SV: Re: [sdiy] Roland DCOs

[Tom Wiltshire]

On 26 Feb 2008, at 15:44, karl dalen wrote:
>
>> 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.

This doesn't have to be a problem. It looks like the JX-3P and 8P do  
use the counter to generate the pulse waves directly (at least, I  
can't see any comparators) and would therefore suffer as you describe  
from decreasing resolution. However, the Juno 106 uses a comparator  
fed with the Ramp waveform and a PWM CV in the traditional manner,  
which avoids the problem completely. It depends whether you want PWM  
as a feature of the chip or a part of the waveshaper stage following it.


> 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.

I agree with Rainer about this. Moving errors into domains where they  
are less perceptible to humans is one of the techniques for dealing  
with them. The human ear is more sensitive to frequency than to  
amplitude or harmonic spectrum, so the errors in the DCO are in the  
'right' places.
If you ensure that the compensation CV tends to be on the low side  
rather then the high side, then the Ramp wave will always be a  
perfect ramp, but it's amplitude might vary a bit. Still, at least  
you've eliminated any harmonic effects.

That said, you're right that getting a decent amplitude compensation  
CV over a full MIDI range does present some challenges.

Regards,
Tom