frequency tracking

[Martin Czech]

> hello
> 
> i am looking for a good way to build a frequency follower.  all i need from
> it is a CV output.  what i am thinking of doing would be running the input
> signal accross a simple RC filter and then tapping the voltage off of one of
> the components (depending on whether i wanted inverse or direct scaling;
> haven't decided yet.. maybe both), and then buffering and rectifying it.
> would this work, or are there more complex considerations i'm unaware of?  
> 
> this isn't for any particular project (yet), just a point of interest for me..
> 

Well, this could work with an ideal sine wave input signal, but I'm
shure you want to use the pitch2volt converter also for real world
applications, and in this case the incomming signal has not at all
constant amplitude or shape, so the above mentioned idea won't work.

All pitch2volt converters I have seen so far convert the incomming
signal into a pulse train. The "magic" is to isolate the fundamental
which may be very weak for some instruments. 
There are two basic methods:

a) Time Domain
There is for example a moog patent (ibm patent server)
that decribes how to use an arrangement of  peak detectors in order to
isolate the fundamental in the hope that the waveform maximum and minimum will indicate
the fundamental frequency (which may work with rather soft sounding
input, but certainly not with, say a guitar signal as input).

b) Frequency Domain
Another one by Bode which proposes  a multiple bandpass filter bank
with amplitude measurement and square wave converter for each channel,
so that the lowest channel (fundamental) will suppress all higher
channel output, which seems to me quite reasonable. And moreover it
allows for buzz/hiss detection know by vocoders, ie. you can hope to
detect noise at the input in order to shut down the audio output of the
whole system (because in this cause the frequency measurement will be
bogus) or to sample and hold the last meaningfull result. (by the way,
the EMS5000 vocoder has a pitch extractor and beeing a vocoder, the
filter bank is allready there... , so it is very likely that the
EMS5000 uses this spectral measurement method, could somebody confirm
?)

m.c.

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