[sdiy] Wikipedia DCO article

[Bob Weigel]

Neil,

I did the write up on opera6 like I said..   When I initially looked at 
that part of the circuit years ago I wasn't real familiar with how other 
DCO's were set up and I saw an op amp circuit with a CV input and 
reasoned it must be just selectively filtering it without thinking 
throught the details on that part. (which it does after all over each 
cycle in a way of thinking.  Filtering the square pulse into a gradually 
changing voltage. :-) )   But of course it is a fairly similar circuit 
to the others and it sounds like not from my description so I'll fix 
that.  Thanks for alerting me. -Bob. 

Neil Johnson wrote:

>Hi Tom,
>
>  
>
>>http://en.wikipedia.org/wiki/Digitally-controlled_oscillator
>>    
>>
>
>Excellent work!
>
>Some comments, if I may, based on my studies of the Jen SX1000 and Siel Opera 6.
>
>1/ The Jen SX1000, also released in 1978, uses a single 74LS221 multivibrator HF VCO (yes, 'V' because tuning and vibrato are analogue control voltages applied to the '221) driving the clock input of an M110 monophonic voice IC.  This single chip scans the keyboard and, together with a portamento clock, provides four output square waves (2', 4', 8', 16') and a DC current proportional to frequency.
>
>The range switch selects one of the four clock outputs.  Divided by 2 provides the required 4'-32' range.  The square wave output is tapped off at this point.  It is also used to reset an integrator to provide a sawtooth output - the current driving the integrator capacitor is the DC current provided by the M110 (a switchable gain block maintains the amplitude of the sawtooth on different ranges).
>
>The portamento clock determines the rate at which the M110 changes pitch from one note to the next.
>
>So I would say it is close companion to the Crumar DS-1, but stating 'VCO' on the programming sheets.
>
>2/ Your description of the Siel Opera 6 is not quite right.  Yes, the basic pitch clocks are generated from the SSM2031 HFO and assigned to a channel through two TMS3631 tone generators.  However, the output of the tone generators is used to reset a - you guessed it - capacitor integrator generating a sawtooth.  There is no 'active filter' as such.
>
>If you look at the schematic for the Opera 6 voice:
>
>http://www.angelfire.com/ms/sander/siel/opera6/Siel_op6schem.pdf
>
>on page 10 in Waveform Gen 'A' you have a 12n integrating capacitor, a reset transistor T3, exponential current source based on half of a '3046 with linear control input as well, and a comparator, IC10, for the PWM output.
>
>Cheers,
>Neil
>--
>http://www.njohnson.co.uk
>
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