PV_PlayBuff and offset

Questions
1

Hi all,

I’m trying to play a FFT buffer at specific points.
I know precisely the tempo of the source audio file.
And I would like to compute the offset value for playing the buffer from beat 2, 3, 4 …

The doc says that the offset is:
offset: An integer number of frames to offset into the playbuf file. Defaults to 0.0.

So my approach was to compute the number of frames for one beat and use that as a multiplier in a Pbind. But that’s not working and I don’t understand how to compute the right offset:

b=Buffer.readChannel(s,"some.wav",channels: [0]);
t=TempoClock(100/60).permanent_(true);

~fftsize = 16384; ~hop = 0.25; ~win = 1; // fft parameters

f = Buffer.alloc(s, b.duration.calcPVRecSize(~fftsize, ~hop)); // buffer allocation for the fft analysis

d=t.beatDur.calcPVRecSize(~fftsize, ~hop); // Number of frames for the duration of 1 beat

// a regular FFT Synth
(
SynthDef.new(\avdjm,{
	var start=\start.ir(0), rate=\rate.ir(1), out=0;
	var sig, chain, localbuf; 
	localbuf = { LocalBuf.new(~fftsize) }; 
	chain = PV_PlayBuf(localbuf, f, offset: start, loop: 0); 
	sig = IFFT(chain, ~win); 

	sig = sig*EnvGate.new;
	Out.ar(out,sig!2);
}).add;
)

// A PBind playing at every beat
(
Pdef(\sample,Pbind(
	\instrument, \avdjm,
	\start, Pseries(0,d,16).trace,
	\dur, 1
));
)

Pdef(\sample).play
Pdef(\sample).stop

This approach works fine at start = 0, and stops working for start > 0.

So computed, d, the offset I compute for 1 beat, equals 131075.0.

The system works for values of d ≈ 20. No idea why.

How could I compute a precise value for d ?

Thanks

2

This is an offset in samples, isn’t it? But PV_PlayBuf expects an offset in FFT frames.

No time to write out the derivation (basically, for questions like this, the exercises from middle/high school physics class where you manipulate and convert units are a good method) but I think it will be sampleRate / frameSize / hop.

Larger frames = fewer frames per unit time, hence division.

1/4 hop should have twice as many frames as 1/2, so again, division.

Then you’d do beats / tempo * framesPerSec (as calculated above).

hjh

3

Thanks. I’ll try in that direction.