[Fpga-synth] Delay Line Theory

[Scott Gravenhorst]

I've looked on the net for examples of delay line implementation using a
RAM, but there doesn't seem to be anything.  I manufactured my own method
based on "clock math".  The void of information leads me to one of two
conclusions - 1) it's as simple as I think it is - or 2) it's IP that
nobody wants to divulge because it's so cool.

I just did a little experiment that I thought would show the error of my
ways, but it does not (I suspected my wrapping algorithm).

This new little project doesn't use an arithmetic wrap algorithm.  It makes
the length of the delay line the entire RAM address space.  This allows me
to let the system simply wrap back to zero after the maximum address
instead of using arithmetic to compute when to wrap to address zero.

I set up a pointer (ptr) that is the end point (for read) and start point
(for write) of the delay line.  I also have a pickup offset value (PU) that
is added to ptr to access the location of the pickup.

In a nutshell, the state machine first accesses and stores in a register
the pickup using ptr+PU as the RAM address (where binary wrapping will
happen).  Next, the data pointed to by ptr is accessed to feed the input
register of the filter.  At the same clock edge, the filter's output is
written to the location in RAM pointed to by ptr, storing the previous
cycle's IIR output in the RAM.  Lastly, in it's own state, ptr is
incremented by one and allowed to simply overflow.

To my understanding, this should result in a fixed length waveguide that
resonates when impulsed and where PU should access a location that is a
constant number of locations away from ptr (given that PU is not changing).
 The output should then be the same frequency, regardless of the value of
PU as long as it is constant.  I'd expect doppler _while_ PU is changing,
but after it's stopped changing the frequency should return to the natural
resonant frequency. 

Do I have this right?  Because it surely isn't working like that.  When I
increase the value of PU, the pitch rises and remains higher even after PU
stops changing. 

Please someone tell me that my understanding of this is muddled...

-- ScottG

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-- Scott Gravenhorst
-- GateMan I - Xilinx Spartan-3E Based MIDI Synthesizer
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.