Re: [Jack-Devel] Introducing delay into simple_client.c

On 10/30/2012 07:59 AM, Jason Martyn wrote:
> Hi everyone,
> 
> I'm brand new to JACK and its implementation, so please bear with me.
> I'm looking to modify the example script simple_client.c to introduce
> a predetermined time delay from the input to the output.  I.e.,
> unmodified, simple_client.c simply copies the input to the output, and
> instead, I would like to take the input, delay it by t seconds, and
> then send it to the output.
> 
> To do this, I thought I would declare an array of type
> jack_default_audio_sample_t and then store the input into that array
> until the critical time where it should start playing it to the
> output. The new input then overwrites the old input by cycling through
> the array from the beginning.
> 
> Here's my process callback (with comments on my reasoning):
> 
> //Initialize counting integers
> int i = 0;
> int j = 0;
> 
> //Initialize storage array
> jack_default_audio_sample_t *prev[200]; //Note: 200 is just a test number
> 
> int process (jack_nframes_t nframes, void *arg)
> {
>    //For the first 200 cycles, store input into array, don't output anything
>     if (i<200){
> 
>         prev[i] = (jack_default_audio_sample_t *) jack_port_get_buffer
> (input_port, nframes);
>         i++;
>     }
> 
>     //For all following cycles, play the input, starting from the
> beginning of the array
>     else {
> 
>     jack_default_audio_sample_t *out = (jack_default_audio_sample_t *)
> jack_port_get_buffer (output_port, nframes);
>     memcpy (out, prev[j], sizeof (jack_default_audio_sample_t) *
> nframes); //Note: j is the new counting variable
> 
>    //Overwrite recently played input with new input:
>     prev[j] = (jack_default_audio_sample_t *) jack_port_get_buffer
> (input_port, nframes);
>     j++;
> 
>    //Reset array counter to 0 when it reaches the end
>     if (j = 200){ j = 0; } //reset j
>     }
> 
> return 0;
> }
> 
> If this looks way off (as in I'm missing the point of how JACK, etc.
> works), I'd appreciate any resources that might help me get started
> with regards to understanding how this all works.  I have gone through
> the tutorials listed on the JACK site, and have a lot of troubles
> trying to find anything else.
> 
> Regards,
> Jason

Hi Jason,

welcome aboard! The issue with above code is
    prev[i] = jack_port_get_buffer(..);

The buffer returned by jack_port_get_buffer() is only valid for the
current process cycle. You need to memcpy() that to a temp. queue.

While easy in principle, things can get a bit complicated because the
period size can change. When the delay-line buffer wraps-around and the
delay-line buffer-size is not an exact multiple of the period-size, you
will need two memcpy()s for writing and two for reading to make it seamless.

While it's a good exercise to code that, the easiest way is to use
libjack's ringbuffer which does exactly that:
http://jackaudio.org/files/docs/html/ringbuffer_8h.html

HTH,
robin

1351588660.7966_0.ltw:2,a <508F9B26.4000309 at gareus dot org>