[CELT-dev] Bad artifacts at 32kbps

[Benjamin M. Schwartz]

On 04/14/2011 11:35 AM, Andrew Lentvorski wrote:
> So, I decided to run with 24KHz sample rate with 16 bit samples.

Depending on how you set the sample rate, this could be a problem.  It
might be good to post how you invoked the encoder.  You might find that
48KHz performs better, even if you know your audience won't hear the top
frequencies.

> Then I tried some bitstream rate changes 128Kbps, 64Kbps, and 32Kbps.
> 128K and 64K are fine.
>
> 32K is *terrible*.  Really, really horrendous.
>
> Is this expected?

For mono at 20ms frames, no.  32Kbps is not enough for transparent
48KHz-samplerate mono audio, but it should be pretty good.  For stereo
32kbps is going to sound crappy.  For shorter frame sizes it is also not
so good.

> Is there a program I can run to estimate the error or
> something?

A good first step would be to post some decoded samples.  The type of
artifact may identify a problem.  There are also tools like PQEvalAudio,
but that is probably overkill.

> I expected degradation, but this just falls off a cliff with
> really objectionable artifacts.

In general, good lossy codecs often have a very small bitrate transition
zone between "great" and "ugly".  Also, a power of 2 is not exactly a
small fractional change.

> My application is going into the embedded space, so I'm quite a bit
> resource conscious.  It's not that I couldn't live with 64kbps, but
> every factor of 2 helps.

You may want to try changing by smaller factors.

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