definition of Syncing

[Patrik Erikson]

At 20:03 1996-11-25 +0100, you wrote:

>Hi!
>
>Can anyone please tell me WHAT is being sync-ed when you sync two
>oscillators ?

Just read it all in my favourite book...

Many popular VCO use a sawtooth generator for the basic waveform. The 'sync'
input of such an oscillator resets the rising sawtooth wave by the falling
edge of the sync pulse. 

     /|   /|   /|   /|   /|
    / |  / |  / |  / |  / |
   /  | /  | /  | /  | /  |
__/   |/   |/   |/   |/   |  VCO normal waveform

     /|      /|      /|
    / |     / |     / |
   /  | /| /  | /| /  | /|
__/   |/ |/   |/ |/   |/ |/  VCO synced waveform

        /|      /|      /|
       / |     / |     / |
      /  |    /  |    /  |
     /   |   /   |   /   |
    /    |  /    |  /    |
   /     | /     | /     |
__/      |/      |/      |   Sync input signal

This causes the output frequency of the synced VCO to be synchronized to the
sync input signal, mostly taken from a second VCO. This syncing will affect
the synced VCOs waveform, and by having two slightly mistuned VCOs, one
synced to another you'll have the basics for all those "sync solo" sounds.
"Aaeeeooouuuwwww....." Then add some modulation to the sync source and "cool
stuff happens..."

There's also sometimes an option to choose between "hard sync" and "soft
sync". The "hard sync" is the one described above. "Soft sync" means that
the synced VCO only resets if the waveform is <5% close to it's ordinary
discharge level.

>How is this done (electronics explanation please) ?

In a sawtooth VCO, like for example the Elektor Formant the waveform is
generated by a linearly rising voltage across a capacitor, which at the
proper level discharges by a transistor switch. This is activated by a
Schmitt-trigger (7413). There should be little difficulty making a sync
input in parallel there.  I'll give it a try some day...

To make a "soft sync" input, all that's needed is some kind of comparator
cirquit set up to determine the right threshold level for resetting. (See
fig. 4-14, p.100 EMS)

There are some fine examples and cirquits, as well as very good explanation
in Barry Kleins' book "Electronic Music Cirquits" p.93-95 and 98-101. 


                Tut Blipp Boooiinng!
                Peace!
                        /Patrik




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