[sdiy] Polyphonic issue revisited.

Sat Dec 13 17:50:02 CET 2003

Then there is always the TOG, but that's a different thing with different
drawbacks (such as lack of glide), but certainly doesn't have the tuning
problems mentioned here.

James Patchell <patchell at cox.net> wrote:
>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
>> >
>> > ***************************************************
>> >
>> >
>> >
>>
>>--
>>+++ GMX - die erste Adresse für Mail, Message, More +++
>>Neu: Preissenkung für MMS und FreeMMS! http://www.gmx.net
>
>         -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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