Yamaha PM4000 Owner's Manual (image) - Page 110
Channel Muting and Gain Structure, What Is a VCA, and Why Is It Used?, 2.2 The Distinction Between,
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7.1.9 Channel Muting and Gain Structure As pointed out earlier, adding inputs to a mix will increase mix levels. If optimum mix levels are established with some input channels muted, and those channels are later turned on (either with the channel ON/off switch or with the channel MUTE and MASTER MUTE switches), then the bus levels may increase unacceptably, and all input channels' levels applied to the offending bus or busses may have to be reduced. Similarly, if some Groups are added to the Stereo Master mix or the Mix Matrix after those gains have been calibrated, then Stereo bus or Matrix levels may increase unacceptably, requiring either a reduction in all Group Master levels or minor adjustments of the Stereo Master Fader or MTRX MASTER controls. 7.2 Further Hints & Conceptual Notes 7.2.1 What Is a VCA, and Why Is It Used? A VCA, or Voltage Controlled Amplifier, is a special type of amplifier whose gain (the amount of amplification) is adjustable by means of an externally applied DC voltage. This is in contrast to a conventional amplifier, whose effective gain may be adjusted by means of altering a feedback resistance or by attenuating audio signal before or after the amplifier. In a conventional console, mixer or other audio processor, a channel fader (or level control) is generally a variable resistor which attenuates the audio signal flowing through it. The Fader is usually preceded a buffer stage and followed by a booster stage, both of which are fixed gain amplifiers. The buffer keeps the fader's changing resistance from loading the input preamplifier, and the booster stage makes up for the fixed insertion loss of the fader resistance when the fader is set to its nominal position (typically 6 dB). The signal then may be routed to a submaster Fader, where it is again subject to insertion loss so that some gain must be "made up" by an additional booster amplifier stage. If the signal path becomes complex, with one or more levels of "submaster" control, more noise and distortion can result due to thermal resistor noise and residual amplifier aberrations. Also, because the audio signal must be physically routed over a longer, more involved path, there is more opportunity for crosstalk, electrostatically or electromagnetically induced noise, and further signal quality degradation. An alternate approach involves the use of a VCA. In the PM4000, there is one VCA in each input module. That VCA takes the place of the post-Fader booster amplifier in a conventional console configuration. The PM4000 channel Fader is a variable resistor, but it does not have audio flowing through it. Instead, it adjusts a DC voltage output (from 0 volts at nominal position, to 0.5 volts at maximum gain, to +10 volts at "infinite" attenuation position). The DC output voltage from the channel Fader is applied to the channel's VCA control input. The VCA is a special amplifier that is designed to operate at unity gain when the fader is at nominal position, can provide some gain with the channel and/or VCA Master Faders set above nominal, but primarily is designed to attenuate the signal as the fader is lowered. (You can think of VCA as Voltage Controlled Attenuator, although technically that is a distinctly different device.) So far, there is no big advantage to this VCA approach over the conventional console, where the audio flows through the channel fader. The VCA's advantage is realized when grouping is used. The VCA Master Faders are really just like the channel faders in that they output a DC voltage. When one or more input channel VCA Assign switches are engaged, the voltage(s) output from the corresponding VCA Master Fader(s) combine with the channel fader output voltage, and the sum of these voltages determine the channel's VCA gain. The audio signal does not actually flow through any VCA Master Fader, and no matter how many VCA Masters affect the channel, the audio path remains the same... simple and direct with no added noise, distortion or crosstalk. For reasons described in Section 7.2.2, conventional group master Faders are also provided in the PM4000. 7.2.2 The Distinction Between The Group Busses and The VCA Master "Groups" The PM4000 affords the operator with two different means to control multiple input channels from a single fader. One approach is to assign multiple inputs to a given Group with the Group Assign switches [1], and to then use the Group Master Fader [42] to control those signals. With this approach, the actual audio output signal from each of the assigned input channels is applied to a bus wire via 18K ohm summing/isolation resistors. The signal on the group bus is then fed into a combining (summing) amplifier in the Master module, is routed through the GROUP INSERT IN/OUT jacks [118], is then controlled by the Group Master Fader, and is fed to GROUP OUT [130] and any other postGroup Master Fader circuits. An alternate approach to control multiple input channels from a single fader is to use the VCA system. The audio signal in each input channel does not actually pass through the channel Fader [25]. Instead, that fader applies a DC control voltage to a VCA (Voltage Con- Page 7-4