[sdiy] Deep thoughts on Old drum machines: the "VCA"

[Ben Bradley]

There was certainly a "lot going on" as far as using both linear and
non-linear design for music synthesis as far back as the mid 1960s,
and I would hope/expect an engineer new to the field would look
through patents and such to get ideas. The exponential converter is
the first thing that comes to mind, though that has only to do with
pitch and wouldn't have influenced drum machines.

What next comes to mind is the Moog ladder filter, published in the
patent. It uses both a low-signal-level "linear range" of a transistor
for filtering the signal, and a large change in DC bias current to
make a change in the impedance seen across the base and emitter. Moog
was "extravagant" in using two transistors per stage, with the signal
being differential and the control current being common-mode, so
sudden changes in filter frequency didn't add a thump/click in the
signal. I forget exactly when I first saw this circuit and its
description, about 15-20 years ago, but this was fairly recent in my
time futzing around with electronic music. I was amazed at how it was
done with relatively "simple" circuitry. In the early 80's I had this
crazy idea of making a VCF by taking a twin-t filter (the twin-t
network was in an op-amp's feedback loop) and replacing the resistor
to ground with a diode, and varying the DC current into the diode to
vary the resistance as seen by the filter. I knew it would have lots
of distortion with any "reasonable" signal level, but it worked much
better than I expected.

Similar things had already been done with vacuum tubes as well,
changing bias of RF and IF tubes to change RF gain as part of
automatic gain control in radios, and in "vari-MU" audio compressors.

So I'm not surprised if a lot of "inexpensive" circuits use tricks
like a forward-biased diode as a VCA. There would be a thump as the DC
level shifts on fast on-off transients, but for snare, cymbal and
hi-hat a simple high-pass capacitive filter should be enough to remove
most of it, especially the low-end thump.

It sounds like a challenge: What if, "for whatever reason," there was
some sort of a parts shortage and only the most basic jelly-bean parts
were available, how would you design good, musically useful analog
products?

On Fri, Nov 26, 2021 at 4:00 AM Adam Inglis (synthDIY)
<synthdiy at adambaby.com> wrote:
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> > On 26 Nov 2021, at 5:39 pm, Mikko Helin via Synth-diy <synth-diy at synth-diy.org> wrote:
> >
> > I assume that Tadao Kikumoto (which I think is the author) when
> > mentioning the "non-linear technology" just means the various fuzz and
> > distortion stompboxes (and other) which he didn't know before entering
> > this electronic musical instruments business.
> >
>
> Hmmm, maybe… but I get the feeling that there was something else going on in “the lab” (or whatever was the Roland equivalent) that stretched his EE s  brain into new shapes! The translation makes it difficult to tease out of course….
>
> A
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> > On Fri, Nov 26, 2021 at 12:06 AM Martin Klang <mars at pingdynasty.com> wrote:
> >>
> >>
> >> The active region of a transistor is also known as the linear region.
> >> The linear circuits that the engineer is referring would be biased to
> >> operate in the active region, without saturating or cutting off.
> >>
> >>
> >> Martin
> >>
> >>
> >> On 25/11/2021 22:14, Brian Willoughby wrote:
> >>>
> >>> On Nov 24, 2021, at 14:21, Adam Inglis (synthDIY) <synthdiy at adambaby.com> wrote:
> >>>> On 25 Nov 2021, at 7:39 am, Mikko Helin via Synth-diy <synth-diy at synth-diy.org> wrote:
> >>>>> Interesting story from the TR-808 makers here:
> >>>>> https://rc-808.com/episodes-of-the-mid-o-series/
> >>>> Wow, thanks for that, I hadn’t seen it.
> >>>>
> >>>> This bit is interesting - I wonder what he means exactly?
> >>>>
> >>>> "Before I was hired to Roland in 1977, starting from around 1972 I was developing multiple systems with MPU such as Intel i8008 and so on. Even before that, I was an expert of linear circuits such as transistor radio, TV, wireless systems etc. After stepping into the electronic musical instruments fields, I was surprised to see this unique technological culture that boldly and cleverly utilizes the non-linear domain of semiconductors that was considered as taboo in the linear system. This non-linear technology enables the distinctively analog sounds, but at the same time, it had large individual variance, and lacked the dynamic range, S/N, temperature dependency, and stability of the products.”
> >>> That paragraph seems like it's partially referring to the difference between digital and analog. Transistors operate in three states: cutoff, active, and saturated. One nice side-effect of saturated and cutoff is that there is basically no voltage drop across the transistor, or no current, and because power dissipation is the product of voltage drop and current, these states don't heat up the transistor as much. So active mode is taboo, where that's actually an option, because active mode involves heating up the transistor.
> >>>
> >>> But active mode is required for most analog circuits, and transistors can be very non-linear without feedback to correct for the non-linearities. I suppose those transistor radio, television, and wireless systems referred to were all using feedback to linearize the signal processing (although the radio decoder circuits probably weren't linear).
> >>>
> >>> I wonder how much of Roland's early electronics were purposely non-linear, versus merely accidentally.
> >>>
> >>> I got a wonderful introduction to electronic drum machines in Holland (thanks, Allert), and it was explained to me that early Roland drum machines would overdrive their own mixer. Roland "fixed" this in later models, but musicians found that the distortion was a useful effect, and thus favored the earlier drum machines that weren't too linear. I got the impression that Roland didn't fully understand the utility of non-linearity.
> >>>
> >>> Brian
> >>>
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