[sdiy] Harry's BBD rant (for newbies...)

[harrybissell]

Hi Rene (et al)....

you are correct in ignoring my BBD bashing... its mostly for fun...  ;^P

... however....

my POINT is that the BBD is the method of choice (like knife, gun, poison)  ;^P
for the newbie who wants to make an "echo unit" like a tape delay...

...often as a first DIY project.

The BBD (as the more experienced are well aware) is not a simple, magic delay
chip. The companders, and anti-alias filters etc are often overlooked, or
underestimated, by the novice.

If they DO get the BBD working (and they are NOT forgiving chips, a simple
error will fry them dead, right now...).. they are often disappointed with the
performance.  (I sure was....)

For me, its a matter of 'bang for the buck'... it might be more effective to get
a used digital delay, and spend your DIY time on a VCO, or VCF instead...
especially for a novice.  I'd consider buying one of those cheap, new, digital
delay pedals from Danelectro and hack that to start with...

(those newbies reading this, check out JH's 'Storm Tide Flanger' for an idea of
how much external circuitry you need to do a BBD right.  If you don't do it
right, you are wasting your time imho...)

(another disclaimer: for those 'lo-fi' types out there, try the BBD you may be
perfectly happy with it....)

So most of the raving about "Harry and the BBD" is just fun and games (for those who
have not heard my genuine concerns before).  Take it for what is is. Mileage
for comparison purposes only, actual mileage way vary....

H^) harry


René Schmitz wrote:

> Hi Scott!
>
> >Yes, you're right, that's what originally got me thinking about the
> >companding.  The fact that the PT2399's minimum delay wasn't going into the
> >minimum delay range that a normal flanger uses diverted me down another path
> >of experimentation, using a different device (rhymes with DDB) =) for that
> >function.   I think the very low noise level of the PT2399 (< -90 dB) is
> >allowing me to get away without companding, though companding may be a way
> >to get some really clean longer delays out of it.  I am assuming that Scott
> >Swartz went with companding to emulate the characteristics of the classic
> >BBD in conjunction with the filtering that he employs in the circuit(?).
> >Now, Harry really needs to talk to this guy - he *wants* the sound of the
> >BBD =).
>
> I don't take Harry serious about his BBD bashing. There is lots of trickery
> you can do with BBDs. People tend to forget that they are sampling systems,
> in which you need an antialiasing pre-filter and a reconstructing lowpass
> post-filter. That gets away of lots of the clock frequency feedthrough.
> Just you can't expect high bandwidth at low sampling frequencies, so
> I usually use these only for very short (some mSec) delays only where they are ok.
>
> The datasheet quotes 74dB SNR for the TDA1022, and that noise can be reduced with
> a compander down to maybe >90dB. So what his point is is still unclear to me.
>
> When properly designed these things can work fine!
> (That means that one needs to think about Mr. Nyquist!)
>
> >Anyway, this is one route that I'm taking to get that flange sound.  Waiting
> >in the wings is experimentation with PT2395 and manipulation of the RAM
> >address lines to get the really low delay range.
>
> Another route, would be two longer delays in parallel. Modulate one with respect
> to the other. The benefit would be through zero possibilities, the downside a
> longer delay will be added to the signal, bad for feedback.
>
> >All of this when I really should be getting that basic synth together.  It's
> >all just so much fun.
>
> It seems to be common, when doing a major construction project to do smaller stuff
> on the side, when you really don't want to work on the big project.
>
> Cheers,
>  René
>
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>