<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 30, 2006, at 12:17 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">Due to limited resources an LFO will have to be a separate design. It's come up often enough that I'd be interested in what folks would like to see in an LFO. A few questions:</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">* How stable does it have to be? (Is 2.5% enough?)</FONT></P> </BLOCKQUOTE></DIV><BR><DIV>Are you talking about frequency jitter, or some other kind of stability? I don't know how accurate an LFO really has to be in terms of frequency -- especially if you have a sync input so you can crack the whip on it with an external time reference.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* Is 1V/Oct necessary, or is linear OK?</DIV></BLOCKQUOTE><BR><DIV>Most cheap and crappy LFO's have linear response, but only because it's cheap to do it that way, not because it's any more musically useful. In fact, with 1v.octave response, it's possible to do very musically useful things such as using CV octave shifts to exactly double or halve the LFO rate. Also, it makes the response of the LFO more uniform and predictable over the entire range on voltage control, eliminating the need for control pots with fancy log or reverse log curves. With 1v/oct response, you just use a linear pot, and you get the same amount of apparent frequency change for the same amount of pot travel, at any point in the entire range of the pot.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* I assume that hard sync would be useful. True?</DIV></BLOCKQUOTE><BR><DIV>Absolutely! This will allow rhythmically synced LFO effects</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* Frequency range?</DIV></BLOCKQUOTE><BR><DIV>Hmmm. Some may say that your LFO should be able to work up into the audio range, for cool FM effects. While I think that would be nice, I think the overall range of the LFO is less important than the frequency control resolution that it allows inside its working range. This is important especially in the slower ranges, where you may be trying exactly match a specific tempo. Without enough resolution, you may be either too slow or too fast, and not be able to hit the sweet spot. Obviously, very slow rates are always fun, but I worry that the waveform stepping may start to become unbearable at extreme settings -- only some testing will tell, right? </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>I think you were claiming that your current design had about a ten octave range, right? (20KHz to 20Hz). Does this wide range come at any expense of frequency control resolution? Or to put it another way, would you be able to get better control resolution if you were to cut the overall range, for instance, in half? </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Anyway, with 1v/octave response, and the same ten octave control range as you have in your audio VCDO, if you set your upper frequency limit at 200 Hz, then your slowest speed would be roughly 0.2 Hz, or about 5 seconds. That would put the typical "sweet spot" for vibrato (about 20-30 Hz) at a control voltage of around 7 volts (if you're working with a 0 to 10 volt control range -- or 0.7 of your total control voltage range if you're working with a different range). If you raise the upper limit to 400 Hz, that would put the sweet 30 Hz vibrato range at a control voltage of about 6 volts -- it would also raise your lower frequency limit to about 0.4 Hz, or about 2.5 seconds for one cycle.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Again, how useful this thing is going to be at the slow ranges is going to be totally dependent upon how much stepping is audible in the output waveform. And I don't think any kind of output filtering is going to be very useful on this. You couldn't even use a very accurate voltage-controlled slew limiter whose slew rate was dynamic and directly proportional to the current LFO frequency to control stepping, because any such slew limiter would not allow sharp transitions at the sharp edges of saws or squares.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Speaking of which -- how sharp and clean are those sawtooth transitions? Could they reliably be used as input for a typical sawtooth > comparator Pulse Width Modulation circuit? You have already told us that voltage controlled PWM isn't possible within the chip in your design. it would be nice to be able to do it outside the chip by traditional means.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Getting back to your question, "Frequency range?", if you're talking about ten octaves of range, then 0.2 Hz to 200 Hz would probably work for me, and would probably be similar to the kind of range we see on LFO's in most modern synths (Korg Triton, etc.). Alternatively, maybe you could give the chip two working ranges, selectable by switch.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* Output voltage range (+/- 5V, or 0-10V?)</DIV></BLOCKQUOTE><BR><DIV>Well, this is a tricky question, isn't it? Traditionally, in analog synths, we like to see different overall DC output ranges for different LFO waveforms, and there are musical, not technical reasons for this. For example, we prefer Sine and triangle waves to have bipolar ranges (+/- 5V). This gives your vibratos a more natural sound (it's how we vibrato when we sing, or when we vibrato on a violin, i.e. both above and below the center pitch). It also ensures that there is no apparent overall pitch shift when we add vibrato -- since the frequency is going both up and down by the same amount, our ears perceive the overall center pitch as not changing. If the sine wave only went positive, you would hear a vibrato as also shifting the overall note pitch sharp. (this, BTW, is what gives that excessive-vibrato heavy metal guitar vibrato that we hear so overused by guitar shredders its characteristic fingernails-on-the-chalkboard quality. Since you can only bend a guitar string's pitch *up*, whenever you do finger vibrato on a guitar, the overall net effect is to raise the perceived pitch of the note. Thus, all finger-vibrato'd guitar notes sound sharp. Do we really want our synthesizers to sound like Yngvie Malmsteen? No, we do not. Therefore, Sine waves and Triangle waves must be bipolar.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>However, it is also useful to have our square waves go only positive (0-10V). Positive-going sawteeth and squares tend to be more musically useful (though not *always*, just most of the time). Particularly in the case of the square wave, since you can easily use a positive-only square wave to create nice musical "trill" effects that trill between the base pitch and some other musical interval (say, a fifth or an octave). If your square went both positive and negative, that would make it MUCH harder to dial in the two pitches that you wanted in your trill, since raising the LFO's amplitude would change both the upper and lower frequencies of the trill. You would have to not only find the correct amplitude to get the fifth or the octave, but you would also have to re-tune the pitch of your target oscillator to bring it back to the original center pitch. i hope I'm explaining this clearly enough. If not, please ask and I'll try to clarify.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>With the case of the sawtooth, I think it usually just sounds better if the a positive-going sawtooth (ramp traveling upwards) goes positive only, and a negative-going sawtooth (ramp traveling downwards) goes negative only. In your case I guess that would mean 0 to +5V for the positive ramp, and 0 to -5V for the negative ramp. This is how it's done in many traditional modular synths (and on VC( 3 of the Minimoog, which is its LFO). I think this leads to very logical and musically useful results.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>If, for some technical reasons, you can't have different ranges for your different waveforms, and must choose just one range, then definitely use the +/- 5V range. It's essential that your sine and triangle are bipolar. If you have to choose just one range, but you still have a couple of spare output pins, you could use those pins to control external analog switches that could be used to do the necessary DC level shifting you might want for the sawteeth and square waves (two pins would allow up to four different selections).</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* Waveforms?</DIV></BLOCKQUOTE><BR><DIV>I'm a traditionalist, I guess. Sine, Triangle, Square, Saw Up, Saw Down. Those are the ones that will be used 90% of the time. I wouldn't waste time including a half-ass psuedo-random "sample & hold" wave, nor would I include any of those fake "noise" waves. Hopefully, your sawtooth will be clean enough to use to make a PWM wave with external means, and your square could easily be used to clock an extrenal *real* sample and hold circuit. Leave the weird stuff to the 2% of the population who thinks that a Buchla synth is a "meat and potatoes" instrument. I would like to see you just build the best traditional LFO possible. </DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">* Other features?</DIV></BLOCKQUOTE><BR><DIV>Two things: </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>1. A sync output pulse would be very handy. One pulse per cycle. Or better yet, selectable numbers of pulses per cycle (1, 2, 4, 6, 8, 12, etc.) These could be used for rhythmic syncing of external events. Alternatively, you could simply have a 24 pulses per cycle output pulse, then it could be used as a DIN sync signal, or could be easily divided down into musically-useful rhythmic units (quarters, eighths, triplets, etc.)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>2. MIDI sync -- sync to external MIDI clock.. Yeah, I know I'm pushing it here. Whatever you do end up making will be cool, and I'm glad you're working on this.</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>