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I'm wondering if there is a way to select part of a sample at a given frequency. The only way I can think to index the sample by frequency is using an FFT, but doing that seems to mess up the sample so that it's not actually playable anymore. I was wondering how else one might select the part of a sample at a given frequency whilst keeping the sound intelligible?

Edit: The exact instructions were "synthesize an example of each vowel of pitch 150 Hz and duration 5 seconds".

Edit: I completely misunderstood what I needed to do originally. New question is here: Synthesizing vowel from existing audio sample jin matlab

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The General
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2 Answers2

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The exact phrasing suggests you are being asked to synthesize, ie create a new signal, not filter, or modify an existing signal. Moreover it asks about a fundamental frequency of 150 Hz (It uses the word pitch and not frequency. I'm assuming that fundamental frequency is good enough and/or what they meant :).

So, let me try rewording the question for you:

Do the following for each vowel sound (A, E, I, O, U, etc):
    Create a 5 second sound with a fundamental frequency of 150 Hz.

I can think of two ways to solve this problem: 1. sum up some sine waves (all of which will be a multiple of 150 Hz) at different intensities. Knowing the intensities is the trick here. or 2. Start with a pulse of 150 Hz and filter it. Knowing the exact filter to use is the trick here, although using the right pulse will probably have some impact as well. Either way, you don't need or want an FFT in the generation stage. If you can't or don't want to look up the unknowns above, you could use an FFT to analyze a real person saying those sounds and use the results of the analysis to fill in the gaps. It wouldn't be too hard to do that, but it's probably covered in an advanced textbook on phonetics and/or acoustics.

If you need a more detailed answer, perhaps you should create a new question and link it here for help answering that. I suggest the following tags, if they exist:

  • Speech synthesis
  • Filtering
  • audio
  • phonetics
Bjorn Roche
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  • Yes, that makes a lot more sense - I'm pretty sure fundamental frequency is what they mean. OK, I've already recorded and smoothed audio samples of me saying to sounds so I'm guessing working from those would be the easiest way to synthesize the sounds I'm looking for. I've posted a new question at: http://stackoverflow.com/questions/19910606/synthesizing-vowel-from-existing-audio-sample-jin-matlab – The General Nov 11 '13 at 16:01
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You should define "at a given frequency" more precisely, but it seems that what you want is a filter with a narrow pass-band tuned at the desired frequency.

However, the narrow frequency requirement is opposed to intelligibility. In the limit, a single frequency would just give you a sinusoid, and intelligibility would be completely lost.

Luis Mendo
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  • The exact phrasing is: "synthesize an example of each vowel of pitch 150 Hz and duration 5 seconds". – The General Nov 11 '13 at 00:37
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    @TheGeneral That's a different problem. A vowel is by no means a single frequency. It has lots or harmonics. In fact, those harmonics are what distinguish a vowel sound from another – Luis Mendo Nov 11 '13 at 00:44
  • Evidently I am misunderstanding the question then. I'm really not understanding how to synthesise sounds of a fixed pitch if I'm not taking a single frequency.. – The General Nov 11 '13 at 01:41
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    @TheGeneral "Pitch" means fundamental frequency (150 Hz). The perceived sound ("timbre") depends on other harmonics as well (300 Hz, 450 Hz, ...) http://en.wikipedia.org/wiki/Timbre#Harmonics – Luis Mendo Nov 11 '13 at 10:09
  • Ah, yeah, you're absolutely right; that makes more sense. Thanks – The General Nov 11 '13 at 15:49