Re: Pops at start of song

[Apostolos Dimitromanolakis <apostolos@aei.ca>]

Hi,

This is not a hardware issue. The problem is that the DAC of your card
gets stopped in some other level than 0V and when you restart playback
the voltage differential causes the pop (because the output of the
digital to analog converter is AC-coupled). To perform a clean stop and
start make sure that when you start the playback, the first few bytes
are zero and when you stop the last few. If they are not, do a gradual
change of the samples (say no more that +/- 500 difference over the
previous sample). I think the problem is more evident in high end
sound-cards which have a flat frequency response down to a few Hz.

Apostolos


Steve deRosier wrote:

> All,
>
> We're having a problem with some of our audio applications.  When a 
> song starts playing we will often hear a loud pop out of the speakers 
> before it plays audio.  It seems dependent upon where we stoped and 
> the specfic content of the last song, if we hit our "stop control" and 
> abort in the middle of the song it may pop uppon restarting or 
> starting a new song.
>
> One of our engrs found if he fills the entire buffer with zeros before 
> quitting the program, on the next start it won't pop (though that 
> moves the pop to the end of the previous due to a disconnect/jump in 
> the created waveform).  Unless this is a hardware issue, this doesn't 
> seem to be the right solution to me.  So:
>
> * What are things that we can do in our program using the alsa pcm 
> functions to eliminate this?  Maybe a specific function we need to 
> call or a particular sequence?
> * Or, is this something inherent in hardware and other than filling 
> with zeros before we quit there's nothing we can do?
>
> I figure some of you have encountered this before, so maybe there's 
> some ideas out there?
>
> If you're wondering what we do when a "stop" command comes in: pretty 
> much nothing.  Basically my "ALSASender" routine ends and calls:
> void CDSP::ClosePCM( void )
> {
>   // some other bookkeeping goes here <snip>
>
>   snd_pcm_close( hPCM );
>
>   if( mPCMStatus )
>   {
>      snd_pcm_status_free( mPCMStatus );
>      mPCMStatus = NULL;
>   }
>
>   // Non-alsa related cleanup goes here <snip>
> }
>
> Also, note that we do get the same pops with using aplay.
>
> Thanks,
> - Steve
>
> PS Takashi -> I haven't given up on my serial driver patch, I was 
> working on it when I got interupted with something else, it should be 
> on its way soon; thanks for your help.
>
>
>
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