[sdiy] digital delay, was ...

[harrybissell]

Hi John: (inline)

john mahoney wrote:

> ----- Original Message -----
> From: "Harry Bissell Jr" <harrybissell at prodigy.net>
>
> > Hah... since I am the worlds foremost proponent
> > of BBD circuits... I'll suggest how to do this
> > (ROTFLMFAO)  ;^P
>
> As the WFPOBBDC, do you like the currently available chips?

(inside joke).  I have been one of the worst opponents of the BBD...
usually because the degree of difficulty of these circuits vs the performance
is not too good. Usually I limit my diatribe to people who say "I have no
previous diy experience so I thought I'd start with a BBD delay :^)

As I have said before... Juergen Haible is one of the few people I trust with
a BBD (see his 'storm tide' phaser)... but to be fair I also trust Mike Irwin
with these chips as well...

> > Make a VCO that has a real frequency for zero volts
> > and send an FM signal through the BBD. The CV is
> > represented as frequency and the delay is the BBD
> > clock. ...
>
> This would act as a delay? I'm not getting it. I must be too digitally
> brained. :-)

The BBD 'is' the delay... but it cannot pass a good or repeatable DC level.
So the idea is to move the voltage information to the frequency domain.
Before affordable data acquisition systems... tape recorders were used with
voltages expressed as frequencies... which the tape could reproduce.

>
>
> > ... If you used sine waves... I think you could
> > do away with most of the anti-alias filters as well.
>
> The need for filters is a function of the sampling rate and the bandwidth of
> the devices downstream. Unless you are running square waves thru the BBD,
> you may need an a-a filter.

ahhh... no.  you need a filter that cuts off at 1/2 the BBD clock at the input,
for sure
(the 1/2 the clock rate is called the Nyquist limit).  This will only guarantee
that the
correct polarity will be preserved... ie the output (after delay) will have the
same polarity
as the corresponding input.

A sine wave at the input would need NO filter, up to 1/2 the BBD clock rate.
The output
would still need some filtering.

A square wave at the input would absolutely guarantee that you'd NEED an
anti-alias
filter... unless the input frequency was an exact subharmonic of the clock. BBD
and square
wave inputs are a super-bad idea :^P

> But hey, a plain old cap makes a simple low pass
> filter, doesn't it? (Or do you also need a resistor?)

Usually an RC...

> A bigger guess: A cap
> limits the slew rate, but that's not a big problem with an LFO. It's also
> not a problem with a Gate or trigger, at least not in this case (where we
> are delaying that signal in the first place) unless it's such a big cap that
> the trigger is suppressed too much. Also, a real anti-alising filter has
> some resonance around the ol' Nyquist frequency, but that would be overkill
> in this circuit.

A gate or trigger could be delayed easily with a one shot multivibrator if that
was what you wanted... its just a 1 bit data system (yes?)...  in all these
cases
we are talking delay, and therefore 'phase shift' in the output... so the delay
of the output recovery means is just one more delay.

In the recovery... lets say we had a 1-10 volt CV causing a 1KHz to 10KHz
step... that occurs in NO time (ok... 1us).   The FM generator at the input
could conceivably slew at this
rate... but at the output we'd need to convert back from frequency to voltage,
and that will take
real time.  The tachometer circuit would take a long time, and or have a lot of
ripple in the output
CV.  The PV-1 circuit (a board that I did based on Bob Moog's Etherwave pitch to
voltage
converter - used with permission) uses a ramp and hold circuit, that would
settle in 2 cycles of
the input frequency (1-10KHz in our example)

>
>
> > Use a tachometer circuit for 'slow response' CV
> > recovery... or a PV-1 Pitch to Voltage converter
> > for faster response (two cycles of the FM
> > frequency...)
>
> I'm still trying to catch up! I think I get the tach circuit, but not the
> PV-1 thing.
>
> --
> john (learning and spewing! thanks for the tolerance ;-)

glad to chat anytime
H^) harry