[CELT-dev] Bad artifacts at 32kbps
Benjamin M. Schwartz
bmschwar at fas.harvard.edu
Thu Apr 14 08:58:58 PDT 2011
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
> 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
> Is there a program I can run to estimate the error or
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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