tAdd note on syncing audio and video - adamsgaard.dk - my academic webpage
(HTM) git clone git://src.adamsgaard.dk/adamsgaard.dk
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---
(DIR) commit dd1fafe054a5b921d85668cb7a09a05eafe4bbe3
(DIR) parent 5b74834ab0d5bb546c96734bf5fa36c6b2c7353b
(HTM) Author: Anders Damsgaard <anders@adamsgaard.dk>
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(-)
---
(DIR) 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 "$f"
+done
+</code></pre>
+
<figure class="pagefigure">
<video poster="video/screencast.jpg" style="object-fit:fill;"
controls preload="none" class="mediaframe">
(DIR) diff --git a/pages/004-screencasts.txt b/pages/004-screencasts.txt
t@@ -19,37 +19,37 @@ script serves the purpose of starting and stopping recording:
lockfile=/tmp/screenrecord.pid
startrecording() {
- out="$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 start'
- 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 start'
+ 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