[opus] Question on opus_decoder output sampling rate

[Jean-Marc Valin]

On 03/15/2016 11:54 AM, Dragos Oancea wrote:
> "The problem is that for resampling factors close to 1.0, the
> interpolation factor can be quite large. For example, in the case
> described above of changing from the sampling rate from 48 kHz to 44.1
> kHz, the ratio is only 0.91875, yet the interpolation factor is 147!"

The quote above assumes a totally stupid resampler, which does not
represent the Speex resampler. Note that there's a quality parameter in
the Speex resampler that allows you to trade quality for complexity.

	Jean-Marc


> My guess is that Opus would perform similar to Speex if you'd have to
> have it resample to 44.1 khz.
> 
> Cheers,
> Dragos
> 
> ------------------------------------------------------------------------
> *From:* Julien Chavanton <jchavanton at gmail.com>
> *To:* Jean-Marc Valin <jmvalin at jmvalin.ca>
> *Cc:* opus at xiph.org
> *Sent:* Tuesday, March 15, 2016 1:18 PM
> *Subject:* Re: [opus] Question on opus_decoder output sampling rate
> 
> Hi, another question on the same topic
> 
> Speex resampler at 44.1kHz seems to be very CPU intensive on Android
> (even more than the Opus encoder)
> 
> While Speex at 48kHz is just fine.
> 
> I wonder any alternate solutions or ideas ?
> Improve it, look for alternate solution ...
> 
> I am guessing the NEON optimization are still used for both, etc.
> 
> 
> On Thu, Apr 2, 2015 at 4:46 PM, Jean-Marc Valin <jmvalin at jmvalin.ca
> <mailto:jmvalin at jmvalin.ca>> wrote:
> 
>     The encoder and decoder can handle, 8, 12, 16, 24 and 48 kHz
>     input/output. If doesn't matter what it gets encoded to/decoded from.
>     you can initialize a decoder at 8 kHz and it'll still decode 48 kHz
>     audio fine (you just won't get the high frequencies obviously). For
>     sampling rates other than 8/12/16/24/48, then you'll have to do
>     resampling. Have a look at the speexdsp resampler if you don't already
>     have one.
> 
>     Cheers,
> 
>             Jean-Marc
> 
>     On 02/04/15 10:42 AM, Julien Chavanton wrote:
>     > Hi, is there any way to tell the decoder the output sampling Fz we
>     want ?
>     >
>     > opus_decoder_create = Sampling rate of input signal (Hz)
>     >
>     > Considering this example (VoIP-out from WebRTC/RTP)
>     >
>     > MICROPHONE(44.1/48kHz) >> [encoder created at 48kHz but with
>     > internalSampleRate set to 8kHz]>> INTERNET >> [decoder(created with
>     > 48kHz)] >> 48kHz(?) >> G.711(8kHz)
>     >
>     > This leaves us with the only option to re-sample even if the internal
>     > sample rate was set to 8kHz.
>     >
>     > This may not seem like a big problem since we could simply
>     resample but
>     > on a server with a lot of load this could be significant ?
>     >
>     > https://tools.ietf.org/html/draft-ietf-payload-rtp-opus-08
>     > Is not very clear on these points.
>     >
>     > Regards
>     > Julien
>     >
>     >
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> 
> 
> 
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