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	tAdd note on syncing audio and video - adamsgaard.dk - my academic webpage
	git clone git://src.adamsgaard.dk/adamsgaard.dk
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	---
	commit dd1fafe054a5b921d85668cb7a09a05eafe4bbe3
	parent 5b74834ab0d5bb546c96734bf5fa36c6b2c7353b
	Author: Anders Damsgaard <[email protected]>
	Date: Tue, 17 Mar 2020 12:36:24 +0100

	Add note on syncing audio and video

	Diffstat:
	M pages/004-screencasts.html \| 20 ++++++++++++++++++++
	M pages/004-screencasts.txt \| 81 +++++++++++++++++++----------…

	2 files changed, 70 insertions(+), 31 deletions(-)
	---
	diff --git a/pages/004-screencasts.html b/pages/004-screencasts.html
	t@@ -96,6 +96,26 @@ for f in "$@"; do
	done
	</code></pre>

	+<p>If there is a delay between video and audio, this can also be
	+adjusted using ffmpeg(1). I correct for a 0.3 s delay that I
	+encounter when recording on my laptop:</p>
	+
	+<pre><code>#!/bin/sh
	+
	+synchronize() {
	+ ffmpeg -y -i "$1" \
	+ -itsoffset 0.300 \
	+ -i "$1" \
	+ -map 0:v -map 1:a \
	+ -c copy \
	+ "${1%.}_out.${1##.}"
	+}
	+
	+for f in "$@"; do
	+ synchronize "$f"
	+done
	+</code></pre>
	+
	<figure class="pagefigure">
	<video poster="video/screencast.jpg" style="object-fit:fill;"
	controls preload="none" class="mediaframe">
	diff --git a/pages/004-screencasts.txt b/pages/004-screencasts.txt
	t@@ -19,37 +19,37 @@ script serves the purpose of starting and stopping recordi…
	lockfile=/tmp/screenrecord.pid

	startrecording() {
	- out="$HOME/screenrecord-$(date '+%Y-%m-%d_%H:%M:%S').mkv"
	- ffmpeg -y \
	- -f x11grab \
	- -framerate 60 \
	- -s "$(xdpyinfo \| grep dimensions \| awk '{print $2}')" \
	- -i $DISPLAY \
	- -f sndio -i default \
	- -r 30 \
	- -c:v libx264rgb -crf 0 -preset ultrafast -c:a flac \
	- "$out" >/dev/null 2>&1 &
	- printf '%s' "$!" > "$lockfile"
	-
	- sleep 1
	- if [ ! -f "$out" ]; then
	- echo 'error: ffmpeg recording did not start' >&2
	- notify-send -u CRITICAL "${0##*/}" 'ffmpeg recording did not s…
	- rm -f "$lockfile"
	- exit 1
	- fi
	+ out="$HOME/screenrecord-$(date '+%Y-%m-%d_%H:%M:%S').mkv"
	+ ffmpeg -y \
	+ -f x11grab \
	+ -framerate 60 \
	+ -s "$(xdpyinfo \| grep dimensions \| awk '{print $2}')" \
	+ -i $DISPLAY \
	+ -f sndio -i default \
	+ -r 30 \
	+ -c:v libx264rgb -crf 0 -preset ultrafast -c:a flac \
	+ "$out" >/dev/null 2>&1 &
	+ printf '%s' "$!" > "$lockfile"
	+
	+ sleep 1
	+ if [ ! -f "$out" ]; then
	+ echo 'error: ffmpeg recording did not start' >&2
	+ notify-send -u CRITICAL "${0##*/}" 'ffmpeg recording did not s…
	+ rm -f "$lockfile"
	+ exit 1
	+ fi
	}

	stoprecording() {
	- kill "$(cat "$lockfile")"
	- rm -f "$lockfile"
	- notify-send "${0##*/}" 'recording ended'
	+ kill "$(cat "$lockfile")"
	+ rm -f "$lockfile"
	+ notify-send "${0##*/}" 'recording ended'
	}

	if [ -f "$lockfile" ]; then
	- stoprecording
	+ stoprecording
	else
	- startrecording
	+ startrecording
	fi

	On Linux systems, the sound driver sndio should be replaced by alsa
	t@@ -63,9 +63,9 @@ command. The following script toggles the video feed:
	#!/bin/sh
	# remember to `chown $USER /dev/video0`
	if pgrep video >/dev/null 2>&1; then
	- pkill video
	+ pkill video
	else
	- nohup video -s 320 >/dev/null 2>&1 &
	+ nohup video -s 320 >/dev/null 2>&1 &
	fi

	On Linux, the command mpv /dev/video0 can take place of the video(1)
	t@@ -80,14 +80,33 @@ concievable loss in quality:
	#!/bin/sh

	encode() {
	- ffmpeg -y -i "$1" \
	- -c:v libx264 -threads 0 -preset faster -pix_fmt yuv420p \
	- -c:a aac -crf 10 \
	- "${1%.*}_out.mp4"
	+ ffmpeg -y -i "$1" \
	+ -c:v libx264 -threads 0 -preset faster -pix_fmt yuv420p \
	+ -c:a aac -crf 10 \
	+ "${1%.*}_out.mp4"
	}

	for f in "$@"; do
	- encode "$f"
	+ encode "$f"
	+done
	+
	+If there is a delay between video and audio, this can also be
	+adjusted using ffmpeg(1). I correct for a 0.3 s delay that I
	+encounter when recording on my laptop:
	+
	+#!/bin/sh
	+
	+synchronize() {
	+ ffmpeg -y -i "$1" \
	+ -itsoffset 0.300 \
	+ -i "$1" \
	+ -map 0:v -map 1:a \
	+ -c copy \
	+ "${1%.}_out.${1##.}"
	+}
	+
	+for f in "$@"; do
	+ synchronize "$f"
	done

	[3]Example screen recording using ffmpeg(1) and video(1) with the