[sdiy] Help-Blacet midi-din sync chip problems!

[Rob B]

Problem is, this 50% duty cycles screws
*some* 808s and 606s up pretty badly,
and they begin to slip.

I propose a scheme by which the duty
cycle can be set somehow. The 808s and
606s tend to work better on about a 30%
duty cycle. I used a pwm circuit to get
the Xbase to sync a 606 properly, funny
thing is that it synced a 202 and a 303
just fine with the 50% duty cycle of the
Xbase.


Rob

----- Original Message -----
From: "Colin Fraser"
<colin at colinfraser.com>
To: "'David D'" <shockadd at hotmail.com>;
<synth-diy at dropmix.xs4all.nl>
Sent: Sunday, February 10, 2002 6:51 AM
Subject: RE: [sdiy] Help-Blacet midi-din
sync chip problems!


> > -----Original Message-----
> > From:
owner-synth-diy at dropmix.xs4all.nl
> >
[mailto:owner-synth-diy at dropmix.xs4all.n
l] On Behalf Of David D
> > Sent: 09 February 2002 9:11
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] Help-Blacet midi-din
sync chip problems!
> >
> >    So I got the midi to sync
convertor chip from Blacet and
> > have constructed
> > the midi to 24ppqn circuit as
described in figure three of the data
> > sheet/schematics that Blacet has on
their web site and sends
> > out with the
> > chip. Ive followed the normal
conventions for creating such a
> > circuit and
> > have at least twelve years
experience with such-
> > so I am not concerned about my
ability but I just cannot seem
> > to get that
> > circuit to output a nice stable
24ppqn 1ms long pulse! If I sub
> > a 1meg resistor for the 100k that
the schemo calls for it is
> > pretty close to
> > a 1ms pulse but it is still not very
stable- it lags and
> > rushes at a very
> > high rate.
>
> For the Blacet chip I would have used
a 74LS123 to stretch the pulse -
> it should be much more reliable.
> I woudl also recommend at least a 2ms
pulse.
> In my own MIDI to SYNC IC (
http://www.colinfraser.com/m2d/m2d.htm )
, I
> used the internal timer to generate
the pulse.
> There are a number of devices that I
have found don't sync well with
> only a 1ms pulse.
>
> Something that has concerned me
recently is that the internal clocks of
> the 303, 606 and 808, probably others,
generate a sync clock with a 50%
> duty cycle.
> At a tempo of 120 BPM, this would give
a pulse length of about 10ms
> (20ms period * 24 ppqn * 120bpm = 60
seconds)
> The 303 uses the trailing edge of the
clock pulse as a timing reference
> for turning the gate off (each untied
note is 3.5 clocks).
> This means a 303 synced using a fixed
length pulse of 1 or 2ms, will not
> have the same gate length as if it
were clocked internally.
> 8ms @ 120bpm isn't much of a
difference, but if you subscribe to
> Synth-DIY, you have to be pedantic
about these things ;-)
>
> So, an ideal MIDI to SYNC convertor
should track the tempo, and maintain
> a 50% duty cycle if you want all
synced devices to behave exactly as
> they would with their internal clock.
> I'm looking at doing a new SYNC chip,
based on an AVR, with 50% duty
> clock output, 48ppqn output,
arpeggiator output etc.
>
> Colin f
>
>