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

[Martin Klang]

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
>