VCOs, VCAs, and vocoders...

[Magnus Danielson]

> I'm currently working on/thinking about two little projects, and I was
> wondering if you guys could offer a little advice...
> 
> I have built the Paia (clever design!) Shepard Function Generator,
> http://www.hyperreal.com/machines/manufacturers/Paia/images/ and now I
> want to produce the "barber pole" tones, so I need 8 VCOs and 8 VCAs.

It's a clever design yes...

> What topology would give me a simple exponential VCO for a bank of them?
> Should I just use a stripped-down version of Gene's ASM-1 VCO?  Also,
> can anyone recommend a simple VCA topology?  I have a bunch of 3080's,
> so I'd like to use them, if possible.

4 ASM-1 cards would do the trick for you, it's a simple thing to stripp it 
down,
just don't stuff componentes... the VCAs are 3080 based so you are in luck :)

Stuff the filters and let them sweep along...

Stuff a envelope generator and let it control the envelope of all VCAs...

> While on the subject of banks of VCA's, I've been thinking about
> building a vocoder.  I was reading the archive, and I came across some
> posts saying that an 8 channel vocoder isn't the best...  How many
> channels should a good vocoder have?  How many octaves should it cover,
> with how many bands per octave?  If anybody would be willing to send me
> an example band, I'd really appreciate it.

Good vocoders usually have 16-20 bands. My Korg VC-10 has 20, the big EMS
Vocoder 5000 has 22. I think that Juergen migth have some comments here...

> Also, for the ASM-1, can the LM394 be substituted for the MAT-02?

>From a pure electrical point will the LM394 be usable as far as I can see.
The metal cap version LM194H, LM394H, LM394BH and LM394CH will work rigth away
from a pin view... but the 8 pin DIP/DIL LM394N and LM394CN will be a bit more
problematic since it will not directly work with the surrounding components.
You will need to cut away the pin 4 and 5 and let the 100k resistor be mounted
over the chip since other wise they will collide physically.

I have not tested the LM394 and have none to try it out. I do use the MAT-02
with great success. My VCOs goes from below 0.2 Hz to around 110 kHz so there's
about 19-20 octaves...

Cheers,
Magnus