[itu-sg16] TR: [WP3Aud] FW: Question on G.726

[yusuke hiwasaki]

Dear Mr. Yamamoto,

Thank you for reporting your findings on G.726. As an Associate
Rapporteur of Q10/16 and Editor of G.191 (STL), I would like to give
some feedbacks on this report. Please accept my apologies for this
slow response.

Firstly, to give you some idea on the narrow-band codecs available in
G.7xx series, those codecs usually expects band-pass filtered signal
between 300 and 3400 Hz, and are characterized using speech signal
(and perhaps modem signal) that lies within that frequency range.

This means that extremely low frequency input signal such as 1 and 2
Hz is not expected. For the case of G.726, there is a backward
adaptive predictor. I do not have your signal and might be wrong in
saying this, but my assumption is that feeding a low-frquency signal
puts the codec state in an extreme divergence that results in large
amplitude impulses as you report.

My suggestion for this would be to have either a band- or high-pass
filter that eliminates signal below 300 Hz before the encoding
block. An example implementation of such filter is also available in
STL. 

I would be happy to answer any further questions, but I think it is
appropriate to discuss it at WP3 audio reflector
(wp3audio at yahoogroups.com).

Best regards,
yusuke
--
============
Yusuke Hiwasaki, NTT Cyber Space Labs.
hiwasaki.yusuke at lab.ntt.co.jp
Tel: +81-422-59-4815 (Fax: +81-422-60-7811)

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From: itu-sg16-bounces at lists.packetizer.com [mailto:itu-sg16-bounces@
lists.packetizer.com] On Behalf Of Yamamoto, Tetsuya
Sent: Friday, 16 July 2010 8:46
To: itu-sg16 at lists.packetizer.com
Subject: [itu-sg16] Question on G.726

Dear Q10/16 Experts,

Sorry to bother you.

We have found unexpected results when using G.726.

When particular waveform audio data are input to G.726 encoder, the decoded
data from G.726 decoder

include large amplitude impulses.

We can reproduce the same phenomena at any of 16, 24, 32 or 40kbps bitrate
when we put sine wave signal

with very low frequency such as 1Hz or 2Hz into G.726 codec.

Our codec module is generated from the C source code provided by G.191
software tools as is

and the all test sequences provided by G.726 Appendix II are passed with this
module.

It would be appreciated if anyone could give me some suggestions regarding
this matter.

Thanks and best regards,

Tetsuya Yamamoto

Sony Corporation
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