[sdiy] SAW core VCO flyback time

[Andrew Simper]

On 30 August 2016 at 03:09, Donald Tillman <don at till.com> wrote:
>
>> On Aug 25, 2016, at 9:42 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>>
>> I stopped building sawcore VCOs because I didn't like flyback.  That's why I only build tricores now, and shape them into saws.  They sound better anyway, and sync in a more interesting way.
>
> While deriving the standard waveforms from a triangle core is a little more tricky, there is a very interesting advantage.
>
> With a sawtooth core, the triangle is derived from the sawtooth by sending it through a full wave rectifier circuit (or call it an absolute value circuit) which folds the bottom half of the sawtooth up to the top.  The original sawtooth is sharp and pointy, and has a finite reset time, and the transitions can show up as glitches.
>
> From a wide angle view, the saw->triangle shaper is taking a sharp pointy wave with a lot of harmonic content and making it into a less pointy wave with very low harmonic content.  A sawtooth wave has 39% harmonic content by power, and a triangle wave has 1.5% harmonic content by power.  And because there's so little harmonic content in the intended triangle wave, any faults will stand out for all the world to hear.
>
> But if you do it the other way, start with a triangle wave core and derive the sawtooth, any glitcheroos in the process will be swamped out by the regular harmonic content of the sawtooth wave.
>
> So the triangle->saw converter has math and psychoacoustics very much on its side.
>
> Practical result: you can hear differences between VCO implementations much better by listening to the triangle waves.
>
>   -- Don
>
> --
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com

Totally agree here Don, but one thing left to talk about is PWM. I
have noticed that on a lot of Tri cores the Sqr is derived from the
Tri, not the Saw, is there any reason for this? If you use the Tri you
can't get that lovely PWM vibrato sound. In fact perhaps even have a
switch so you can pick between Saw or Tri for the source of the Sqr.

There is still the downside of traditional hard sync, yes David
"interesting" is great but what if you want regular old hard sync? How
much hassle would it be to implement a regular hard sync on a Tri
core?

If you could get regular PWM and hard sync from a Tri core then it
does sound like the way to go. Then for all those people that love
buzzy Tris they can add a pulse to get it.

Cheers,

Andy