sync'ing continuous waveform to clock pulses (was:voltage control lable harmonic generator idea ( Was Re: clock multiplier )

[Haible Juergen]

	>If you don't want to use a pll for clock multiplying (large run-in
time
	>constant, but on the other hand... if the clock rate were very
high...)
[...]
	>Most pieces of music won't change the tempo more then some %
	>anyway.

The bad thing about generating a continuous wave (for the
waveshaping / frequency multiplying we talked about, or
for other things) from a pulse input with varying frequency is
that the pulse does not carry *any* information about altered
frequency before the next slope appears. So you are always
one period late, regardless what device you use. But you
might have some choice *how* the error would manifest. 

I ran into this problem when I thought about external syncing of
an analogue sequencer. (Not yet built, so this is only theory, mind
you!)
I wanted to have an internal saw wave clock generator, in order to
get voltage controlled gate lengths similar to pulse width modulation.
And I also wanted to have voltage control on the (saw) clock generator,
and wanted to have a sync input to clock it from external sources.
I decided against a PLL, because I wanted the sequencer to respond
*immediately* to changes of the external clock. (clock rate suddenly 
doubled or  divided by two is a very "musical" thing, after all.) 
So my decision was to change the slope of the saw wave rather slow
(within the possible limits) with a f/V converter or the like on the input 
pulses, but do a hard sync of the clock generator to the input pulses
at the same time. So the slope (and height) of the saw wave would 
start as an approximation and crawl to its new value slowly, but the
tempo change would happen immediately. So I would have a pulse
width (gate time, note lenght) error, but no timing error. The good thing
is that this works around a standard saw-based VCO with hard sync.

We could use a similar approach for frequency doubling, based on
Kevin's idea with the monoflops. The monoflop time constant could
be adjusted slowly by some control loop, but we'd still have a
"frame" of the input beats that would be tracked immediately.

No idea if this is of real use. The effect should be like a drummer
that plays the tempo change accurately at the strong beats, but
performs some freedom in between. No idea if this sounds pleasant
or not. Would like to hear it.

	JH.