Questions from a novice regarding MIDI, CV, Gate, and EFM...

[Magnus Danielson]

From: CasioRZ1 at aol.com
Subject: Questions from a novice regarding MIDI, CV, Gate, and EFM...
Date: Sat, 19 Feb 2000 19:18:03 EST

> Hi All,

Hi Derek!

> I'm new at the whole synth DIY thing, so please bear with me if any of my 
> questions /concepts/terminology seem particularly silly or if this post 
> rambles a bit.  I really would appreciate your feedback! 

Cool down, we have to start somewhere, don't we?

Fire away!

> I have constructed a pretty cool keyboard style controller based on the paia 
> touchswitch diagram at <a href="http://www.paia.com/touchsw.htm"> paia's 
> touchswitch stuff</a>.  The touchswitches take the places of keys, 
> controlling 48 SPST relays along a resistor bridge type thing w/3.9 volts 
> across it (idea taken from the EFM site).  

Hmm... it's not the ideal way of doing things, but it will work.

> Right now, my setup (I'm guessing) will only supply CV, right?  I guess I 
> need to be able to generate "gate"  too, but I really don't know what gate 
> is... my understanding is that it is a +voltage pulse generated w/each 
> keypress to start an envelope?  If so, is the length of the pulse generated 
> supposed to be independent of the time that the finger is held on the key?  

You either have trigg or gate. A trigg input will react on either the rising
or falling edge of the input. It will not care about the other edge, so you
only get the trigg event and you have to go from there all on your own.
For the gate input you will use both the rising and falling edges. If you
take a gate signal and let that control your VCA, it will "gate" the signal
through the VCA. However, a gates signal is not covering the full spectrum
of sound experiences we can have with synthesizers, so we use the rising edge
to trigger the Attack/Decay timing section of the Envelope generator, then
we use the falling edge to trigger the Release timing section. Or in a piece
of ASCII:
         _
        | |
Trigg   | |
     ___| |________________________
         ________________
        |                |
Gate    |                |
     ___|                |_________

          /\_____________
Env.     /               \_
     ___/                  \_______

Notice here that the release point needs to be "programmed" as a delay from
the trigg or attack-peak.

> Next group of questions:  Since none my synths have CV in, I lack a VCO to 
> test my touchswitch controller with.  SO, I went down to local parts store 
> and bought/built an "op amp function generator kit", a tiny (3 by 3 inch, 
> maybe) project.  It actually sounds pretty cool (really!), and I can kinda 
> control it w/my touchswitch setup... but 1.  it is really, really unstable 
> (tuning drifts) and 2.  its frequency is controlled exponentially whereas my 
> touchswitch controller (as I understand it) is linear.  Is there a schem 
> somewhere for converting the latter to the former?   And, in regard to the 
> pitch drift, is there anything I could do to stabilize it?   The circuit has 
> very few components(<15), the only IC being an LM348. I have changed out all 
> the resistors to 1% tolerance which didn't really help, and, the only other 
> components are 2 ceramic caps and 4 diodes.... I realize that it is probably 
> difficult to diagnose w/o seeing the circuit, but is the LM348 the likely 
> problem?  I have ordered another VCO kit (linear and intended for synths) but 
> am still curious as to why the ptich of the function generator drifts, if it 
> is correctible, and how to convert linear CV to exponential.

Well, to start of with the scale thing. If your CV rises linearly with each
note, giving you a 1 V rise per octave, then you have a 1V/oct CV signal.
This is really the logarithmic scale of the requested frequency. So, you need
to somehow change the scale from logarithmic to linear, for that you use an
exponential curcuit. They come in many tastes, but they all rely on the
exponential behaviour of a NP-junction. Not too uncommon is the 2-transistor
solution where the transistors have a common emitter (they are usually NPN's).

The 2-transistor solution can convert a linear voltage into an exponentail
current. Either you just go with the current (many oscillators do) or you
convery the current into a voltage using an op-amp and a resistor.

Either way, the NP-junction will change it's behavior with temperature, so
you will find us here discussing how to figth this problem the "best" way.
Some of us use these special temperature sensitive resistors that we learned
to love under the nick-name "tempco". Tempco really refers to the fact that the
resistor has been made to have a specific temperature coefficent, that is, the
relative amount of resistance raised as a result of increased temperature.
The most common tempco have a tempco value of +3300 ppm/C, but we also see the
use of +3500 ppm/C. The magic about using those values owe to the temperature
sensitivity being experienced by NP-junctions in room-like temperature. Yeat,
others go with schemes where they force the transistors into a certain
temperature and enforces that temperature, this is usually called "owen" even
if you don't actually put anything around it. Then others invent clever schemes
to sense the temperature and compensate for them.

There are many things which could make your particular circuit unstable and
temperature is just one contributing factor. One really has to see the
schematic in order to help you out. But you could try to provoke your curcuit
and tell use what you learned.

> Almost the last question:  On the Electronics for Musicians (EFM) site, there 
> are a bunch of projects that appear to be for sale... I would like to 
> purchase one of them, but can't seem to find the e-mail addy for the guy who 
> designed the projects, a certain "tom g".  Anyone know how to get in contact 
> w/him?

The good man Tom is on this list, so I am sure he will reply to you.

> The last question:  (looking ahead):  How does one convert CV to MIDI?  I've 
> seen lots of MIDI->CV schems/products, but I haven't seen anything that goes 
> the other way.  Once I've finished my controller/synth project, I would like 
> to be able so send MIDI (even just note on/off) with it if possible... could 
> someone point me in the right direction?

Well, it is possible, but there is not really direct path. It depends on what
you want. You could just A/D the CV and then use the gate to generate note on
and then note off. That would be simple. However, you can't bend the tones
with that. No, for that you would have to convert the A/D into running MIDI
values for the pitch bend wheel aswell. The trouble with that is that you are
not sure that it will be in scale with how the CV went. Sigh!

Then there is allways the CV for other things like filters, then you would
have to convert that into some suitable controler message and update that...

You can do it, but you run into walls of questions just like that.

> Thanks for reading this, and for any info you'd be so kind to send!

Hey, we all started out like that and somehow along the way we picked up a
scrap of info here and a scrap of info there. By having people asking "stupid"
questions you for us others to think again on how things really work, why we
do like we do things etc. No worries.

Oh, BTW. Take a peak at the ASM-1 stuff aswell:
http://home.swipnet.se/cfmd/synths/friends/stopp/

I really need to update my pages since I know of alot of lovely stuff that I
haven't linked in yeat.

Cheers,
Magnus