[FFmpeg-user] Increasing lags when using ffmpeg for streaming audio from microphone

[Alex R]

Hi everyone,

I'm attempting to use ffmpeg as a DIY baby audio monitor. Although I have a
working prototype, there is a delay that gets progressively greater.
The lag is of a few seconds in the beginning, but it reaches a few minutes
after several hours.

My set-up is:
- Raspberry Pi 2B with a USB microphone is the streaming server
- My client is usually VLC running on Android or on a computer elsewhere in
the house
- I only have one client at a time,
- but clients are different, depending on who watches the child (so the
solution to stream directly to a specific IP is not suitable)
- All the devices are in the same network
- ffmpeg is started by another process with these parameters:

ffmpeg -re -f alsa -i plughw:1,0 -vn -acodec libmp3lame -b:a 8k -ac 1 -ar
22050 -f mp3 -

The parent process continuously reads stdout and exposes the chunks over
HTTP. This makes it convenient, as the stream can be played in a browser
too. The tool that does it is micstream:
https://github.com/BlackLight/micstream/blob/main/micstream/server.py


I've tried tweaking the ffmpeg parameters and have gotten some small
improvements, by reducing the bit-rate, for example. However, I believe the
approach needs to be reviewed, because the delays still pile up over time.




Hypotheses I've had:
1. The client has a buffer of its own
However, VLC allows me to set the cache size by specifying a duration,
which is currently at 1000ms. I tried lower values too, but there was no
noticeable difference.

2. The hardware is not fast enough
I doubt it because next to the RasPi 2B streaming the microphone, I have a
RasPi ZeroW that streams video from a camera - it is very smooth, and the
delay is ~1s even after weeks of uptime.
Further, if I inspect what ffmpeg writes to stdout, the text at the bottom
says `size=  343273kB time=97:38:43.52 bitrate=    8.0kbits/s speed=   1x`,
which I suppose means that encoding in real-time works well.

3. The network itself
Although both RasPis mentioned above work over Wi-Fi and are in a remote
part of the house - the video stream works reliably. Also, if I disable the
video, audio still lags. Moreover, if I modify ffmpeg parameters to stream
directly to another address over RTP: `ffmpeg -re -f alsa -i plughw:1,0 -vn
-acodec libmp3lame -b:a 8k -ac 1 -ar 22050 -f rtp rtp://192.168.1.10:5002`
and on 192.168.1.10 (the receiver, not the same system as the streamer) I
run netcat to see what I'm getting `nc -u -l 5002` - I do see small chunks
of what appears to be mp3 headers and some payload arriving at regular
intervals, and there are no periods where these datagrams stop arriving.



I am wondering what else I could try to establish the root cause of the
problem and reduce the delays. Perhaps the culprit is `micstream`, I'd be
happy to replace it with something else that is known to work better.
However, on the Internet I found references to `ffserver` which is not
available anymore, while tutorials I've found for Apache and nginx are
tailored for video streaming.


Is the scenario I described feasible at all? What troubleshooting steps
could I try?


Best wishes to everyone, and I look forward to your feedback,
Alex