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

[Colin Fraser]

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl] 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