Alsa Library for transcoding

Question

Can I use ALSA library for transcoding of audio from one format to another.

i.e S32 2 channels to S16 1 Channel audio from file as input source to file output

In one of link I saw statement, Plugin: File This plugin stores contents of a PCM stream to file or pipes the stream to a command, and optionally uses an existing file as an input data source (i.e., "virtual mic")

How to setup such pipeline file input and file output, when it is input from file from where does it pick information like format and channels since, API "snd_pcm_file_open" doesn't ask for format related information ? I am hoping this to come from configuration file .asoundrc but, I don't see any doucmentation on how to do it ?

Answer 1

It can't be done since configuration file (asoundrc) as only option to set input file and format as raw or wav as below

pcm.file {
        format raw
        infile "/home/root/raw.pcm"
}

since you can't specify input format you can't transcode, it reads in format that you request from that file. i.e it reads S32 if you request S32 through arecord even if file contains S16.

But, alternative is to use tools like "sox" for transcoding.

If it is only to check how does ALSA does transcoding when real mic is present,then dump "snd_pcm_dump" (arecord -v) will give conversion function(equation) that it uses