[sdiy] PWM Outputs

[Tim Ressel]

Man, it's the little things that get you. I like the notion of leaving the 
output TTL. That way it can be used as a gate or clock. Perhaps the thing to do 
is either have both outputs available, or else provide a jumper for user 
selection of ac/dc outputs. 


--tr




----- Original Message ----
From: Ian Fritz <ijfritz at comcast.net>
To: Tim Ressel <madhun2001 at yahoo.com>; Synth-Diy <synth-diy at dropmix.xs4all.nl>
Sent: Sun, November 7, 2010 10:45:30 AM
Subject: Re: [sdiy] PWM Outputs

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