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

[Paul Albrecht]

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?

> -- 
> 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
> 
-- 
Paul Albrecht