I knew what you were talking about and the terminology is correct. I wan't sure if you were looking for an analog representation or had planned a DSP approach all along. There is also an inverse comb filter which has a succession of notches that filter out particular frequencies.Hi Benta. I want to detect the fundamental frequency of each tone sung but in the lowest register human voices contain no fundamental frequency. The way humans recognise them with our ears is to detect their harmonics and our brain does the rest. So I want to emulate the detection of the harmonics electronically. The range of fundamental frequencies involved exceeds that of the lowest register so it includes some of the notes in the next two registers. There the problem is not so difficult because there the signal will contain a fundamental. Only one note will be sung at a time.
Papabravo I called what I want a comb filter because its frequency response will look like the teeth of a comb. Sorry if I chose the wrong word.
AK is correct in his analysis of doing an analog comb filter. You should go the digital route so you can easily experiment with alternative approaches.At the concept level, you can make a comb filter with an analog (!!!) delay line and a summing amplifier. The frequencies and widths of the comb tines are a function of the delay period and the fundamental frequency. For something where the fundamental is known and stable, like the 3.58 MHz color subcarrier frequency in NTSC video, a comb filter can do an excellent j0b of picking off (or notching out) the subcarrier and its harmonics. For speaking or singing voice, this is much more difficult. Since you said this would be for one note at a time, are you looking for something that automatically locks onto the note, or can your application tolerate manual adjustments?
This can be done without a DSP, but a frequency-tracking delay line is not simple stuff. Also, a voltage-variable delay line that covers the periods of a full octave for first formants is doable in theory, but probably not in practice without going to a digital delay or quasi-digital like a bucket brigade. A possibility is an acoustic delay like the springs in a reverb amp, but those are almost always fixed delays. A DSP has several advantages, not the least of which are software based delays of any length and very steep bandpass filters without a zillion precision matched components. But the software is *very* heavy lifting.
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