MOTM

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
it!

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)?

--PBr

> -----Original Message-----
> From:	jhaible [SMTP:jhaible@...]
> Sent:	Friday, April 21, 2000 5:06 AM
> To:	motm@egroups.com
> 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.
>

> Unfortunately, I don't quite understand this safety gap.  Can you explain
it
> further...?

The two clock pulses for BBDs should not ovelap. Even when you make
sure they won't overlap in your clock circuit, they might as soon as you
connect
the BBD. That's because of the BBD's input capacitance and the limited
current output of the clock circuit. Acts like a slew limiter.
You have to insert a "gap" between the two clock pulses, a small time when
none of them is on. The better your clock driver, the smaller the required
gap.

JH.

In a message dated 4/21/2000 6:48:00 PM, jhaible@... writes:

>The better your clock driver, the smaller the required
>gap.

I'm trying to remember whether it was this chip that was being discontinued 
by Panasonic (for those who remember this discussion maybe 6 months ago 
regarding Moog and Blacet VC Delay modules coming out).

Again, it is really good to have you're voice added to the list Juergen, 
since our flights of fancy (with these dream modules) can often take us into 
outer space with no way to actually realize them.

And for the (recent) record, I'd like to see VC Delay, Interpolating Scanner, 
Analog Shift Register, and any other module that MOTM produces.

Very few stooge posts lately!
JB

Hi All !

Hi input capacitance ?  What about buffering the clocks with an opamp 
that easily drives capacitive loads .  I seem to remember the TL272 
was good for this .  Could be another series though .

Bye for now .


Keep 'em oscillating :)


Hugo
=

> Hi input capacitance ?  What about buffering the clocks with an opamp
> that easily drives capacitive loads .  I seem to remember the TL272
> was good for this .  Could be another series though .

We're talking about the 750mA it takes to charge a 5nF capcitor to
15V in 100ns.

JH.

>We're talking about the 750mA it takes to charge a 5nF capcitor to 15V
in 100ns.

There used to be a lovely little chip called the DS0026 made by National
that was made for this job. Mainly designed for driving MOSFETs in big
SMPS but it would do the trick here... well, it would if National hadn't
discontinued it.

But, I may have a look at other drivers when I finally get around to
updating my very noisy but nice enough sounding chorus unit in my poly.

Regards,

Tony Allgood  Penrith, Cumbria, UK

http://www.techrepairs.freeserve.co.uk/projects.htm