[opus] running at 44.1K but with standard frame sizes

[Marc Lindahl]

Hi all,

I'm implementing the opus codec for a proprietary RF link (for fullband audio) and want to make sure I understand something.
The link is currently running at 44.1KHz - realtime (i.e. streaming from an A/D at one side, ultimately to a D/A at the other).
Rather than muck with all the infrastructure, I'm looking at how to run Opus at 44.1K.
I have flexibility in the frame sizes of the unencoded audio, and packet sizes on the RF link.

So, I was digging through the code, and I didn't see any attempt to measure in realtime the actual audio rate (makes sense, since this codec can work offline too).

Opus_custom seems to be needed if you have a certain frame size constraint with low latency.  I don't have that.  I can use the stock frame sizes ( 120, 240, 480, 960, 1920, and 2880 sample/frame).

My conclusion is that I could set up Opus for 48K (stereo), and in reality run it at 44.1K, as long as I use stock frame sizes, and it would be fine.

The only issue I can think of is any of the perceptual stuff will be off by -8%, e.g. crossover/mask frequencies, etc.  Or is that true?  
And with Opus_custom is all that stuff recalculated?

If I do need to go to opus_custom, it seems that it's still best to use the stock frame sizes, true?

Thanks!

Marc