Decimator

  • Author or source: ed.bew@raeybot
  • Type: Bit-reducer and sample&hold unit
  • Created: 2002-11-25 18:13:49
notes
This is a simple bit and sample rate reduction code, maybe some of you can use it. The
parameters are bits (1..32) and rate (0..1, 1 is the original samplerate).
Call the function like this:
y=decimate(x);

A VST plugin implementing this algorithm (with full Delphi source code included) can be
downloaded from here:
http://tobybear.phreque.com/decimator.zip

Comments/suggestions/improvements are welcome, send them to: tobybear@web.de
code
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// bits: 1..32
// rate: 0..1 (1 is original samplerate)

********** Pascal source **********
var m:longint;
    y,cnt,rate:single;

// call this at least once before calling
// decimate() the first time
procedure setparams(bits:integer;shrate:single);
begin
 m:=1 shl (bits-1);
 cnt:=1;
 rate:=shrate;
end;

function decimate(i:single):single;
begin
 cnt:=cnt+rate;
 if (cnt>1) then
 begin
  cnt:=cnt-1;
  y:=round(i*m)/m;
 end;
 result:=y;
end;
code
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int bits=16;
float rate=0.5;

long int m=1<<(bits-1);
float y=0,cnt=0;

float decimate(float i)
{
 cnt+=rate;
 if (cnt>=1)
 {
  cnt-=1;
  y=(long int)(i*m)/(float)m;
 }
 return y;
}

Comments

Nothing wrong with that, but you can also do fractional-bit-depth decimations, allowing
smooth degradation from high bit depth to
low and back:
---------------------------------------

// something like this -- this is
// completely off the top of my head
// precalculate the quantization level
float bits; // effective bit depth
float quantum = powf( 2.0f, bits );

// per sample
y = floorf( x * quantum ) / quantum;

---------------------------------------

it looks to me like the c-line

long int m=1<<(bits-1);

doesnt give the correct number of quantisation levels if the number of levels is defined as
2^bits. if bits=2 for instance, the above code line returns a bit pattern of 10 (3) and not
11 (2^2) like one would expect.

please, do correct me if im wrong.

/heatrof