Add support for transcode audio file to PCM format

[feilongfl]

Support transcoding audio to PCM format; this allows the client to play directly through the I2S bus without decoding. This feature is particularly useful when playing with MCUs.

Changes

Add support for transcode audio file to PCM format

1. set file ext name to pcm when audiocodec is pcm_*
2. add pcm params for ffmpegs

[nyanmisaka]

@feilongfl In which client have you tested this?

[feilongfl]

I test audio both on linux and a MCU platform，Jellyfin is running in linux

linux

I couldn't find a direct method to test PCM music on Linux,
so I used the following command to convert PCM to MP3 and then listened to it:

wget -O wget.pcm http://192.168.10.109:8096/Audio/84f7cdda408d432361c29d6bda9cbc27/universal?Container=pcm&TranscodingContainer=pcm&AudioCodec=pcm_s16le&audioBitRate=16000&api_key=d1e79804ea3b4215aecfc7d9a2043e23
ffmpeg -y -f s16le -ac 2 -ar 16000 -acodec pcm_s16le -i wget.pcm mp3.mp3
mplayer mp3.mp3

MCU platform

I'm submitting this PR primarily for the following repository:
https://github.com/feilongfl/aipi-eyes-s1-jellyfin-demo

It's in Chinese, but based on your github profile, I think you can understand it.
This provides a method to play Jellyfin audio on an MCU device.

Test Video Here: https://www.bilibili.com/video/BV1j94y1h7kn

There are some noises on the MCU platform in video,
which are caused by the MCU's network communication processing.

There is no problem with the converted sound using ffmpeg on linux.

@nyanmisaka

[nyanmisaka]

LGTM. Since these PCM de/muxers have been around for over 10 years, I think it should be safe to just use them without checking.

• https://github.com/FFmpeg/FFmpeg/blame/f95c928f80f19535e43671e6cf568a192b73afde/libavformat/pcmdec.c
• https://github.com/FFmpeg/FFmpeg/blame/22238d0b9440cf55aafda42c60f4413514d4aeb8/libavformat/pcmenc.c