<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 24, 2006, at 6:08 PM, Eric Brombaugh wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica"><SPAN class="Apple-converted-space"> </SPAN>So did they wrap a servo loop around from the comparator to the charging current input to keep the amplitude stable?<SPAN class="Apple-converted-space"> </SPAN></FONT></P> </BLOCKQUOTE></DIV><BR><DIV>In most cases, they usually also have an analog voltage that rises in proportion to the frequency of the programmable divider. They use that varying voltage to charge the ramp, resulting in a ramp that stays fairly uniform in amplitude. It's not perfect, but pretty good enough. Typically, they also use the output of that ramp to feed a comparator to make the pulse wave. They give the comparator a CV-variable triggering threshold that allows the DCO pulse wave to have PWM. Essentially, this gives the DCO an entirely analog output. (the fact that it's locked to a clock, however, gives it part of its characteristic "cold" DCO sound -- the frequency never drifts)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Also, if they were using counters for the reset pulse, how did they compensate for the reciprocal relationship between the count value and the frequency? </DIV></BLOCKQUOTE><BR></DIV><DIV>I'm not sure I understand your question. Just to clarify, these chips are used as programmable frequency dividers, not as "counters", per se. The official function name for the 8253 chip is "triple programmable timer" (there are three independent timers in each chip, each one can be used as a separate DCO voice). The count value is a binary word that's latched into the count value input of the counter chip. The actual counter output that's used by the synth is simply a square wave at the frequency of the DCO. More often than not, the synth actually uses the direct counter output as the Square wave for the voice, besides using it ti derive the reset pulse for the ramp generator. </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>They can also use the rising and falling edges of the 50% square wave to toggle a switch to alternate wave direction in a ramp-to-triangle waveshaper, if they want to also provide a triangle wave (not very common, though). Most of this waveshaping stuff is done inside of the CEM3394 chip that's used in the Oberheim Matrix 6, Matrix 1000, and some of the Akais that had DCO's. All the chip needs is the square wave input and the CV's to control and select the waveshape and the proportional saw amplitude. This greatly reduces the amount of ancillary circuitry needed by the system. The Roland DCO synths required a lot of analog circuitry per DCO.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Plus the fact that the frequency resolution decreases as the count value decreases? Did they just use very fast clocks to drive the counters?</DIV></BLOCKQUOTE></DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>I don't know if it's true that the frequency resolution decreases at lower frequencies. If it does, I've never noticed it in the sound, It is true, however, that these systems use fairly high frequency master clocks for the counters. Most of these synths seem to have pretty decent fine pitch control (for pitch bend, fine tuning, DCO detuning, etc.). This type of system even allows for pretty convincing keyboard portamento over fairly wide pitch intervals (created by gradually changing the count value).</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>The best of them actually provide two separate, free-running (analog) master clocks for the counter banks, and use the two clocks for the two DCO's of each synth voice. This gives the system a more natural "analog" sound. Since the two DCO's are not locked to the same master clock, the end result is kind of imperfect like a VCO. The Oberheim Matrix 6 is a good example of all of this, as are some of the Roland DCO synths (JX-10, MKS-70, etc)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Mike B.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV></BODY></HTML>