[opus] Encoding ultrasonics

[Basil Mohamed Gohar]

On 01/16/2013 02:45 PM, Corey Shay wrote:
> It's my understanding that the CELT layer of Opus has a maximum input
> sample rate of 48k, and frequencies above 20k are effectively not
> encoded. I've been trying to get up to speed on the specification, and
> studying its operation, but as far as I can infer, there is a fixed
> set of 21 bands distributed logarithmically to encode DC to 20k. If I
> were inclined to encode at say, 96k, and pass ultrasonics up to 40k, I
> suppose I could in theory lie to the encoder about its input rate so
> it thinks it's 48k, and I could restore its 96k rate after decoding,
> but obviously this would result in doubling the necessary bit rate. Is
> there a more efficient way to encode information over 20k without
> breaking the standard?
>
> You may ask, why do you care about ultrasonics? Well, because the
> intention is to encode once to a file, and play back multiple times,
> at arbitrary playback speeds, and in some cases, half speed or even
> lower. This would result in the ultrasonics coming into the audible
> range, and for certain recordings, it would be preferable to hear them
> in detail rather than a low pass filtered version. Why use Opus for
> this? Video games, of course. Memory constraints.
I think this is a neat idea, if only to see how the behaviors come out,
but I'm still confused why you would need to be able to play back things
at different speeds.  Do you mind elaborating more on this, please?  I
am just interested in knowing the applications of this.

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