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

[Paul Coverdale <[e-mail]>]

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.
>>
>>
>