[sdiy] Polyphonic issue revisited.

[James Patchell]

Well, this comes under the heading...."Its a big Job".  And, I am still 
working on this problem myself (have been now for about 5 years).  In fact, 
I am going to be making another testing run to check the stability of 3 
oscillators today to see how well they are tracking.

You have to separate the two types of drift you get with exponential 
oscillators.  There is scale drift and offset drift.  The scale drift is 
the number of octaves you get per volt.  Traditionally, this is compensated 
for by either controlling the temperature of the expo pair, or using a 
+3300ppM resistor to control the gain of your summing amplifier.  Scale 
drift is very difficult to compensate for, as this means you will generally 
have to retune the oscilator.

Offset drift is easy to compensate for (relatively).  You can just use the 
knob on the front of your synth to adjust the pitch to bring things back in 
tune.

The only way to really know if you have solved the problem is to build a 
poly with the oscillators in question and see how it works.  If you have a 
bad scale drift problem it shows up real fast (and makes the thing sound 
horrible)...I know, the first set of VCO's I did for the poly I built a 
long time ago had this problem.  They were replaced with VCO's that used 
the CEM3340, which did not have that problem.

The ASM-1 Core VCO is pretty good.  Any drift it does have will show up as 
an offset drift.  This is why I have been concentrating on expo 
design.  Scott Bernardi and I seem to have come up with a pretty good way 
of compensating for the scale drift without the use of heaters 
or  expensive temperature compensating resistors, at the cost of higher 
circuit complexity (not really that much more complex, but there are 
varying opinions on that subject).

If you do go the temperature compensating resistor route, Ian Fritz has 
come up with ways of dialing in the tempco as I recall, so you might want 
to check that out.

At 01:12 PM 12/13/2003 +0100, MAKoot MAKoot wrote:
>And how about the AMS-1 Oscillators? Are they stable enough to handle such a
>job? Because I actually also inteded to make my synth 4Voices-polyphonic.
>
>cheers,
>Michiel
>
> > Building a polyphonic DIY synth is a big job....but not imposible.  I
> > built
> > a 4 voice one some 25 years ago...I used a Polymorphic 88 computer the
> > keyboard controller...but, that is another story....
> >
> > Your biggest engineering challenge is going to be the hardware.  20 years
> > ago, Curtis Electro Music and Solid State Music had chip sets to build
> > synthesizers...these greatly reduced the parts counts when building a
> > poly.  Those don't, for practical purposes, exist any longer, you we are
> > stuck with building our synths out of discrete components, so to speak.
> >
> > Another problem that you will have to solve is temperature stability of
> > the
> > oscillators. Not exactly an easy job.  But it may force you to make a hard
> >
> > choice between linear and exponential oscillators...(linear  and expo
> > oscillators both have their own unique problems to solve).
> >
> > It is a big job....one that so far, I have not had the guts to re-tackle
> > again...although, I am getting close... :-)
> >
> > At 08:12 AM 12/12/2003 -0500, Tim Johnson wrote:
> > >Hi hate to bring this issue up again but I have been wondering about the
> > >possibility of building a poly synth. Just a couple of notes 8 or 10  you
> >
> > >know. (Just kidding ==8-O).
> > >Are any gurus familiar with the Korg Poly6? (I loved that board.) I
> > worked
> > >on one a while back (I have a schematic for interested parties to peruse)
> >
> > >and noticed that every time a key is played the led on the circuit board
> > >jumped from one of the 6 oscillators to the next.
> > >... relatively feasable for a DIY or should I just buy one? Heaven
> > forbid.
> > >
> > >(Donning rain gear for the tomatoes.)
> > >
> > >Jim
> > >
> > >_________________________________________________________________
> > >Take advantage of our best MSN Dial-up offer of the year — six months
> > >@$9.95/month. Sign up now! http://join.msn.com/?page=dept/dialup
> >
> >          -Jim
> > ***************************************************
> > I'm a man
> > But I can change
> > If I have to
> > I guess.
> >
> > Man's Prayer
> > Red Green
> >
> > ***************************************************
> > http://www.oldcrows.net/~patchell
> >
> > http://members.cox.net/patchell
> >
> > ***************************************************
> >
> >
> >
>
>--
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         -Jim
***************************************************
I'm a man
But I can change
If I have to
I guess.

Man's Prayer
Red Green

***************************************************
http://www.oldcrows.net/~patchell

http://members.cox.net/patchell

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