[sdiy] BBDs and crap.

[Batz Goodfortune]

Y-ellow All.
         Excuse me I'm very tired. Or is it the drugs? I can never tell.

I'm pretty sure, that on my web page (God I don't even know what's on my 
own web page.) is a circuit for a DDL which YOU ALL. are probably going to 
find interesting at this point. I know I certainly did.

It is possibly the simplest, kick-ass-est, way to achieve 
long/short/sideways digital delays for about 10 bux in parts with quality 
that'd knock 9 colours of snot out of a BBD.

I never got the thing perfected onto a PCB because I needed to redesign 
some kind of logic sequencer instead of the simple pulse delay (RC-CMOS 
gate) arrangement. And my brain just wasn't in it at the time. The RC pulse 
delay works as an echo type unit but for flanging etc, you really have to 
lock it down with some hard logic.

Basically it's a very simple sigma/delta modulator. Also known as 1bit 
conversion. It uses a single 4164/41256 etc, memory chip as the storage 
unit. OK I'm tired here so don't shot down this attempted description but 
basically what you do is you differentiate at the front end and you 
integrate at the other. You do this across a suitable capacitor at each end.

When the thing powers up it finds it's own level. A CMOS gate output 
constantly switches 1-0-1-0 at a given rate. When the gate output is "1" 
the cap charges up. When it's "0" it discharges. The voltage across the cap 
is compared with the incoming voltage with a comparator. If the cap voltage 
is lower, the gate output is instructed to pump the cap up. If the cap is 
higher,  it's discharged till it's equal. It does this at a reasonably high 
speed. Though slower than you might think. The status of the comparator is 
sampled at the clock rate and stored successively in memory locations in 
the RAM chip. There is only one CAS/RAS count register as I recall. The 
first cycle rips the data out and the second cycle replaces it with new 
data. It just runs round the length of the RAM chip from one end to the 
other. The faster the clock the shorter the delay. You can also cut down 
the number of addresses used as well of course.

At the ass end, it just does the opposite to the mouth end. The outgoing 1s 
and zeros charge/discharge a cap and this doesn't even require any aliasing 
because the cap is the aliasing filter as well.

Clear as mud so far? Well go take a look. It's a lot simpler than it 
sounds. And it does sound rather impressive considering. I messed with it 
as a data communications system at one time too. Instead of the RAM chip I 
just used a length of wire basically. You could use an RS485 driver or 
something and a little bit of logic to encode the clock and you have a 
robust digital audio transmission medium. Complete cheap-assed-bastard style.

I never got back to this project because of the sequencer logic. My aim was 
to find a way to program some of these little PALs I've got and put most of 
the logic inside a single chip. But I still have no clue as to how to get 
any joy from those things. I'm not sure if a 2051 would be fast enough. An 
AVR probably would be but whether I get round to investigating AVRs is 
another story entirely.

This was not an original idea BTW. I saw the original circuit in, ooo 
Wireless and Radio world???? It's on the web site anyway. I think I put the 
original circuit up there (redrawn of course) not my bastardized attempts, 
because there is still ample room for experimentation with this thing. But 
I can tell you that I was pretty impressed with it and it certainly knocked 
the blocks from under a BBD delay. More importantly, unlike the PT chips 
(Which in all likelihood are just a sigma delta scheme similar to this one 
anyway) there is ample scope for tayloring the circuit to the specific 
needs of the device you're trying to build. For a Flanger/chorus unit, 
you'd pump up the clock and  chop off some addresses to the RAM chip. If 
memory serves I calculated in the sub 2mS delay time with good quality. Or 
if longer delays are required you can stuff in bigger RAM chips. Prolly 
getting huge delays if you wished.

I can't think of anything more I can tell you off hand. You'll just have to 
go look it up. But if anyone takes it on and comes up with something 
interesting I'd love to hear about it.

Hope this helps.

Be absolutely Icebox.

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