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Subject: RE: [motm] BBD delays

From: "Brousseau, Paul E (Paul)" <noise@...>
Date: 2000-04-21

I personally would be happy with a flanger / chorus module. I mean, I'd
like to have an echo as well, but if it means compromising quality, forget

Unfortunately, I don't quite understand this safety gap. Can you explain it
further...? Would this safety gap apply to the DRAM application, as
suggested by Tony Allgood (Since BBDs are becoming hard-to-find)?


> -----Original Message-----
> From:jhaible [SMTP:jhaible@...]
> Sent:Friday, April 21, 2000 5:06 AM
> Subject:[motm] BBD delays, and module suggestion
> > 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.