Re: [codec] draft test and processing plan for the IETF Codec

[Koen Vos <[e-mail]>]

Hi Paul,

> The filtering described in the test plan [..] is there to establish 
> a common bandwidth (and equalization characteristic in some cases) 
> for the audio chain (be it NB, WB, SWB) so that subjects can focus 
> on comparing the distortion introduced by each of the codecs in the 
> test, without confounding it with bandwidth effects.

I believe it would be a mistake to test with band-limited signals, for 
these reasons:

1. Band-limited test signals are atypical of real-world usage.  People 
in this WG have always emphasized that we should test with realistic 
scenarios (like network traces for packet loss), and the proposal goes 
against that philosophy. 

2. Band limiting the input hurts a codec's performance.  In the Google 
test for instance, Opus-WB@20 kbps outperformed the LP7 anchor --
surely that wouldn't happen if Opus ran on an LP7 signal.  That makes 
the proposed testing procedure less relevant for deciding whether this 
codec will be of value on the Internet. 

3. Audio bandwidth matters to end users.  Real-life experiments show 
that codecs with more bandwidth boost user ratings and call durations.  
(E.g. see slides 2, 3 of http://www.ietf.org/proceedings/77/slides/codec-3.pdf)  
So if a codec scores higher "just" because it encodes more bandwidth, 
that's still a real benefit to users.  And the testing procedure 
proposed already reduces the impact of differing bandwidths, by using 
MOS scores without pairwise comparisons. 

4. Testing with band-limited signals risks perpetuating crippled codec 
design.  In order to do well in the tests, a codec designer would be 
"wise" to downsample the input or otherwise optimize towards the 
artificial test signals.  This actually lowers the performance for 
real-world signals, and usually adds complexity.  And as long as 
people design codecs with a band-limited response, they'll argue to 
test with one as well.  Let's break this circle. 

I also found it interesting how the chosen bandwidths magically match 
those of ITU standards, while potentially hurting Opus.  For instance, 
Opus-SWB has only 12 kHz bandwidth, but would still be tested with a 
14 kHz signal.

best,
koen.


----- Original Message -----
From: "Paul Coverdale" <[e-mail]>
To: "Koen Vos" <[e-mail]>
Cc: [e-mail], "Anisse Taleb" <[e-mail]>
Sent: Saturday, April 16, 2011 6:25:04 PM
Subject: RE: [codec] draft test and processing plan for the IETF Codec

Hi Koen and Jean-Marc,

The filtering described in the test plan is not meant to be for anti-aliassing, it is there to establish a common bandwidth (and equalization characteristic in some cases) for the audio chain (be it NB, WB, SWB) so that subjects can focus on comparing the distortion introduced by each of the codecs in the test, without confounding it with bandwidth effects.

Regards,

...Paul 

>-----Original Message-----
>From: Koen Vos [mailto:[e-mail]]
>Sent: Saturday, April 16, 2011 4:07 PM
>To: Paul Coverdale
>Cc: [e-mail]; Anisse Taleb
>Subject: Re: [codec] draft test and processing plan for the IETF Codec
>
>Paul Coverdale wrote:
>> You mean that VoIP applications have no filtering at all, not even
>> anti-aliassing?
>
>The bandpass filter in the test plan runs on the downsampled signal,
>so it's not an anti-aliasing filter.
>
>Also, the plan's bandpass for narrowband goes all the way up to Nyquist
>(4000 Hz), whereas for wideband it goes only to 7000 Hz.  So if the
>bandpass filters were to somehow deal with aliasing, they are not being
>used consistently.
>
>I presume the resamplers in the plan use proper anti-aliasing filters
>representative of those in VoIP applications (and described in
>Jean-Marc's post).
>
>best,
>koen.
>
>
>----- Original Message -----
>From: "Paul Coverdale" <[e-mail]>
>To: "Koen Vos" <[e-mail]>, "Anisse Taleb"
><[e-mail]>
>Cc: [e-mail]
>Sent: Saturday, April 16, 2011 4:42:06 AM
>Subject: RE: [codec] draft test and processing plan for the IETF Codec
>
>Hi Koen,
>
>You mean that VoIP applications have no filtering at all, not even
>anti-aliassing?
>
>...Paul
>
>>-----Original Message-----
>>From: [e-mail] [mailto:[e-mail]] On Behalf
>>Of Koen Vos
>>Sent: Saturday, April 16, 2011 1:04 AM
>>To: Anisse Taleb
>>Cc: [e-mail]
>>Subject: Re: [codec] draft test and processing plan for the IETF Codec
>>
>>Hi Anisse,
>>
>>I noticed your plan tests with band-limited signals: Narrowband signals
>>are
>>filtered from 300-4000 Hz, Wideband from 50-7000 Hz, Superwideband from
>>50-14000 Hz.
>>
>>However, VoIP applications have no such band-pass filters (which
>degrade
>>quality and add complexity).  So results will be more informative to
>the
>>WG
>>and potential adopters of the codec if the testing avoids band-pass
>>filtering as well.  We want test conditions to mimic the real world as
>>closely as possible.
>>
>>Instead of band-pass filtering, tests on speech could use a simple
>high-
>>pass
>>filter with a cutoff around 50 Hz, as many VoIP applications do indeed
>>have
>>such a filter.
>>
>>best,
>>koen.
>>
>>
>