MOTM

On Mon, 25 Oct 1999, J. Larry Hendry wrote:

> From: "J. Larry Hendry" <jlarryh@...>
> 
> OK, I am going to stick my neck out and show my "modular" ignorance again. 
> Some of guys more in the know please explain to me the musical usefulness
> of a multiplier and how you might apply it in your modular.

  The module I'm proposing is a "ratio multiplier/divider", often
shortened to "ratio multiplier".  The idea is this:

  You have some input rate and want to create, say, 3 output pulses for
every 5 input pulses.  A ratio multipler will take the input rate,
multiply it by 5, then divide it by 3 and present the new rate on an
output.

  The multiplication is done by a common communications circuit known as a
phase-locked loop.  This beast is essentially two things: a VCO and a
comparator.  The VCO frequency is set based on a control voltage (often
called the "error voltage") that the comparator output--passed through a
1st-order LP filter (thus allowing the rate of change to appear as a
voltage the VCO can deal with) outputs.  What the comparator is comparing
are the "edges" of the input rate/frequency with the the VCO's output.
The trick here is that the VCO output goes through a divider, the value of
which is the rate of multiplication. Once the edges line up for each input
cycle the two signals are "in-phase" and the multiplier VCO is outputting
a frequency known to be an exact multiple of the input.

  This frequency is then sent through another divider, thus creating the
ratio.

  The uses are vast in the communications world.  Using a single
"reference" input frequency of, say, 1MHz, the ratio divider can generate
any output in the range of the PLL's VCO.  All modern TVs, radios,
cellphones, etc.  uses one or more PLLs in their tuning and CRT
raster-control circuits.

  Musically, there are two primary uses: creating harmonics at intervals
higher or lower in pitch, and creating sequencer clocks.  Say you have a
quarter-note temp and need a triplet tempo for a sequencer.  Send the rate
through a ratio multiplier of 3:1 and you have your triplets.

  Interesting things can be done by running the module with loop filter
settings that aren't necessarily "ideal".  Harmonic-lag processing, for
one.

  --Crow

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