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

[Marc Lindahl]

Hi Jean-Marc,

On Jun 15, 2013, at 2:23 AMEDT, Jean-Marc Valin wrote:

>> 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.
> 
> You should probably consider resampling. It's not that expensive and it
> would make things easy. But otherwise, see below.

Yes, considering your and Ben's remarks, I'm considering this.  I saw the 'tools is using the speex resampler, I'll look at it.

> 
> On 06/14/2013 06:23 PM, Marc Lindahl wrote:
>> 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).
> 
> What do you mean by "measure in realtime the actual audio rate"?

Meaning using the CPU clock via time() to calculate the realtime data rate.  Nevermind, it's a red herring.

> 
>> 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.
> 
> Do not do that, ever. Everything is calibrated for 48 kHz and you will
> likely cause audible noise.

How would it cause audible noise, I don't understand that part?  After all the frequency calculations are off by 8%, that's not too extreme...

> Besides, running at the wrong rates means
> you lose all compatibility anyway, so you might as well use the custom
> modes.

Except, based on Ben's previous comments, possibly the quality isn't as good?

> Using custom modes means you would probably want to choose a
> frame size that's a power of two and save a few extra cycles (maybe even
> reuse a platform-specific power-of-two FFT).

That's interesting… there isn't anything in the compression (estimators, etc.) or FEC that really 'likes' multiples of 120 samples?

> 
>> 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?
> 
> Yes, custom modes properly adjust all the psychoacoustics to the exact
> sampling rate.

Got it.

> 
>> If I do need to go to opus_custom, it seems that it's still best to 
>> use the stock frame sizes, true?
> 
> There is no "stock frame size" with Opus custom. Modes are generated on
> the fly (or you can always pre-generate them) with any frame size. The
> frame size is that it can't have prime factors above 5, but that's about
> the only constraint.

What I meant was, 120, 240, 480, 960, 1920, or 2880 samples, the sizes imposed in the normal mode.

So I still wonder, if you set up a custom mode, but then had all the settings the same as a normal mode, would the codec perform worse, or the same?

Thanks for your thoughtful replies.

Best,
Marc