`audio-buffer` option won't set buffer for DirectShow audio devices

I'm trying to achieve the lowest latency playback on a set of input devices (audio+video) but found myself to be locked out by the DirectShow audio buffer of 500ms.

The demuxer (ffmpeg) allows you to set the DirectShow audio buffer with the -audio_buffer_size <value> (under mpv would be option demuxer-lavf-o=audio_buffer_size=<value>) and I believe --audio-buffer=<value> is the equivalent option for mpv. However, neither actually sets the buffer size.

• mpv version 0.35.0-327-ga45995f6
• Windows Version 11 - 21H2 (build 22000.1696)
• Source of the mpv binary shinchiro on SourceForge
• If known which version of mpv introduced the problem N/A
• Possible screenshot or video of visual glitches N/A

Reproduction steps

Run any command with a DirectShow audio device, and set the audio buffer. i.e.:

mpv.com av://dshow:video="this is a video device:audio=this is an audio device" --hwdec=no --profile=low-latency --no-cache-pause --video-sync=display-resample-vdrop --framedrop=decoder+vo --video-latency-hacks=yes --untimed --demuxer-lavf-o=audio_buffer_size=128

or

mpv.exe av://dshow:"audio=this is an audio device" --no-cache-pause --untimed --audio-buffer=0.1

For reference, DirectShow devices on your system can be shown by running the command ffmpeg -list_devices true -f dshow -i dummy.

Expected behavior

Playback has a latency of less than 500ms

Actual behavior

Playback has a latency of 500ms or more

Log file

Ran mpv.com --demuxer-lavf-o=audio_buffer_size=128 av://dshow:"audio=Microphone (Blue Snowball )" --log-file=%tmp%\log.txt

log.txt

Additional notes

Since ffmpeg is the demuxer it might be worth to note how ffmpeg/ffplay handles this.

According to ffmpeg documentation, the default DirectShow audio delay is 500ms (which can be perceived if running ffplay -f dshow -i audio="this is an audio device"). The option -audio_buffer_size allows one to adjust the buffer size. Adding this parameter to ffplay with a buffer size lower than 500 allows for lower latency playback (e.g. ffplay -f dshow -i audio="this is an audio device" -audio_buffer_size 64).

mpv should set the demuxer audio buffer using either --audio-buffer=<value> or demuxer-lavf-o=audio_buffer_size=<value>. It doesn't return an error nor set the buffer to the expected value.

Possible relevant DirectShow sources:

Setting Audio Capture Properties
IAMBufferNegotiation interface (strmif.h)
mpv-player/mpv#9703

Start by reproducing the DirectShow command with --demuxer-lavf-o=audio_buffer_size=128 and compare it with ffplay's -audio_buffer_size behavior. Trace the DirectShow audio-device path and the demuxer option handling; done means the configured buffer is applied and playback latency falls below the default 500ms.