature:

Now suppose that, rather than outputting ones and zeroes in equal
measure, one outputs a sequence of ones followed by three times as many
zeroes. This is a pulse wave, an asymmetrical version of the square
wave. In this case, 25% of the pulse is made from ones, and the rest
from zeroes, and so the pulse wave has a duty cycle of 25%. Its tonal
characteristics are similar to those of the square wave, although a
Fourier analysis reveals a different frequency spectrum, where M is the
number of successive ones in the N sample points that represent a
complete cycle of the wave:

Continuing in this manner, the number of ones in each cycle of the wave
can be reduced further to create smaller and smaller duty cycles,
varying the frequency spectrum and tone of the sound, until the beeper
is sent just a single positive bit followed by a stream of zeroes. This
signal is a binary impulse, and its Fourier transform is a constant. In
other words, an impulse contains all possible frequencies at equal
magnitude.

It is not possible to hear an impulse on its own, but it is possible to
hear its effect on a speaker, the so-called impulse response. Any
speaker exhibits a degree of inertia, taking a short but finite time to
move from rest to maximum displacement and back again, and it is this
response that can be heard as a noticeable click. By sequencing a
series of binary impulses together separated by short gaps, an impulse
train emerges, a pitched tone, the frequency of which is determined by
the period between successive impulses, and which contains all of the
harmonics of the signal at equal strength, as shown in Figure 6.
[59]Figure 6 – Outputting an impulse train generates a signal whose
frequency spectrum contains an equal amount of energy across all
harmonics. The top plot here shows a time domain representation of the
sound, showing a series of impulses separated by silence. The lower
plot shows a frequency domain representation of the same sound, which
has its energy distributed equally across all of its harmonics.

Figure 6 – Outputting an impulse train generates a signal whose
frequency spectrum contains an equal amount of energy across all
harmonics. The top plot here shows a time domain representation of the
sound, showing a series of impulses separated by silence. The lower
plot shows a frequency domain representation of the same sound, which
has its energy distributed equally across all of its harmonics.

Invaders uses binary impulse train synthesis to create all of the
sounds in the game, ensuring that the speaker is tied up for as short a
period as possible while still allowing for continuous in-game sound,
the game processing taking place in the fractions of a second between
impulses. Moreover, the story does not end there. By using clever
sequencing of the sounds, similar to that of the implied polyphony
discussed in Section 4 above, Invaders manages to create multiple sound
effects playing synchronously with the underscore.

The first, and most frequent of the game’s sound effects is the alien
explosion, which is cued whenever a player shot collides with one of
the alien invaders. The explosion lasts for approximately 85ms, and is
always triggered sequentially with the underscore. If an explosion
sound coincides with one of the soundtrack tones, the sound that was
triggered first, either the tone or the explosion effect, takes
priority, and the subsequent sound is delayed until the first sound has
completed. This results in a maximum delay for the explosion sound of
around 25ms, which is barely perceptible in the context of the game.
For the underscore, however, the worst-case situation could result in a
delay of around 80-90 ms, which is enough to cause a degree of
jerkiness to the underlying note sequence, although not so much as to
cause it to break down.

The second effect is triggered by a bonus mystery ship, which travels
across the top of the screen. Here, the sound effect plays continuously
while the ship is onscreen, which takes approximately between 6 to 7
seconds, and is created by toggling the speaker on and off at 25ms
intervals. During the mystery ship effect, when an underscore tone is
due to be triggered, the game stops generating the mystery ship
impulses and prioritizes the underscore grain, before picking up the
mystery ship sound when the underscore tone has finished. The blip
effectively masks the discontinuity in the mystery ship sound effect,
creating an illusory continuity of tone in the latter. The final two
effects are the player explosion and a level-start siren effect. These
are played strictly sequentially, and cause the other elements of the
soundtrack to stop playing.

Aside from the slight lumpiness to the underscore caused by the
prioritized sequencing of the soundtrack elements, and the curious
omission of sound effects for the player ship’s laser fire, the game’s
soundtrack is very effective, not just referencing the original sound
effects from the coin-op, but also in creating a real sense of
continuous two- or three-channel sound, something that it achieves by
the clever handling and sequencing of the short impulse trains.

Extending this idea further, it wasn’t long before developers were
using the technique to play two simultaneous musical lines by
alternating between two or more grain pitches, and that grainy, bubbly
quality became firmly established as part of the Spectrum sound.
Rockman (Carter, 1985), for example, features an arrangement of the
first movement of Mozart’s Eine Kleine Nachtmusik, although the use of
50ms sound grains and lengthy inter-grain silences results in an
unconvincing multi-voice effect, in the same way that a slowing a film
sequence to below about 15 frames per second spoils the illusion of
continuity of motion, and the viewer becomes aware that they are seeing
a series of time-sampled images. More successful was Imagine’s port of
the Konami coin-op, Yie Ar Kung Fu (Beuken & Thorpe, 1985), which uses
the effect to play the main game stings in double-octaves, and Dynamite
Dan (Bowkett, 1985), which uses alternating and arpeggiated grain
pitches to recreate Mozart’s Rondo a la Turca. Durell Software featured
two-voice granular music tracks on two of their 1986 releases, Thanatos
(Richardson, 1986a) and Turbo Esprit (Richardson, 1986b).

The music on Turbo Esprit is a fine example of the technique. Its Jan
Hammer-styled melody complements perfectly the Miami Vice-like
gameplay.

Singing to the tune of two

In 1983, Matthew Smith, a schoolboy from the seaside town of New
Brighton in the North-West of England, was loaned a Spectrum by
Liverpool-based publisher Bug Byte to develop three games. His first
title, Styx (1983a), was a fairly simple action maze game based on a
single, repeating screen that became progressively more difficult each
time the player completed a level. It was his second game, Manic Miner,
which became a runaway success, making Smith an unlikely superstar, and
introduced the Spectrum’s first truly iconic character, Miner Willy.

Manic Miner was based on Miner 2049er (Hogue 1982), a platform game
that featured a Canadian Mountie, Bounty Bob, navigating his way
through ten different screens and inspecting each area before his
oxygen runs out. Several elements of Miner 2049er appear in Manic Miner
(the underground setting and the oxygen-level as a timer, for example),
but in creating Miner Willy, Smith injected a particularly British spin
on the game, with an absurd humor to the level and character design,
and a Pythonesque boot [60]^3, which descends to squash Willy when the
game is over.

On loading, the game displays a dynamic title screen showing the sun
setting behind an idyllic cliff-top house, below which an animated
keyboard plays, pianola-style, the notes of a delightfully-clangorous
two-channel rendition of The Beautiful Blue Danube by Johann Strauss
II. Although the music routine includes an algorithm that uses the note
data to display the notes onscreen, the keyboard graphics show a
shortened octave (C to E) to the left of middle C, making it almost
impossible to use this as a visual point of reference for transcribing
the music.

[61]Figure 7: The title screen from Manic Miner. Note the short octave,
C to E, just to the left of the middle C in the graphic, making it
difficult to use a visual point of reference for transcribing the
music.

Figure 7: The title screen from Manic Miner. Note the short octave, C
to E, just to the left of the middle C in the graphic, making it
difficult to use a visual point of reference for transcribing the
music.

Smith (2014) notes that:

“The game needed music, as I felt it was an integral part of the
attraction. The title song, I had an old, simple piano arrangement [of
The Beautiful Blue Danube] in sheet music so it was easy to transcribe.
I did everything as quickly as possible, got the loop running as fast
as possible, but I never got too prissy about exact timings”.

A RAM disassembly of Smith’s code reveals that he used impulse trains
as the basis of the title music routine. The music was stored in memory
as a series of 95 groups, each containing three data bytes. Each
triplet corresponds to a separate beat (or sub-beat) in the
arrangement, and each is encoded as a duration and a pair of pitch
values, or more accurately, as counter values, which are used to
calculate the period between successive impulses using a technique
known as frequency divider, or divide down synthesis (Roads 1996, p.
925).

This technique generates a waveform by counting the pulses of a master
clock, and triggering an impulse when a chosen divisor (the counter
limit) is reached. The counter is then reset and begins again. This
generates a periodic impulse train at a frequency that can be
calculated as follows:

By rearranging the equation, one can calculate the counter limit that
corresponds to any given frequency. In the case of Manic Miner, the
counter is updated on each cycle of the theme-music subroutine, and so
the timing of each master clock tick is determined by two factors: the
clock speed of the Z80 CPU, which runs at 3.5 MHz, and the length of
time taken by the CPU to execute each of the machine instructions in
the loop, which can be obtained experimentally. Smith was thus able to
construct a frequency table that mapped the notes of the musical
arrangement to a series of counter values, and it is these values that
provide the note data for his routine.

Smith’s music routine uses two counters to calculate two simultaneous
impulse trains. The routine writes the two counter values stored in the
data triplets into two memory registers, and calculates the period
between successive impulses, effectively interleaving the two impulse
trains on playback to create two channels of playback. For single
melody notes, Smith encoded the pitch as a pair of counter values
separated by 1 to create a phasing effect. Chords are encoded as two
distinct frequency values. The phasing effect works well, creating a
harmonically rich, time-varying tone on the single notes with a
characteristic sweeping effect at the beat frequency. However, when the
effect is used to trigger two simultaneous distinct pitches, the
routine introduces a degree of pitch ambiguity that results from the
relative amplitudes of the harmonics of the individual tones.

As noted above, single notes are encoded as pairs of counter values
separated by a single unit, the effect of which is to create two binary
impulse trains separated in frequency by only a few Hertz. This results
in a frequency spectrum that is very close to a harmonic series, as
illustrated in Figure 8.

[62]Figure 8: A spectral plot of the two near-coincident impulse trains
shows a pseudo-harmonic series, although the concordance between the
harmonics of the lower tone, illustrated by the dark bands, and the
upper tone, illustrated by the lighter bands, decreases with increasing
frequency. This harmonic character makes it easy to identify a definite
sense of pitch.

Figure 8: A spectral plot of the two near-coincident impulse trains
shows a pseudo-harmonic series, although the concordance between the
harmonics of the lower tone, illustrated by the dark bands, and the
upper tone, illustrated by the lighter bands, decreases with increasing
frequency. This harmonic character makes it easy to identify a definite
sense of pitch.

When two impulse trains are interleaved at distinct frequencies, this
pseudo- harmonic spectrum breaks down, as shown in Figure 9 below. This
spectral plot illustrates a major third interval. As before, the dark
bands correspond to the harmonics of the lower tone in the interval,
and the light bands to the harmonics of the upper tone. It can be seen
immediately that there is no regular structure to these frequency
components. The spacing between spectral components is variable, and
includes a number of very closely clustered components, which
introduces an unpleasant beating to the tone. Also, because each of the
harmonics of each tone has equal magnitude, one of the key auditory
cues that we normally use to locate and identify pitch, the
fundamental, which is usually the strongest of these frequency
components, is not evident. Every frequency component therefore
arbitrarily becomes the dominant one as the ear focuses in on different
regions, creating a very vague and indistinct sense of pitch. The
overall effect is to create a sense in the listener of a rough, complex
tone, rather than two discrete and distinct pitches.

[63]Figure 9: A spectral plot of two non-coincident impulse trains
shows a more complex relationship. There is variability in the spacing
between components and some clustering, leading to beating. The uniform
magnitude of the components makes it very difficult to identify
discrete pitches.

Figure 9: A spectral plot of two non-coincident impulse trains shows a
more complex relationship. There is variability in the spacing between
components and some clustering, leading to beating. The uniform
magnitude of the components makes it very difficult to identify
discrete pitches.

Smith’s approach, then, was innovative and, to an extent, very
effective. He had managed to move beyond implying polyphony on a macro
level, by manipulating the temporal arrangement of fairly large-scale
sound grains, to implying it on a micro level by interleaving impulses,
the smallest units of binary sound. This took ZX Spectrum music into
similar territory to that which was explored by electronic music
pioneers like Pete Samson, whose work with MIT’s TX-0 and PDP-1
computer systems, explored similar methods some twenty years earlier
(Levy, 2010, p 17-18), and suggested a direction for other developers
to continue innovating.

Pulse-Width Modulation

In 1984, Quicksilva’s Zombie Zombie (White & Sutherland, 1984) became
the first spectrum game to address the failings of Manic Miner’s
two-channel routine and coax two completely independent channels of
tunable square waves from the spectrum using pulse-width modulation
(PWM). As discussed earlier, sending different sequences of ones and
zeroes to the beeper allows the creation of a series of related wave
shapes, from trains of binary impulses through to pulse waves of
varying duty cycle. This idea can be taken one step further by
returning to the idea of speaker inertia, which is the notion that a
speaker cone cannot change its state discretely and instantaneously.
When driven, it takes a short but finite time to reach maximum
displacement and must move through all its intermediate states between
fully off and fully on. The speaker behaves in a similar, though not
identical way, as it returns to rest. Modulating the width of the
signals (by varying the amount of time that the speaker is driven
relative to the time that it is not) sent to the beeper, the speaker
can be driven to intermediate points between off and on, thereby
simulating the effect of a continuous analogue voltage. There are, as
you might imagine, many ways to achieve this, but the most common
method for the Spectrum was to use pre-calculated lookup tables to
convert note frequencies to counter values which could be stored in
memory and used to synthesize pulse trains in a similar way to the
binary impulse trains discussed earlier. Using this form of PWM, the
speaker cone could be made to dance in very elaborate ways to create
very complex multi-voice tracks. This process tied up the CPU
completely, though, meaning that the effect was only possible for the
title screen and breaks in gameplay.

The sound routine in Zombie Zombie generates two-channels of sound
without any volume or timbral control, and is based around an eighth
note quantization scheme, with longer notes consisting of multiple
eighth notes at the same pitch and triggered sequentially. The game
features three main music sequences. The first is a triumphal,
march-like setting of Ten Green Bottles, which morphs in bar 9 into an
unsettling arrangement in parallel augmented 4ths, a reference to the
common eighties horror soundtrack trope of the distended children’s
song or nursery rhyme. The game also features a simple, yet triumphal
arrangement of Bizet’s March of the Toreadors on completion of the
game, and a track that combines White’s two-channel routine with the
implied polyphony technique described in Section 4, combining bass and
a simple arpeggiated accompaniment to create the suggestion of three
simultaneous voices.

Having established PWM as a viable approach to music-making on the
Spectrum, some games applied the technique with varying degrees of
success, while Melbourne House’s Wham! The Music Box (Alexander, 1985),
a fairly sophisticated music sequencer and percussion synthesizer
provided users with an easy-to-use graphical interface that would be
familiar to users of most digital audio workstations today. The
Spectrum’s beeper, however, had yet more to give, and it was Tim
Follin, a young programmer from St. Helens, in the northwest of
England, who really embraced PWM, and took the Spectrum and its 1-bit
voice to a whole new level. Follin developed his sound routine on his
earliest titles, Subterranean Stryker (Follin, 1985), Star Firebirds
(Follin et al, 1985a) and Vectron (Follin et al, 1985b), so that by
1986 with Agent X, both his signature sound and his technical
implementation, which had reached a channel count of five, along with
percussion, enveloping, portamento and phasing, were already very well
developed. This did, however, come at the expense of audio fidelity.

In retrospect, Follin’s earliest soundtracks showcase the incremental
development of both his sound engine and his emerging musical style.
The soundtrack for his first Spectrum game, Subterranean Stryker, is
interesting only insofar as it demonstrates some of his engine’s
nascent capabilities. It features a single-channel melody line, which
drifts stylistically and with little in the way of melodic coherence,
the programming equivalent, perhaps, of a guitarist noodling on a
fretboard. Beneath the notes, however, can be heard amplitude
enveloping, a far-from-trivial task on a speaker that can only be
either on or off, and a phasing effect, creating a dynamically-changing
timbre, both features that Follin would continue to develop. For his
next title, Star Firebirds, Follin introduced a portamento effect,
creating quite dramatic Emersonian pitch glides in places, but it was
Vectron, a 3D maze game inspired by the Space Paranoids sequence from
Disney’s Tron (Lisberger, 1982), where both the engine and Follin’s
musical style really begin to shine through. The soundtrack in Vectron
manages three independent voices during playback and begins with a
phased, enveloped synth leading into an electronic fanfare, before a
fast blues-scale riff, not unlike the percussive organ lines of Keith
Emerson and Rick Wakeman, begins. The score then breaks style, directly
referencing Wendy Carlos’s original score from Tron, before returning
to a series of blues-scale sequences.

Follin published his three-channel music routine as a hexadecimal
type-in program listing in Your Sinclair magazine (Follin, 1987),
making it freely available for use in non-commercial programs. The
listing contains just 167 lines of code, and the entire routine,
complete with note data weighs in at just over 1K in size. The article
noted that, at the time, Follin was working on a new 6-channel routine
with chorus, bass, echo, portamento and full ADSR, all elements that
would turn up in his later soundtracks as his commercial engine
continued to develop.

In 1986, with the release of Agent X, Follin upped the channel count to
5, although this came at the expense of some audio fidelity. With the
processor pushed to its limits, the music is very lo-fi, something
Follin acknowledged in an interview with Eurogamer, noting that “It’s
hard to actually hear [the music in Agent X], I think I’d pushed the
processor too far actually!”. Follin’s Agent X engine works by using
five of the Z80’s registers, sections of RAM inside the main CPU that
can be used to store and rapidly operate on frequently-used data,
prioritized areas of memory that allow for rapid access by the
processor, in a loop, all of which count down from a series of
predetermined values to zero. When each loop is complete, it generates
a pulse, the width of which determines the speaker level. The
constantly shifting pulse-widths affect both the level and timbre,
adding noise in the sense that the changing harmonic content introduces
an undesirable roughness to the sound and causes tuning problems as the
channel count rises.

Summary

That peculiar quality of sound of the ZX Spectrum, its quality of
sound, the grungy fuzziness, came to define the sound of the Spectrum
for a generation of gamers, becoming an important feature of the style,
in much the same way that the warmth of tape saturation came to
characterize the sound of recorded music throughout the 1960s and 70s
to such an extent that modern developers now devote significant time
and resource to create effects algorithms that degrade pristine digital
recordings to simulate some of that analogue character.

It was a sound, however, that evolved gradually, through a series of
logical steps, each of which is rooted elsewhere in the annals of
electronic music history. Interestingly, however, my conversations with
those early game music pioneers and game music historians, including
Rob Hubbard, Ben Daglish, and Chris Abbott, suggest that these
innovations happened independently. These were young, creative
programmers looking for a way around a technical problem. In the same
way that they weren’t aware of copyrights, nor were they aware of Max
Matthews’ and Peter Samson’s innovations in electronic music that had
taken place in the preceding decades.

Following the demise of the Spectrum in 1992, 1-bit music continued to
feature in many games, largely thanks to the PC speaker, which provided
the default sound output for many early PC games. LucasArts’ The Secret
of Monkey Island (Gilbert, 1990) is a fine example of such early PC
soundtracks, using a combination of the techniques outlined above to
create an engaging title theme.

With the introduction of dedicated PC soundcards, Frequency Modulation
and sample playback synthesis gradually replaced PSGs (Programmable
Sound Generators) as the source of video game sound, and video game
soundtracks became more cinematic, often increasingly relying on
multiple channels with orchestral timbres, both in concept and in
execution, and yet the chirpy 1-bit sound continued. Music trackers,
such as the DOS-based Monotone (Leonard, 2008) and Pulse Tracker
(Larsson, 2012), put these 1-bit music techniques in the hands of
musicians rather than programmers. Emulators and hacked code allowed a
new generation of musicians to continue to push the capabilities of the
Spectrum, and demoscene meets and compos (competitive events that
encourage the creation of sophisticated real-time generative art and
music using obsolete and limited hardware) continue to provide
platforms for creative performance.

The growth in recent years of open development systems like the
Raspberry PI, which was introduced to promote the teaching of basic
computer science in schools, has kick-started the same sort of
experimental approach to coding that happened during the first wave of
the microcomputer revolution. With just a few lines of code and a small
Mylar speaker wired to the digital output pin of an Arduino, a new
generation of coders has been able to experiment with 1-bit music
techniques.

Recent developments in music technology over the last 30 years have
seen an explosion in the range and scope of music creation and
production tools. Virtualization has taken esoteric studio hardware
that previously would have been the preserve of international-class
studios and converted them to code, allowing all-comers to build
flexible virtual processing racks, driven by carefully designed presets
that allow the devices easily to integrate into any production session.
Classic synths have similarly been modeled and virtualized, and primed,
both with sounds and loopable MIDI sequences, to allow their users to
channel the sounds of, for example, Kraftwerk, the Prodigy, or Emerson,
Lake and Palmer, with a few simple selections from a drop-down menu.
Such is the democratizing effect of this technology that armed with a
laptop, a suitable digital audio workstation (DAW) and a little time
and enthusiasm, it is possible to create quite authentic-sounding
electronic music tracks with relatively little effort. In many
respects, this is a very positive development. It has provided a
creative outlet for many and has made music making and production more
accessible. This accessibility, however, comes at a cost.

Constraint is what the lo-fi sound of the 8-bit microcomputer can
provide. With simple, raw waveforms, limited polyphony and few
options for dynamic articulation, chip musicians have no option but
to go right back to the very basics and address the fundamentals
that make music engaging and entertaining.

Historically, scholars such as Amabile, (1983) have argued that too
much constraint on creative freedom decreases the intrinsic motivation
to create. However, recent work has demonstrated a clear distinction
between constraints that obstruct creativity (for example by
encouraging conformity, as may be the case when composing new work from
preconfigured musical patterns and presets), and those that promote it
(see, for example, Stokes, 2005).In addition, recent research has
suggested that the “Paradox of Choice” (Schwartz, 2004) can have
similarly deleterious effects on intrinsic motivation (Iyengar Lepper,
2000) and originality (Chua Iyengar, 2008). While, on the one hand, it
is wonderfully liberating to have complex in-the-box software solutions
that enable musicians to compose, arrange and produce, on the other
hand, the tyranny of choice that is presented can be crippling, leading
to creative procrastination as one searches for ‘just the right sound’,
rather than ploughing on with the process of creation. It is just as
Devo sang back in the 80s: “Freedom of choice is what you got; Freedom
from choice is what you want,” (Mothersbaugh, 1980).

Constraint is what the lo-fi sound of the 8-bit microcomputer can
provide. With simple, raw waveforms, limited polyphony and few options
for dynamic articulation, chip musicians have no option but to go right
back to the very basics and address the fundamentals that make music
engaging and entertaining. There is nowhere for half-formed ideas or
weak arrangements to hide. It is electronic music in its most
fundamental state; it is about simple ideas expressed well.

In 2003, Malcolm McLaren declared 8-bit to be the new punk (2003). It
has that same, lo-fi DIY aesthetic and, just as punk raised a defiant
middle finger to the worst excesses of prog rock and glam rock, so too
8-bit and the associated lo-fi subculture stands in stark contrast to
the over-produced sound of much of current commercial music. The
Spectrum embodies that spirit perfectly and, as a small but vibrant
part of the retro computing scene, the demoscene and the chipscene
suggest that there are, even now, many new musical chapters to be
written in Z80 assembly.

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Author’s Info:
Kenneth McAlpine is a musician, author and academic at Abertay
University, Dundee, Scotland, and is a Member of the Editorial Board of
The Computer Games Journal. His research focus includes video game
music and the role of technical constraint in the development of its
aesthetic, and the implementation and analysis of real-time adaptive
music used in interactive contexts.

Endnotes:
1. For an overview of the early period of video game music see, for
example, Collins (2008), and Collins and Greening (2016) [65]▲
2. BASIC, or Beginners All-Purpose Symbolic Instruction Code, is a
high-level interpreted programming language that provides simple
English-like commands that allow the end-user control over certain
aspects of the machine’s hardware. POKE was one such Spectrum BASIC
command, which allowed users to write data values directly into the
machine’s addressable memory registers. By addressing certain
memory registers, the ZX81’s tape interface, which used square wave
tones to encode and save digital data to analogue cassette tape,
could be made to play simple melodies. [66]▲
3. “Pythonesque boot” is a reference to the surreal animation of a
gigantic squashing boot that regularly appears in the television
series of the British comedy sketch group, Monty Python, in order
to segue various sketches. [67]▲

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[71]“Heute gehört uns die Galaxie”: Music and Historical Credibility in
Wolfenstein: The New Order’s Nazi Dystopia.
[72]Driving the SID chip: Assembly Language, Composition, and Sound
Design for the C64
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G|A|M|E (ISSN: 2280-7705)
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