Pitch Tracker Update

[gstopp at fibermux.com]

     On Friday I moved the exponential VCO PLL pitch tracker off of the 
     protoboard and onto a perf circuit board, and it stopped working. 
     Seems that the extra capacitance (or something) on the protoboard 
     allowed the NAND gate phase comparator to work, whereas on the circuit 
     board the R/C edge recovery levels aren't quite good enough. Over the 
     weekend I thought about it and came in this morning with a solution.
     
     The protoboard is one of those white "matrix of groups of five holes" 
     that you stick components and wires into to interconnect things. 
     There's probably a lot of capacitive coupling between adjacent rows. 
     Anyway for the circuit board I replaced the 74LS00 with a 74HCTLS00 
     and put a 10K pullup on the VCO input gate, and things were back up 
     and running again.
     
     In anticipation of live signal tests I added an external signal 
     conditioner. This consists of a dual-stage microphone preamp, followed 
     by a passive R/C lowpass (with a pot for the R), followed by a 
     comparator with a pot for adjusting the threshold. The preamp also 
     drives a FWR-type envelope follower, with a gate comparator, again 
     with a variable threshold. I added a sample and hold chip (LF398 
     configured as track and hold) between the loop filter and the VCO, 
     controlled by the gate, to see if I can get the VCO frequency to hold 
     the last pitch tracked when the gate goes low.
     
     For testing I put an external triangle wave into the preamp. 
     Everything looks good on the scope, including the "hold" function. 
     I'll post the test results after I try it out with live signals.
     
     - Gene
     gstopp at fibermux.com