Do audio bitrate, audio channel and audio sampling frequency settings matter in FFmpeg when piping RAW output to another process?

Question

I am using FFmpeg version 3.2.2 and LAME 3.99.5 on macOS Sierra (10.12.2) and I have this command that pipes FFmpeg output (from a FLAC file) to LAME for encoding into an MP3:

ffmpeg -y -v quiet -nostdin -i test.flac -b:a 320k -ac 2 -ar 44100 -f s16le -acodec pcm_s16le - | \
  lame --quiet -r -m s --lowpass 19.7 -V 3 --vbr-new -q 0 -b 96 --scale 0.99 --athaa-sensitivity 1 - test.mp3;

It works fine and I am happy with the results, but when I remove the -v quiet setting from the FFmpeg part of the pipe — so I can actually see what’s happening — I notice the following in the output:

Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> pcm_s16le (native))
size=   81858kB time=00:07:55.18 bitrate=1411.2kbits/s speed=41.2x  

Note the bitrate is being shown as 1411.2kbits during the conversion process and not the set bitrate of 320k from the -b:a 320k option being set.

Is this expected behavior? Is that whole chunk of audio parameters (-b:a 320k -ac 2 -ar 44100) superfluous when piping output via RAW output using -f s16le -acodec pcm_s16le since that bitrate setting is being ignored?

Answer 1

Is this expected behavior?

Yes, bitrate (-b:a) is ignored when outputting an uncompressed format. The resulting bitrate is:

sample rate * bits per sample * number of channels = bitrate

So in the case of yours:

44100 * 16 * 2 = 1411200 bits/s or 1411.2 kilobits/s

Is that whole chunk of audio parameters (-b:a 320k -ac 2 -ar 44100) superfluous when piping output via RAW

The bitrate is superfluous. If you want to change the number of channels or the sample rate for whatever reason you can keep -ac and -ar (but you'll likely never need to).