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

[Scott Stites]

Hi Harry,
Yeah, I understand that a lot of the BBD bashing is done in fun, but with an
underlying point.  I'm pretty new to Synth DIY, rather this is my second
go-round (I briefly waded into it in the late 80's and got sidetracked by
life).  As far back as the early 80's, I've had a lot of experience *using*
BBD based devices, but as for design and/or building, I am most definitely a
newbie.

The BBD devices I had back in my guitar days (back when I lost my favorite
toothpick, only to have Batz find it years later lodged in an effect switch)
I had your basic two types of BBD device - the analog delay (a DOD model,
800 ms) and I had an ADA flanger.  Never did own a chorus.  These were the
pre-digital, post tape delay days when that was as good as it got (at least
in Hutchinson, Kansas), and I thought it was the cat's meow.  I loved the
heck out of both of them, but I remember the performance of the analog
delay - sounded cool, but you needed to noise gate it or eq it or do
something to get rid of that hiss at the high delay times.  The flanger was
a different story - noise was never much of an issue with it.  A certain
amount of the reason between the noise of one and the quietness of the other
probably had something to do with the difference in design quality, but I
have always had the assumption that the overwhelming issue was that the
analog delay was going through a much greater number of delay stages and
was, as a result, noisier.  That is only an assumption and you know what
assumptions can do.....

Now that I'm into synth's, and want to build my own delay.  I realized early
on that to build a long delay, I would probably really have to get down to
the brass tacks and spend a looonnngg time trying to figure out how to make
it quiet enough with enough bandwidth to make a serious synth module out of
it.  Obviously John Blacet succeeded, but I ain't no John Blacet!  The
scarcity of BBD's and the specs of the PT2399 sent me in the direction of
the PT2399, and I've gotten a relatively noise-free delay without climbing
the noise control learning curve too far.  Now, I want to flange and commit
all sorts of other time based phase chicanery in that region of time that
the PT2399 won't go, and it looks like it's going to take a BBD for that.
Or a PT2395, but I thought I'd try one before the other and the BBD won the
coin toss.

The learning curve is going to be steep, I'll probably be unsuccessful, but
I'm bound to learn something useful along the way (like, "Hey, Harry was
right"), and it'll be fun (you didn't know about my mashochistic tendencies,
did you?).  Thankfully, as you point out, JH has that Stormtide out there so
I have an excellent example of what it takes to do it right.  You provide a
good counterbalance to the whole thing in reminding me and any other newbie
out there that the BBD isn't a magical device and that taking it on isn't
going to be all sunshine and roses and Pink Floyd on acid.

So, in jest or not, the famous Harry Bissell loathing of the BBD is a pretty
good head's up, and I appreciate the advice.  And also that bit about the
fry-ability of the BBD - I'll be extra careful.  Though I imagine, the more
BBD's I take out of action, the happier you'll be ;-p.

Take care,
Scott





----- Original Message -----
From: harrybissell <harrybissell at prodigy.net>
To: René Schmitz <uzs159 at uni-bonn.de>; <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, September 01, 2002 4:11 PM
Subject: [sdiy] Harry's BBD rant (for newbies...)


> 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
> >
> >
>
>