[sdiy] Roland DCOs

[Neil Johnson]

Hi,

Sounds _very_ similar to what the SGS-ATES M110 did years ago, and  
what my little M110 replacement module does.  Although in both cases  
the input is a scanned keyboard, and my module adds MIDI input,  
rather than a CV.

M110 datasheet:

	http://www.milton.arachsys.com/nj71/pdf/m110.pdf

M110 replacement module (designed for Jen SX1000):

	http://www.milton.arachsys.com/nj71/index.php? 
menu=2&submenu=4&subsubmenu=1#ModKbd

This little module is based on an Atmel ATMega8.

The sawtooth output is produced by the external VCO circuit, using  
the exponential current output, a current mirror, and a capacitor.   
If you look on page 2 of the SX1000 schematic, here:

	http://www.milton.arachsys.com/nj71/img/Jen/SX1000/SX1000-sch-2.gif

you can see the current mirror (a couple of BC309C's), integrating  
capacitor, and the discharge transistor (a BC239C).  The exponential  
current output produces, as its name suggests, a current proportional  
to the output frequency, i.e. exponential in note number.
In the replacement module the SW does the MIDI note to timer reload  
conversion through a lookup table and octave shift, and then the same  
number is poked into a 12-bit DAC to produce the current output.

Cheers,
Neil

On 26 Feb 2008, at 13:42, jpdesroc at oricom.ca wrote:

> Here is my next DCO (or DVCO..?) project.. hope this could give some
> ideas..
>
> I want to build the DVCO around a 20mhz Microchip PIC.
> I'm gonna read an external DC control voltage (1v/oct) using
> the PIC inner 12 bits ADC, use the ADC read value to read a lookup
> table
> period value that will generate a narrow pulse (16bits inner counter)
> and discharge a continually charging cap
> at the needed time period.
> This will give a saw wave shape. Ok.
> But the amplitude wil vary according to the wave period time.
> So to correct that I'll charge the capacitor with a controlable
> current source that will keep the amplitude the same for
> all the DVCO range. A simple formula will calculate
> the correct current to charge the cap with according to the
> instantaneous
> frequency period. Doing so I get a clean (no alliasing) very
> temp/freq stable saw waveshape
> of several octave range. The only thing that could vary with
> temperature
> is the wave amplitude but very little. I can do a wave shaping
> further on
> for all other wave shapes.
> I hope I'm clear enough here.
> Thanks for your time.
> JP
>
>
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