I wrote the following script to capture both the screen and the audio from the microphone, both lossless from my laptop:
#!/bin/bash d=$(date +"%Y%m%d_%-H%M") dimensions=$(xdpyinfo |grep dimensions| awk '{print $2}';) ffmpeg -f x11grab -r 25 -s "${dimensions}" -i "${DISPLAY}" -f alsa -ac 2 -i default -c:v libx264rgb -crf 0 -color_range 2 -preset ultrafast -c:a pcm_s16le -strict -2 "${d}.mkv" The sound becomes very intermittent when setting to a very low number of frames per second (the sound practically disappears when r is set to 7) and it improves substantially to an acceptable quality with -r set to 50 or higher. This behaviour is also present with lossy capturing of the screen and audio as with ffmpeg -f alsa -ac 2 -i default -f x11grab -s "${dimensions}" -r 7 -i "${DISPLAY}" "${d}.mkv"
Why does the -r parameter influence the quality of the sound? Is the syntax of my script incorrect? When I record the audio without screencasting, either ffmpeg -f alsa -ac 2 -i default "${d}.mp4" or ffmpeg -f alsa -ac 2 -i default "${d}.wav" works fine giving me satisfying quality of the audio. The output from recording with ffmpeg -f x11grab -r 25 -s "${dimensions}" -i "${DISPLAY}" -f alsa -ac 2 -i default -c:v libx264rgb -crf 0 -color_range 2 -preset ultrafast -c:a pcm_s16le -strict -2 "${d}.mkv" is:
ffmpeg version 5.1.4-0+deb12u1 Copyright (c) 2000-2023 the FFmpeg developers built with gcc 12 (Debian 12.2.0-14) configuration: --prefix=/usr --extra-version=0+deb12u1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libglslang --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librist --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --disable-sndio --enable-libjxl --enable-pocketsphinx --enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-libplacebo --enable-librav1e --enable-shared libavutil 57. 28.100 / 57. 28.100 libavcodec 59. 37.100 / 59. 37.100 libavformat 59. 27.100 / 59. 27.100 libavdevice 59. 7.100 / 59. 7.100 libavfilter 8. 44.100 / 8. 44.100 libswscale 6. 7.100 / 6. 7.100 libswresample 4. 7.100 / 4. 7.100 libpostproc 56. 6.100 / 56. 6.100 [x11grab @ 0x55d20fe24040] Stream #0: not enough frames to estimate rate; consider increasing probesize Input #0, x11grab, from ':0': Duration: N/A, start: 1707046868.487852, bitrate: 1658880 kb/s Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1920x1080, 1658880 kb/s, 25 fps, 1000k tbr, 1000k tbn Guessed Channel Layout for Input Stream #1.0 : stereo Input #1, alsa, from 'default': Duration: N/A, start: 1707046868.768038, bitrate: 1536 kb/s Stream #1:0: Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s Stream mapping: Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264rgb)) Stream #1:0 -> #0:1 (pcm_s16le (native) -> pcm_s16le (native)) Press [q] to stop, [?] for help [libx264rgb @ 0x55d20fe7e9c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2 [libx264rgb @ 0x55d20fe7e9c0] profile High 4:4:4 Predictive, level 4.0, 4:4:4, 8-bit [libx264rgb @ 0x55d20fe7e9c0] 264 - core 164 r3095 baee400 - H.264/MPEG-4 AVC codec - Copyleft 2003-2022 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=0 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=0 threads=24 lookahead_threads=4 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=25 scenecut=0 intra_refresh=0 rc=cqp mbtree=0 qp=0 Output #0, matroska, to '20240204_1241-loseless.mkv': Metadata: encoder : Lavf59.27.100 Stream #0:0: Video: h264 (H264 / 0x34363248), bgr0(pc, progressive), 1920x1080, q=2-31, 25 fps, 1k tbn Metadata: encoder : Lavc59.37.100 libx264rgb Side data: cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A Stream #0:1: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s Metadata: encoder : Lavc59.37.100 pcm_s16le frame= 1 fps=0.0 q=0.0 size= 1kB time=00:00:00.02 bitrate= 286.5kbits/sframe= 21 fps=0.0 q=0.0 size= 1kB time=00:00:00.02 bitrate= 286.5kbits/s[alsa @ 0x55d20fe30440] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8) [alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 33 fps= 32 q=0.0 size= 1kB time=00:00:00.81 bitrate= 7.3kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 46 fps= 30 q=0.0 size= 1kB time=00:00:00.82 bitrate= 7.3kbits/sframe= 58 fps= 28 q=0.0 size= 1kB time=00:00:01.58 bitrate= 3.8kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 71 fps= 28 q=0.0 size= 1kB time=00:00:02.34 bitrate= 2.6kbits/sframe= 84 fps= 28 q=0.0 size= 5376kB time=00:00:02.34 bitrate=18780.5kbits/[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 96 fps= 27 q=0.0 size= 5376kB time=00:00:03.14 bitrate=14003.2kbits/[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 109 fps= 27 q=0.0 size= 5376kB time=00:00:03.93 bitrate=11186.2kbits/frame= 122 fps= 27 q=0.0 size= 5376kB time=00:00:03.93 bitrate=11186.2kbits/[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 134 fps= 26 q=0.0 size= 5376kB time=00:00:04.70 bitrate=9358.3kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 147 fps= 26 q=0.0 size= 5376kB time=00:00:04.88 bitrate=9020.9kbits/sframe= 160 fps= 26 q=0.0 size= 5376kB time=00:00:05.49 bitrate=8010.2kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 173 fps= 26 q=0.0 size= 5376kB time=00:00:06.25 bitrate=7039.7kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 185 fps= 26 q=0.0 size= 5376kB time=00:00:06.39 bitrate=6889.9kbits/sframe= 198 fps= 26 q=0.0 size= 5376kB time=00:00:07.02 bitrate=6268.2kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. [matroska @ 0x55d20fe7d800] Non-monotonous DTS in output stream 0:1; previous: 7108, current: 7104; changing to 7108. This may result in incorrect timestamps in the output file. [matroska @ 0x55d20fe7d800] Non-monotonous DTS in output stream 0:1; previous: 7173, current: 7170; changing to 7173. This may result in incorrect timestamps in the output file. frame= 211 fps= 26 q=0.0 size= 7424kB time=00:00:07.82 bitrate=7776.2kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 223 fps= 26 q=0.0 size= 7424kB time=00:00:07.97 bitrate=7626.0kbits/sframe= 236 fps= 26 q=0.0 size= 7424kB time=00:00:08.58 bitrate=7088.3kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 248 fps= 26 q=0.0 size= 7424kB time=00:00:09.34 bitrate=6510.1kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 261 fps= 26 q=0.0 size= 7424kB time=00:00:09.40 bitrate=6469.2kbits/sframe= 273 fps= 26 q=0.0 size= 7424kB time=00:00:10.14 bitrate=5996.0kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 286 fps= 26 q=0.0 size= 8960kB time=00:00:10.93 bitrate=6711.2kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 299 fps= 26 q=0.0 size= 8960kB time=00:00:10.94 bitrate=6703.8kbits/sframe= 312 fps= 26 q=0.0 size= 8960kB time=00:00:11.69 bitrate=6276.2kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 324 fps= 26 q=0.0 size= 8960kB time=00:00:12.46 bitrate=5888.5kbits/sframe= 337 fps= 26 q=0.0 size= 8960kB time=00:00:12.46 bitrate=5888.5kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 349 fps= 26 q=0.0 size= 8960kB time=00:00:13.26 bitrate=5533.0kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 362 fps= 26 q=0.0 size= 8960kB time=00:00:13.47 bitrate=5445.9kbits/s[matroska @ 0x55d20fe7d800] Non-monotonous DTS in output stream 0:1; previous: 13457, current: 13451; changing to 13457. This may result in incorrect timestamps in the output file. [matroska @ 0x55d20fe7d800] Non-monotonous DTS in output stream 0:1; previous: 13457, current: 13453; changing to 13457. This may result in incorrect timestamps in the output file. frame= 374 fps= 25 q=0.0 size= 8960kB time=00:00:14.05 bitrate=5220.9kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 387 fps= 25 q=0.0 size= 8960kB time=00:00:14.82 bitrate=4952.8kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 400 fps= 25 q=0.0 size= 8960kB time=00:00:15.04 bitrate=4878.7kbits/sframe= 413 fps= 25 q=0.0 size= 11776kB time=00:00:15.57 bitrate=6193.0kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 425 fps= 25 q=0.0 size= 11776kB time=00:00:16.33 bitrate=5905.7kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. frame= 438 fps= 25 q=0.0 size= 11776kB time=00:00:16.48 bitrate=5853.3kbits/sframe= 451 fps= 25 q=0.0 size= 17408kB time=00:00:17.09 bitrate=8340.0kbits/s[alsa @ 0x55d20fe30440] ALSA buffer xrun. [q] command received. Exiting. frame= 455 fps= 25 q=-1.0 Lsize= 21617kB time=00:00:18.16 bitrate=9750.7kbits/s speed=1.01x video:20715kB audio:896kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.025721% [libx264rgb @ 0x55d20fe7e9c0] frame I:2 Avg QP: 0.00 size:950424 [libx264rgb @ 0x55d20fe7e9c0] frame P:453 Avg QP: 0.00 size: 42629 [libx264rgb @ 0x55d20fe7e9c0] mb I I16..4: 100.0% 0.0% 0.0% [libx264rgb @ 0x55d20fe7e9c0] mb P I16..4: 56.4% 0.0% 0.0% P16..4: 0.1% 0.0% 0.0% 0.0% 0.0% skip:43.5% [libx264rgb @ 0x55d20fe7e9c0] coded y,u,v intra: 1.8% 1.8% 2.0% inter: 0.1% 0.1% 0.1% [libx264rgb @ 0x55d20fe7e9c0] i16 v,h,dc,p: 98% 2% 0% 0% [libx264rgb @ 0x55d20fe7e9c0] kb/s:9323.81 The laptop (Debian based system, utilizing X11 for the window manager, 16GB of RAM, Intel i7-10870H CPU 2.20GHz - 16 cores, NVIDIA Corporation GA106M [GeForce RTX 3060 Mobile / Max-Q]) does not lack compute, the loading averages when not recording are: 0.16, 0.14, 0.38.
