Soft Sync Vs. Hard Sync?

[J. Larry Hendry]

> From: Tim Ressel <Tim_R1 at verifone.com>
> Hi. Can anyone explain the difference between hard and soft sync on VCOs?
I
> think I know what hard sync is, but what is soft sync?

OK, Tim, I will bite on this.  I just finished building two of the MOTM-300
VCOs which have both soft and hard sync.  We have discussed this subject in
great detail on the motm mailing list.  So, I feel at least a little
qualified to answer this.  If you understand hard sync, you have made my
job easier as hard sync would be the most difficult for me to explain.  

Hard sync causes the oscillators of the slave VCOs to reset immediately
when a sync pulse is detected.  This can cause some quite nasty sounds as
the frequency of the slave VCO is tuned upwards.  In some cases, it almost
sounds as if a filter is involved.

Soft sync is a synchronizing of oscillators when their frequencies are
naturally close together or at some mathematical multiple between the two
oscillators.  For example, if you wanted two oscillators to sound at
exactly one musical fifth apart and avoid beating of the frequencies, soft
sync would be your solution.  This same is true for octaves, and other
tuning intervals.  This is very useful for keeping VCOs locked at a fixed
interval over a wide tracking range.  Soft sync does have limitations.  If
the frequencies of the two oscillators drift more than a few cycles apart,
soft sync will break and you will hear the beating between the two
frequencies.

In the application of soft sync, I have found that tuning of the
oscillators to a slow beat prior to applying soft sync should always be
done at the highest frequency you intend to use.  Any tracking error
between the oscillators will be smaller (in number of cycles) at the low
end.  Therefore, your soft sync has a better chance of working over the
entire tracking range if initial VCO tuning is done in the upper
frequencies.  I have also found that lower frequencies take longer to sync
up as the zero crossings come less frequently.

I think I have some .wav files of examples between hard and soft sync if
you are interested.  Or, I believe you will find hard and soft sync MP3
files on the MOTM-300 VCO page at synthtech.com.  Be warned, however, that
visiting this site will cause an overwhelming desire to order MOTM modules
(which is really a good thing for me.  I love mine).

Larry Hendry