Converting to Opus: 44.1 kHz resampled to 48 kHz Options

[gurkburk]

So i have a bunch of regular 44100 flac files that i'de like to convert to OPUS file so i have something to listen to when i run, that's not very hard to setup following this guide.
http://www.saunalahti.fi/~cse/Opus/

What does concern me is that opusenc.exe seems to default to a samplingrate of 48000 when you feed it a raw stream (the '-' option as seen in the guide seems to do that) it's easy to see that the .opus files do indeed become converted/upsampled to 48000hz using this method.
So my question is, should i override this behavior by adding --raw-rate 44100 to the encoder?
Does it matter at all that the files get upsampled? Could it introduce noise or other unwanted side effects?

Thanks

(yes i know that there aren't many android players yet avail for playback of opus, but the question still remains)
Also i think this post belongs to this forums, at least it seemed like it was a good fit, but if i'm wrong please move it :\


This post has been edited by gurkburk: Sep 17 2012, 18:41

[C.R.Helmrich]

I have a similar question as the OP. In my case the input file is at 32 kHz sampling rate, and I want this to be encoded at high rates (i.e. CELT-only). Now there are two ways I can do this:

• Encode the 32-kHz file as-is. This gives me a file decoded at 32 kHz, or decoded at 48 kHz if I specify "--rate 48000" in the decoder command-line.
• Upsample the input to 48 kHz externally, then encode it. This gives me a 48-kHz output file which contains some artificial content between 16 and 20 kHz.

Which of the two approaches is more efficient? Intuitively I would choose the first approach, since that one only encodes content actually present in the input, but maybe CELT is more efficient on 48-kHz than on 32-kHz input (and the 16-20-kHz spectral range consumes almost no bits)?

What do the experts say?

Chris

[nu774]

I'm not an "expert" but from what I can see from opusenc.c (and audio-in.c) of opus-tools, it resamples 32kHz input to 48kHz (by speex resampler) before sending to encoder.

CODE

  if(rate>24000)coding_rate=48000;
  else if(rate>16000)coding_rate=24000;
  else if(rate>12000)coding_rate=16000;
  else if(rate>8000)coding_rate=12000;
  else coding_rate=8000;

CODE

  if(rate!=coding_rate)setup_resample(&inopt,coding_rate==48000?(complexity+1)/2:5,coding_rate);

CODE

  st=opus_multistream_encoder_create(coding_rate, chan, header.nb_streams, header.nb_coupled,
     mapping, frame_size<480/(48000/coding_rate)?OPUS_APPLICATION_RESTRICTED_LOWDELAY:OPUS_APPLICATION_AUDIO, &ret);

[nu774]

It might have not been enough explanation...
"rate" in the first part is the actual input sampling frequency, and it's get resampled to "coding_rate".

[C.R.Helmrich]

Yes, I expected Opus to resample to 48 kHz internally. The question was more: if it does, will the CELT coder know that the input was upsampled from 32 kHz by the Speex resampler, and limit its encoding bandwidth to 16 kHz (the bandwidth of the original input file), or is the 20-kHz bandwidth of high-bitrate CELT hard-coded regardless of the input sampling rate?

Chris

[jmvalin]

Yes, I expected Opus to resample to 48 kHz internally. The question was more: if it does, will the CELT coder know that the input was upsampled from 32 kHz by the Speex resampler, and limit its encoding bandwidth to 16 kHz (the bandwidth of the original input file), or is the 20-kHz bandwidth of high-bitrate CELT hard-coded regardless of the input sampling rate?

Opus doesn't actually have a mode for 16 kHz bandwidth (only 12 and 20). However, the new 1.1-alpha release has new code (it may not be 100% reliable) to avoid wasting bits on frequencies that aren't present in the signal being coded.