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Re: Generating a rising sine tone with sinf()
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Re: Generating a rising sine tone with sinf()

  • Subject: Re: Generating a rising sine tone with sinf()
  • From: Adam Bellard <email@hidden>
  • Date: Thu, 19 Jun 2014 12:55:41 -0400
Just a wild guess, maybe you are writing interleaved stereo audio out instead of mono?

Adam Bellard


On Thu, Jun 19, 2014 at 12:42 PM, Jim McGowan <email@hidden> wrote:
I’m writing some code to generate a sine tone that begins at a given frequency and the frequency rises linearly to a maximum over a specified time.  Seemingly straightforward, my approach was to fill a sample buffer in a loop (I’m using the AudioSampleType on OS X, i.e. Float32):

frequency = minFreq + (frequencyRangeLength * (sampleIndex / lengthInSamples));
sineSweepSamples[sampleIndex] = sinf((2.0 * M_PI * sampleIndex * frequency) / sampleRate)

However, this generates a tone where the frequency increases at a faster rate than it should.  When I remove the “2.0 *”, i.e. use pi instead of 2pi, I get the results I want, but I don’t understand why.  Checking the formula (with the 2pi)  in the Grapher app shows it is correct, and by dumping all the values in each iteration of the loop I see that the frequency is being calculated properly.

Can anyone explain this for me?

Thanks,
Jim
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 >Generating a rising sine tone with sinf() (From: Jim McGowan <email@hidden>)

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