[asterisk-dev] downsampling slinear16 to ulaw (or alaw or g729)

[Kevin P. Fleming]

On 08/30/2010 02:51 PM, Paul Albrecht wrote:
> On Mon, 2010-08-30 at 14:39 -0500, Kevin P. Fleming wrote:
>> On 08/30/2010 02:29 PM, Paul Albrecht wrote:
>>> On Mon, 2010-08-30 at 14:15 -0500, Kevin P. Fleming wrote:
>>>> On 08/30/2010 01:48 PM, Paul Albrecht wrote:
>>>>
>>>>> As for AST_FORMAT_SLINEAR16 to AST_FORMAT_SLINEAR translation, I get
>>>>> truncation, that is, instead of the 160 samples I was expecting I get
>>>>> 137 samples. I guess I don't know how to interpret these results, if
>>>>> slinear16/slinear results in truncation that's a bug, right?
>>>>
>>>> Yes. That particular transcoding step is just resampling, and it should
>>>> produce exactly half as many samples as were input (unless an odd number
>>>> were input, of course).
>>>>
>>>>> One more thing to mention, I have translated my silent frame to some
>>>>> other codecs from wide slinear without truncation. They are gsm, speex,
>>>>> and g722. Of course g722 is wide so that's not surprising, but I don't
>>>>> think gsm is wide and it is not truncated.
>>>>
>>>> That's somewhat illogical; all paths to 8Khz codecs should go through
>>>> the same resampling step first, then into the codec. If there are
>>>> samples being dropped during resampling, it should occur for all of them.
>>>>
>>>
>>> I don't know what's causing the problem, but the translated gsm and
>>> speex frames claim 160 samples which is what what I got when I used
>>> AST_FORMAT_SLINEAR. The g729 was truncated in half, that is, only 80
>>> samples, which is much worse than ulaw/alaw truncation.
>>
>> What audio are you feeding in to these translators? It is sampled audio,
>> all zeroes, all ones, something else?
>>
> 
> I'm sending one slinear frame to the translator, and all the frame
> samples are zero. 

Then I have no other ideas... something is terribly wrong, but it can't
be happening to everyone, because lots of people are using mixed
wideband/narrowband calls.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming at digium.com
Check us out at www.digium.com & www.asterisk.org