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

[John Speth]

> 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. The crystals I have tried are all brand new and 
> good. The power 
> supply I am using is a nice regulated stable
> +5 volts at about 300ma. I have all the +5v connections for the chips
> and the ground/common connections. All resistors are the 
> value called for 
> and are 1% tolerance. The caps are exact values and check good.

I recently went through the exercise of making a Blacet clone so I had a
chance to thoroughly analyze the system.  I found that the lagging and
rushing came primarily from the MIDI data stream.  If the data stream is
busy with other MIDI data, the timing bytes need to wait for other bytes to
complete.  A MIDI bytes takes at 320us to send not to mention decode time.
That's one source of jitter.  Also the device generating the MIDI clocks
might not be that accurate also.  I tested it with a cheap Roland PAD5 and I
found it was pretty sloppy in nailing the sync right on.  And finally, the
Blacet chip is probably another source of jitter.  It's running at 4MHz
which carries the extra burden of processing all MIDI data.  When it finally
decides it has a MIDI clock byte, it asserts the clock pin.  This all takes
a finite amount of time.  All sources of jitter add up.

John Speth
mailto:johns at molectron.com 
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