[sdiy] PWM Outputs

[Ian Fritz]

At 09:21 AM 11/7/2010, Tim Ressel wrote:
>A technical question: if you have a PWM, say 0-5 volts from ttl or cmos, 
>is it
>better to cap-couple it to the output jack, or not? My concern is the 
>average DC
>level of PWMs varies over a wide range. This would cause a thump if 
>cap-coupled,
>yes?

I just tried it.  The best test, I think, is to modulate (square wave) 
equal amounts around 50% PW. The spectra are the same in the two cases so 
you pretty much hear a steady tone with clicks.  I don't hear any noticable 
difference in the two cases. Probably beause my mixer is AC coupled 
anyway.  (I don't think you'd want the DC levels ever hitting your speakers.)

So I guess it depends on what levels you want in your synth system.  With 
AC coupling the peaks move up and down on a time scale determined by the 
cap value, so there is some transient click. Also, do you want to use the 
output at very low frequencies?  Then the waveform is mangled.

Another method would be to dynamically offset the the output according to 
the PW CV to get a zero DC component. (Didn't someone do this?)

   Ian