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<TITLE>RE: [sdiy] Help-Blacet midi-din sync chip problems!</TITLE>
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<P><FONT SIZE=2>> So I got the midi to sync convertor chip from Blacet and </FONT>
<BR><FONT SIZE=2>> have constructed </FONT>
<BR><FONT SIZE=2>> the midi to 24ppqn circuit as described in figure three of the data </FONT>
<BR><FONT SIZE=2>> sheet/schematics that Blacet has on their web site and sends </FONT>
<BR><FONT SIZE=2>> out with the </FONT>
<BR><FONT SIZE=2>> chip. Ive followed the normal conventions for creating such a </FONT>
<BR><FONT SIZE=2>> circuit and </FONT>
<BR><FONT SIZE=2>> have at least twelve years experience with such-</FONT>
<BR><FONT SIZE=2>> so I am not concerned about my ability but I just cannot seem </FONT>
<BR><FONT SIZE=2>> to get that </FONT>
<BR><FONT SIZE=2>> circuit to output a nice stable 24ppqn 1ms long pulse! If I sub</FONT>
<BR><FONT SIZE=2>> a 1meg resistor for the 100k that the schemo calls for it is </FONT>
<BR><FONT SIZE=2>> pretty close to </FONT>
<BR><FONT SIZE=2>> a 1ms pulse but it is still not very stable- it lags and </FONT>
<BR><FONT SIZE=2>> rushes at a very </FONT>
<BR><FONT SIZE=2>> high rate. The crystals I have tried are all brand new and </FONT>
<BR><FONT SIZE=2>> good. The power </FONT>
<BR><FONT SIZE=2>> supply I am using is a nice regulated stable</FONT>
<BR><FONT SIZE=2>> +5 volts at about 300ma. I have all the +5v connections for the chips</FONT>
<BR><FONT SIZE=2>> and the ground/common connections. All resistors are the </FONT>
<BR><FONT SIZE=2>> value called for </FONT>
<BR><FONT SIZE=2>> and are 1% tolerance. The caps are exact values and check good.</FONT>
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<P><FONT SIZE=2>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.</FONT></P>
<P><FONT SIZE=2>John Speth</FONT>
<BR><FONT SIZE=2><A HREF="mailto:johns@molectron.com">mailto:johns@molectron.com</A> </FONT>
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