There are two common means of memorizing and controlling the gain of a channel: one which stores the positions of the fader and uses this data to control the gain of a VCA or digitally controlled attenu-ator (DCA), the other which also stores fader movements but uses this information actually to drive the fader’s position using a motor. The former is cheaper to implement than the latter, but is not so ergonomically satisfactory because the fader’s physical position may not always correspond to the gain of the channel.
It is possible to combine elements of the two approaches in order that gain control can be performed by a VCA but with the fader being moved mechanically to display the gain. This allows for rapid changes in level which might be impossible using physical fader movements, and also allows for dynamic gain offsets of a stored mix whilst retaining the previous gain profile (see below). In the fol-lowing discussion the term “VCA faders” may be taken to refer to any approach where indirect gain control of the channel is employed, and many of the concepts apply also to DCA implementations.
With VCA faders it is possible to break the connection between a fader and the corresponding means of level control, as was described in Fact File 5.5. It is across this breakpoint that an automation system will normally be connected. The automation processor then reads a digital value corresponding to the position of the fader and can return a value to control the gain of the channel (see Figure 5.18).
The information sent back to the VCA would depend on the operational mode of the system at the time, and might or might not correspond directly to the fader position. Common operational modes are:
n WRITE: channel level corresponds directly to the fader position.
n READ: channel level controlled by data derived from a previously stored mix.
n UPDATE: channel level controlled by a combination of previously stored mix data and current fader position.
n GROUP: channel level controlled by a combination of the channel fader’s position and that of a group master.
In a VCA implementation the fader position is measured by an analog-to-digital convertor, which turns the DC value from the fader into a binary number (usually eight or ten bits) which the microprocessor can read. An eight-bit value suggests that the fader’s position can be represented by one of 256 discrete values, which is usually enough to give the impression of continuous movements, although profes-sional systems tend to use ten bit representation for more precise control (1024 steps). The automa-tion computer “scans” the faders many times a second and reads their values. Each fader has a unique
130
address and the information obtained from each address is stored in a different temporary memory location by the computer. A generalized block diagram of a typical system is shown in Figure 5.19.
The disadvantage of such a system is that it is not easy to see what the level of the channel is. During a read or update pass the automation computer is in control of the channel gain, rather than the fader. The fader can be halfway to the bottom of its travel whilst the gain of the VCA is near the top.
Sometimes a mixer’s bargraph meters can be used to display the value of the DC control voltage which is being fed from the automation to the VCA, and a switch is sometimes provided to change their func-tion to this mode. Alternatively a separate display is provided for the automafunc-tion computer, indicating fader position with one marker and channel gain with another.
Such faders are commonly provided with “null” LEDs: little lights on the fader package which point in the direction that the fader must be moved to make its position correspond to the stored level. When the lights go out (or when they are both on), the fader position is correct. This can sometimes be nec-essary when modifying a section of the mix by writing over the original data. If the data fed from the automation is different to the position of the fader, then when the mode is switched from read to write there will be a jump in level as the fader position takes over from the stored data. The null lights allow the user to move the fader towards the position dictated by the stored data, and most systems only switch from read to write when the null point is crossed, to ensure a smooth transition. The same pro-cedure is followed when coming out of rewrite mode, although it can be bypassed in favor of a sudden jump in level.
Update mode involves using the relative position of the fader to modify the stored data. In this mode, the fader’s absolute position is not important because the system assumes that its starting position is a point of unity gain, thereafter adding the changes in the fader’s position to the stored data. So if a channel was placed in update mode and the fader moved up by 3 dB, the overall level of the updated passage would be increased by 3 dB (see Figure 5.20). For fine changes in gain the fader can be preset near the top of its range before entering update mode, whereas larger changes can be introduced nearer the bottom (because of the gain law of typical faders).
Some systems make these modes relatively invisible, anticipating which mode is most appropriate in certain situations. For example, WRITE mode is required for the first pass of a new mix, where the absolute fader positions are stored, whereas subsequent passes might require all the faders to be in UPDATE.
Voltage returned
from automation processor V
DC fader
VCA
Control voltage
Audio in Out
A/D
convertor D/A
convertor
Voltage proportional to position of fader
Fader position data Fader position data
FiguRE 5.18
Fader position is encoded so that it can be read by an automation computer. Data returned from the computer is used to control a VCA through which the audio signal flows.
131
A moving fader system works in a similar fashion, except that the data which is returned to the fader is used to set the position of a drive mechanism which physically moves the fader to the position in which it was when the mix was written. This has the advantage that the fader is its own means of visual feedback from the automation system and will always represent the gain of the channel.
Addr.
decode MPX
A/D
Addr.
decode Demux.
D/A From faders
(DC levels)
Switch buffering and addressing
CPU
RAM ROM Disk Display User
IF Timecode
IF To fader VCAs
(DC levels) From
switches To
switches
Address Data
Control
FiguRE 5.19
Generalized block diagram of a mixer automation system handling switches and fader positions. The fader interfaces incorporate a multiplexer (MPX) and demultiplexer (Demux) to allow one convertor to be shared between a number of faders. RAM is used for temporary mix data storage; ROM may hold the operating software program. The CPU is the controlling microprocessor.
Original mix gain profile Updated mix gain profile Initiate
UPDATE Cancel
UPDATE
Return fader to null point Raise gain
by 3 dB
Re-enter READ mode
3 dB
Channel gain (dB)
Time
FiguRE 5.20
Graphical illustration of stages involved in entering and leaving an UPDATE or RELATIVE mode on an automated VCA fader.
132
If the fader were to be permanently driven, there would be a problem when both the engineer and the automation system wanted to control the gain. Clutches or other forms of control are employed to remove the danger of a fight between fader and engineer in such a situation, and the fader is usually made touch-sensitive to detect the presence of a hand on it.
Such faders are, in effect, permanently in update mode, as they can at any time be touched and the channel gain modified, but there is usually some form of relative mode which can be used for offset-ting a complete section by a certain amount. The problem with relative offsets and moving faders is that if there is a sudden change in the stored mix data while the engineer is holding the fader, it will not be executed. The engineer must let go for the system to take control again. This is where a com-bination of moving fader and VCA-type control comes into its own.