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

On 11-04-16 07:42 AM, Paul Coverdale wrote:
> You mean that VoIP applications have no filtering at all, not even
> anti-aliassing?

What Koen means (I assume!) is that there is no deliberate stop band 
starting at 87.5% of the Nyquist rate (e.g. 7 kHz for wideband). These 
days what you will see is either a sharp cutoff with "complete 
attenuation" starting at 8 kHz -- just enough to avoid aliasing -- or 
even using a -3 dB cutoff at 8 kHz and complete attenuation starting 
just above that. While the latter introduces a tiny bit of aliasing, it 
also produces the widest audio bandwidth possible and the aliasing is 
likely to be lower than the coding noise at that frequency anyway. 
Fortunately, I think systems with no anti-aliasing at all are pretty 
rare (and they don't deserve we care about them anyway!).

I have to admit that in Speex I tuned/trained everything assuming these 
3.5/7 kHz cutoff frequencies and I now consider that this was an error 
because very few of my users have such filtering.

Cheers,

	Jean-Marc

> ...Paul
>
>> -----Original Message-----
>> From: codec-bounces@ietf.org [mailto:codec-bounces@ietf.org] On Behalf
>> Of Koen Vos
>> Sent: Saturday, April 16, 2011 1:04 AM
>> To: Anisse Taleb
>> Cc: codec@ietf.org
>> 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.
>>
>>
>
>
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