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Investigating some Unique aspects of the TB-303's sound

A few years ago I wrote for the  Analogue Heaven mailing list on the unique aspects of the TB-303.
This concentrates on the Accent Sweep Circuit, rather than on the filter and the special Env Mod
circuit which also have some unique characteristics.

Robin Whittle 1 June 1999

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Tony Clark (, ) wrote
something that is often said in respect of the TB-303: that it does
not do anything unique.

> I think that the whole point is that other equipment will rule the 
> dancefloor just as well as the 303.  A good "anything" track will get 
> everyone dancing.  The whole discussion of "it's easier to use", "has a 
> better sequencer", "has a unique sound" is stupid simply because there 
> are an infinite number of ways to accomplish the same thing that the 
> little silver box does.  

There are a number of things that only the TB-303 can do - which cannot 
be done with a modular synth using standard modules.

Ignoring for a moment the exact sonic nature of the filter, which may 
be unique, I want to concentrate on the accent system.  I really 
need a block diagram for this.

There are two envelope generators in the TB-303.  I have named them:

The Volume Envelope Generator (VEG)

    Sharp attack, exponential decay, fixed decay and rather long 
    time.  (In the Devil Fish it can be altered between very short, 
    normal and infinite sustain.)

    Drives the VCA and does nothing else.

The Main Envelope Generator (MEG)

    This primarily controls the filter frequency.  It too has sharp 
    attack and exponential decay.  

    MEG has two possible times.  On normal notes, there is a variable
    time, controlled by the Decay pot.  On accented notes, this is 
    shorted out and MEG runs to the relatively short time which is 
    the same as when the Decay pot is fully anti-clockwise. 
    (The Devil Fish provides independent control of MEG time for
    both normal and accented notes.)

    For both normal and accented notes, the MEG affects the filter
    frequency via the Env Mod pot.  However the circuitry does not
    allow this effect to be turned down to zero even when the pot is
    fully anti-clockwise. (The Devil Fish allows this and extends the 
    amount of Env Mod when the pot is fully clockwise.)

    On an accented note, the MEG performs two other functions. In 
    both cases the MEG voltage goes through a switch (which is only
    on during accented notes) and then through the Accent pot:

    1 - It adds to the control current of the VCA.  (Via an RC
        network of a 47k and a 0.033uF to soften the attack a
        little.)  This is the primary reason why accented notes
        are louder.  Note that in a TB-303, the MEG time is 
        fixed and short for accented notes.

    2 - Via an "Accent Sweep Circuit" it adds to the filter
        control current.  Basically this raises the filter
        frequency on accented notes, to a degree controlled
        by the Accent pot.

    All the above is unique to the TB-303. It would be impossible
    to implement such things on any synthesizer I know of - except 
    by building special additional circuitry.

The real fun is in the "Accent Sweep Circuit" (my name).

This involves a diode and a 47k resistor in series driving
the ACW (anti-clockwise) end of a 100k pot, and a
1uF capacitor (to ground) hanging off the CW end.  The wiper 
of the pot goes via a 100k mixing resistor to one of the two 
summing nodes for filter frequency.

If the pot is ACW, then the filter is driven largely with the
output of the MEG, but the capacitor must always drain via the 
100k, since it gets some charge and this is via the diode.
This means that the filter gets a sudden hit of control current
to increase its frequency - largely a direct copy of the output
of the MEG.  (It is basically 100/147 of the MEG voltage
minus the capacitor voltage. The capacitor voltage does
not change a lot when the pot is ACW, since most of the 
current goes out via the wiper of the pot rather than through
the pot to the cap.)

If the pot is fully CW, then the pulse of voltage driving the
filter is smoothed by coming from the 1uF cap as it is charged 
via the diode and 147k of resistance.

   !!! This pot is the second section of the Resonance pot. !!!

What this means with a single accented note:

The filter goes "Wow."  It rises and falls in a quick, smooth curve.

         ***     ***** 
       **             ***** 
      *                    *****         
     *                          ******
    *                                 *********
   *                                           *******************    

This is rather unique - is is not the angular response you would get
with a ADSR envelope generator.

The real fun happens when you have multiple accented notes in 
quick succession.  Since the capacitor has not discharged
fully from the one before, the second and subsequent response 
curves go *higher*:                                               *****
                                       *****                    **     ***
                                    ***     ****              **           
                                  **            ****         *
            *****                *                  ****    *
         ***     *****          *                       ****
       **             *****    *
      *                    ****

Human listeners - and probably a few furry quadrupeds - interpret this 
output of increasingly high pitched audio signals as sounding like 
the cry of a living creature becoming increasingly distressed, 
apparently in response to quickly repeated bursts of stimulus.

Whole essays could develop from this, in terms of how a few simple
passive components in the TB-303 mimic the distress cry of a living,
sentient creature.  This is especially important in the context of
the effect happening more as the filter's resonance is increased. 
There could be speculation as to how this audio response to distress
evolved, both in nature and in the TB-303.  From there, it is 
relatively simple to postulate that humans and other intelligent 
forms of life would evolve to develop an analagous emotional 
response when hearing such sounds.  

All these are left as exercises for the reader.  Meanwhile, 
instinctually, many people are putting the TB-303 to use and millions 
more are dancing as if possessed.  

A few people are staring at computer screens and writing that the
TB-303 has no unique characteristics - and quite a few more beleive
them.  It may well be that the TB-303 has become a fashion statement
and so has become over-rated.  However there is a very real basis for
its popularity amongst musicians and listeners.  It has a fluidity,
uniqueness and a somewhat animal quality which is lacking in most
other electronic instruments.  The bubbly little sequencer helps. 
The accents and to a lesser extent the slides have no parallel in
other gear. 

Like any other form of music which becomes popular, there is an
increasing number of empty-heads doing it and a great deal of
reductionism and "monkey-see monkey-do" generic product - leading to
boredom and saturation.  We have had a few years of that in techno, a
few decades in rock and a few centuries in earlier forms of western

To experience a truly inspired, minimal, energetic, abstract TB-303 
acid romp, check out Hardfloor's 1992 "Aceperience 1".  

I think that acid music has a lot to do with these curves of filter
response rising with repeated accents.  Reverb and slowing the thing
down (via a playing it on a sampler at a lower pitch) probably has
quite a bit to do with it too.  However there are many other relevant
musical elements - most of which resist scientific analysis.

- Robin

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