Re: [Discuss-gnuradio] Audio sink does not throttle sample flow

[Felix Wunsch]

Am 12.08.2012 19:19, schrieb Tom Rondeau:
> On Tue, Aug 7, 2012 at 6:22 AM, Felix Wunsch
> <address@hidden> wrote:
> > Hi all,
> >
> > I recently wrote a block for decoding DRM AAC streams. For testing I put
> > together a small flow graph consisting of a wav source, encoder block,
> > decoder block, (rational) resampler and an audio sink (an image of the flow
> > graph is attached).
> >
> > When I now run this flow graph, the audio is correctly decoded, but the
> > output is not throttled to 44.1 kHz as it should be. It seems to be running
> > at full speed. I also connected a resampler and an audio sink directly to
> > the wav source and there the output is correct.
> >
> > I use default values for the audio sink (alsa, 44.1 kHz), GNU Radio 3.5.1
> > and xubuntu 11.10.
> >
> > Any hints why this is happening and how to solve this?
> >
> > Best regards,
> > Felix Wunsch
> Felix,
>
> I know that the audio sink will throttle the flow graph. What evidence
> do you have that it's not or that it's running at full speed? Is it
> the sound coming from the audio? My first guess is that there's a
> misunderstanding somewhere of the actual sample rate. You're
> resampling by almost 2x, which means you expect the signal coming from
> the decoder to be 44.1e3/2. Is that right?
>
> Tom

Hi Tom,

thanks for your reply.

My first evidence was in fact the sound coming from the audio that is 
running at a very high speed. However, the pitch seems to be normal. The 
flow graph is set to "run to completion" and processes a 3 min wav-file 
in about 15 sec.

The signal coming from the decoder has a sample rate of 24 kHz. I 
verified that by writing the decoder output into a file and using aplay 
for playback with -r 24000. At this point, the sound is still normal.

The next steps are in detail:
- Type conversion short->float
- multiply const (1/32768) for range adjustment
- rational resampler( interp: 441, decim: 240)
- audio sink (44.1kHz)

Did I configure the resampler correctly? I left taps blank and 
fractional bandwidth at 0.

I also attached a file sink to the output of the resampler and tried to 
play it with aplay using -r 44100. It shows the same behaviour like the 
audio sink (normal pitch, very fast playback).

Best regards,
Felix