SV: Re: [sdiy] Roland DCOs

karl dalen dalenkarl at yahoo.se
Tue Feb 26 16:44:13 CET 2008


> AARGH! The nastiest chip I ever came across!!

C stands for completely better then original 8254! :-)
However i agree, its not the simplest of interfaceing!

>Wow...  is there a way to add a waveshaper control to a Juno 6  ?
>Drew

No!

<If it is one of the old arcade game machine sound chips, you might find 

MB87xxxx are Matshushita/Roland custom not an old arcade chip.

>The other thing to watch out for on these DCOs is that the pulse width 
>of the pulse output is fixed. Not fixed ratio, like say 20%, but a fixed 
>time. Thus the harmonic structure changes as you go down the keyboard. 
>The JX10 does not a powerful bass monster make. :-)

?
The PW are fixed yes but its also fixed over the entire frequency range.
So there are no change in the spectrum as if it whould have been if it
its PW was changed over freq! However the PW can be changed over time
and or freq but the higher the freq the less bits of PW are available
making the chip completely useless as DCO **WITH** variable PW.

To have that, one needs at least 32 bit timers. PW are a phase parameter
so ear will be very sensitive to its change therefore you need high
resolution of its control.

Btw the DCO appears at first glance neat and component effective
in the end it can be a lot more components then a ordinary VCO,
also even more complex if all the wave functions are to be included.
One major problem are the integrator current drive, 12bits bare
minimum if direct driven like Roland. Yes i know POLY61 uses 8 bits
with expo but then you add a temp error factor.

DCO's are a effective design used in integrated form as the CEM33xx.

You need av very accurate low end current controll wich means 
**MINIMUM** 12 bits dual range. One low for lo freq, one high for
high freq any mismatch of current will give skewed waveforms.

In other words current errors in ordinary VCO designs give you
freq errors will give you spectral errors in DCO, all you do is
to translate one error source into another error domain.

KD


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