Pitch/Voltage converter board ready at EFM

[danial stocks]

>The problems I noted were these (I tried a simple plug in a mic test...)
>
>1) There is no input filtering, and the human voice usually has many 
>harmonics. If the fundamental frequency
>isn't reasonably pure a garbled output will result.
>
>2) When a note decays it takes several cycles for it to stop. If these 
>drift in frequency, or if the last two detected cycles get chopped up, the 
>sample hold will latch the wrong value.
>
>3) Higher notes have a greater chance of mistracking for reason #2. If you 
>consider that the voice stops in (oh...) 5mS regardless of pitch... then a 
>200Hz input would be gone within 1 cycle, and the right value would still 
>be latched. But at 1KHz 5 cycles would occur, and the chances of latching 
>bad data would increase.
>
>There is a latch input that can be driven by a positive signal... you could 
>use a footswitch, or output from an envelope follower to latch the 
>sample/hold before the decay of the note. Then with careful singing it 
>would work...
>
>I want to do vocal tracking also. It will need additional circuitry. Its 
>not as easy as Guitar which will only need a little processing... because 
>the vocal note could get louder at any time (guitar always gets quieter on 
>the average...) I made this board with the intention of have a reliable 
>module to experiment with. The parts that are sure NOT to change are there, 
>the squaring circuits, the ramp, sample hold, and antilog converter. If you 
>experiment you MUST have these functions... and the "Ramp" type P/V 
>converter is always faster than the "tachometer" type.
>
>H^)

I used an unusual technique for pitch vtg conv a while ago...

1. square the i/p - feed it thru diode clipper... nice sq wave.
2 feed it thru a 6db/oct lpf
3 use a precision op amp rectifier like the suggested ones in nat semi 
LM3914/5/6 display drivers data/appnotes
this gives a nice [inverse] linear frq to vtg relationship - ie a 1 octave 
down will give double the vtg..
If you want it the right way around ie vtg inc's w/freq then you need to 
take the reciprocal of this vtg.. I designed a linear reciprocator with a 
couple of op amps and a transcond. op amp, haven't built it yet.. If anyone 
wants to see the diagram I'll see if I can find someone with a scanner...
the thing about this technique is .. it's fast .. it doesnt glitch on 
harmonics etc..
you can't effectively do it the other way around ie with a hpf istead of lpf 
because of harmonics in the sq wav giving wrong readings [and you need the 
sq wave for input amplitude+ harmonic immunity.
Cheers,
Dan

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