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<p><font face="arial"><span style="font-size: 14pt;">Please do NOT call a 4 Quad Multiplier a VCA. A VCA is and should be only two quadrants. The reason for this is that if you have 4
</span><span style="font-size: 18.6667px;">quadrants</span><span style="font-size: 14pt;">, the only way for the VCA to turn OFF is to have an envelope go very precisely to zero. If the envelope</span><span style="font-size: 14pt;"> remains </span><span style="font-size: 14pt;">slightly
positive, the signal leaks through, and if it goes </span><span style="font-size: 18.6667px;">slightly</span><span style="font-size: 14pt;"> negative, the signal leaks trough (inverted). Keep in mind the extremely wide dynamic </span><span style="font-size: 14pt;">range
of the ear. What you want is to be able to "bury" the tail of the envelope ever so slightly negative so that it shuts down (a 2QM). In addition to the
</span><span style="font-size: 18.6667px;">difficulty of trying to get exactly</span><span style="font-size: 18.6667px;"> to zero, it is contended that burying the tail sounds more natural. Apparently, most physical musical instruments shut down</span><span style="font-size: 18.6667px;"> (friction?)
ahead of a theoretical exponential decay. </span></font></p>
<p><font face="arial"><span style="font-size: 18.6667px;"><br>
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<p><font face="arial"><span style="font-size: 18.6667px;">So, the OTA is rather inherently a 2Q multiplier (a VCA). A</span><span style="font-size: 18.6667px;">s</span><span style="font-size: 18.6667px;"> originally presented in National's app note on the
LM13600, it can be made to work as a 4QM. This we can suggest as an adequate "ring modulator". The RM effect need not be exact - ARP once used a left-over logic gate for what they called a RM! The OTA 4QM is not suggested for something more precise such
as a frequency shifter. </span></font></p>
<p><font face="arial"><span style="font-size: 18.6667px;"><br>
</span></font></p>
<p><font face="arial"><span style="font-size: 18.6667px;">The OTA 4QM is in fact analyzed in
</span><span style="font-size: 18.6667px;">E</span><span style="font-size: 18.6667px;">lectronotes </span><span style="font-size: 18.6667px;">EN#107 - pages 13-15. Unfortunately, I can't at present post this - my (old) </span><span style="font-size: 18.6667px;">Lexmark
scanner is not supported by my (new) Windows 10. If someone can scan these three pages and attach them to me, I will add them to the requested section of the EN site. </span><span style="font-size: 14pt;"> </span></font></p>
<p><font face="arial"><span style="font-size: 14pt;"><br>
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<p><font face="arial"><span style="font-size: 14pt;">Bernie </span></font></p>
<p><font face="arial"><span style="font-size: 14pt;"><br>
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<p><font face="arial"><span style="font-size: 14pt;"><br>
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