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

[Kevin P. Fleming]

On 08/30/2010 10:53 AM, Paul Albrecht wrote:
> On Mon, 2010-08-30 at 10:30 -0500, Kevin P. Fleming wrote:
>> On 08/30/2010 10:16 AM, Paul Albrecht wrote:
>>> On Mon, 2010-08-30 at 10:02 -0500, Kevin P. Fleming wrote:
>>>> On 08/30/2010 09:39 AM, Paul Albrecht wrote:
>>>>
>>>>> I have a question about asterisk transcoding from wide slinear to ulaw
>>>>> (or alaw or g729). Specifically, the result I get when I translate from
>>>>> AST_FORMAT_SLINEAR16 to AST_FORMAT_ULAW is truncated, that is, I don't
>>>>> get a 160 samples in the output frame. Is this a bug or should I have
>>>>> expected the translator to truncate the result?
>>>>
>>>> How many samples were in the input frame? Is there a smoother involved?
>>>>
>>>
>>> The slinear16 frame contains 320 samples and I'm getting 137 samples of
>>> ulaw out which is not what I expected. I was looking for a full 160
>>> samples of ulaw.
>>>
>>> I don't know if a smoother was involved. Here's how I do the
>>> translation:
>>>
>>> trans = ast_translate_build_path(AST_FORMAT_ULAW,AST_FORMAT_SLINEAR16);
>>> out = ast_translate(trans,in,0)
>>
>> That certainly sounds like a bug then; there's no smoother involved with
>> that type of construction. Can you post the contents of the ast_frame
>> structure called 'in' (but not the data)?
>>
> 
> The translated frame is silent, that is, all the samples are zero so
> that's easy. Here's the code for how I set the input frame:
> 
> static struct ast_frame f;
> static unsigned char data[AST_FRIENDLY_OFFSET + 640];
> 
> f.data.ptr = (void *)(data + AST_FRIENDLY_OFFSET);
> 
> f.frametype = AST_FRAME_VOICE;
> f.subclass = AST_FORMAT_SLINEAR16;
> f.samples = 320;
> f.offset = AST_FRIENDLY_OFFSET;
> f.datalen = 640;
> 
> Is this enough or do you want me to write particular fields to the log
> file or is there another way to dump the frame?

No, that's enough, and it appears to be correct. I don't see any obvious
reason why that would be failing. To debug it further, I'd change it to
output AST_FORMAT_SLINEAR, and see if you get 160 samples; if not,
you've found the step that is causing the problem, and if so, you've
still found it :-)

-- 
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
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