[gst-devel] amplitude modulation plugin

berti bertrand.haut at brutele.be
Thu Aug 25 15:45:05 CEST 2005

I'm trying to make a amplitude modulation of a sound signal, i.e. simply 
multiply the sound by sin(2*pi*f*t) where t is the time.

My plugin have the following caps 
"audio/x-raw-int, width = (int) 16, depth = (int) 16, signed = (bool) true, 
endianness = (int) 1234, rate = (int) 44100, channels = (int) 1"
so that I'm sure I will be able to connect it to sinesrc & osssink to simply 
test it.

The main part of the code is :

  out_buf = gst_buffer_copy (buf);
  gint16 * data = (gint16 *) GST_BUFFER_DATA (out_buf);
  int num_samples = GST_BUFFER_SIZE (out_buf) / sizeof (gint16);
  int i=0;
  double t=((double)  GST_BUFFER_TIMESTAMP (buf)) * 10e-9; //the "time" of 
this buffer in "s"
  for (i=0; i < num_samples ; ++i){
    data[i]=ByteSwap((gint16) ((double) ByteSwap(data[i]) *cos(2.0 * PI * 
(double) filter->f * (t+ i / ((double) filter->rate)) )));

 gst_pad_push (filter->srcpad, GST_DATA (out_buf));

-filter->f is the frequency of the modulation
-The ByteSwap simply convert a gint16 between Big and Little endian 

The code compile and work, I can hear the variation of the frequency BUT there 
is a lot of noise on the top of the sine signals. Where does this noise come 
from ? The sound directly from the sinsrc element is perfect so I thought it 
was possible to make a filter with perfect output.
ICQ 48978874 
pgpkey B9514013C02E33FA
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