Archive of the former Yahoo!Groups mailing list: MOTM

previous by date index next by date
  topic list  

Subject: BBD delays

From: "jhaible" <jhaible@...>
Date: 2000-04-21

> Yes. I guess the trick is using a high enough sample rate on the top-end
> to compensate for the low end.

I've done some research on BBD circuirs a few months ago.
IMO, if you want echo (300ms) and flanger (very short), you
either need more than one BBD chip (expensive), or you will
have to find some compromise, especially on the Flanger end.
BBDs can be clocked much higher than their data sheet suggests,
but even then there is a limit.
The tempting idea is to take a long BBD chip (2000 or 4000 stages)
and clock it fast enough to get into flanger range. What I found is that
you can achieve ∗some∗ flanger effects that way, but you won't cover
everything that a short BBD chip (500 stages) can do.
One reason for this is the high input capacitance of long BBD lines,
which call for a "safety gap" in the clock where neither of the two phases
are active. Therefore there are two mechanisms that make if harder
to get short delays from long BBDs:
You need N times the clock rate for N times length to achieve the same
delay time, and
you need N times the safety gap as well.
So the "difficulty" to realize short delay times goes quadratic with N.

Ok, I see the arguments coming. In reality it's not quite as bad. You can
use a high current clock driver to keep the safety gap small even with
5nF load and 800kHz clock rate. But you need this clock driver, and
you need some means to keep glitches away from the power supply rails.
(As an excercise, calculate the peak currents needed for 800kHz and
5nF at 15V supply.)
Many BBD circuits don't care for the safety gap at all. This still works,
somehow, but quality will be degraded, especially at high clock rates.
And I think we agree that if we're using BBDs, we should squeeze the
maximum possible quality out of them.

My suggestion is to concentrate on short (chorus and flanger) delay
times if we build a MOTM BBD module, and either build an extra echo
module or (much better) use outboard devices for that. Which brings up
the idea of a module for a universal voltage controlled send / return path,
where you can connect any outboard echo or reverb you like, and still
have voltage control over the wet / dry mix, panorama, etc.