{"id":7035,"title":"Voltage Control and the Analog Synthesizer - from Experimental Television Center Studio System Manual","kind":"biblio","url":"https://www.videohistoryproject.org/voltage-control-and-analog-synthesizer-experimental-television-center-studio-system-manual","version":"dba2595eb9190f1ae90d901e1f137531b8b3a4d068acdffb5f51b697d2bd40cf","modified":"2026-09-13T21:51:53.8865065+00:00","credits":[{"role":"Author","name":"Richard Brewster"}],"authorKeys":["Brewster, Richard"],"subjects":["Tool Texts"],"metadata":[{"label":"Contributors","value":"Brewster,Richard","url":null},{"label":"Date","value":"1984 1978","url":null}],"citations":{"page":"\u0022Voltage Control and the Analog Synthesizer - from Experimental Television Center Studio System Manual.\u0022 Video History Project, Experimental Television Center, 2026. https://www.videohistoryproject.org/voltage-control-and-analog-synthesizer-experimental-television-center-studio-system-manual.","publication":"Brewster, Richard. Voltage Control and the Analog Synthesizer - from Experimental Television Center Studio System Manual. 1984 1978.","risUrl":"https://www.videohistoryproject.org/citation/7035.ris"},"links":[],"sections":[{"name":"body","source":"VHP record body","generated":false,"characters":32307},{"name":"summary","source":"existing VHP generated summary","generated":true,"characters":705}],"content":{"section":"body","source":"VHP record body","generated":false,"text":"The Center\u0027s Analog Synthesizer is a modular system for creating different types of changing voltages. The final outputs of the Synthesizer will be used in two distinct ways: 1. as control signals for the image processing modules 2. as synthesized audio for sound tracts INTRODUCTION TO VOLTAGE CONTROL The system has been designed specifically to produce slowly changing voltages suited to controlling various parameters on the imaging modules such as fading, keying, colorization and so on. One way to view the Analog synthesizer is as a machine that combines various simple changes (represented by changing voltages, which are usually periodic in nature) for the purpose of obtaining interesting and complex changes in imagery. Our model has been the modular electronic music synthesizer and we have retained many specific standards from that industry. The Analog Synthesizer is compatible with most commercially made audio synthesis equipment, so you are encouraged to bring your own synthesizers to the studio if you wish. If you plan to do so, please check first with the Center\u0027s staff to be sure that the appropriate cables are available. To use this machine you will have to think about voltage control. We can think of voltage as a quantity that can be measured discretely, for example a given number of volts, 5V. For this to be meaningful we need a point of reference. Usually ground (earth) potential is used as a reference as 0V. All sources of constant voltage (DC which stands for direct current) are signed. They have a positive and negative terminal, like a battery. If the negative terminal were connected to ground, then our 5V source will have \u002B5V with respect to ground at its positive terminal. If the positive terminal were connected to ground instead, then we would have -5V at the negative terminal with respect to ground. In the Analog Synthesizer we use both polarities of voltage with respect to ground and the term \u0022bias\u0027 to describe this relationship. Our control voltage standard is -5V to \u002B5V. Both the Analog Synthesizer and the video image processing modules have been built to respond to this voltage range. This means that a certain aspect of a module\u0027s output will be varied from one useful extreme to the other by a sweep of -5V to \u002B5V. This range can be expressed as 10V peak to peak, abbreviated as 10V P-p. For example: in controlling video pedestal, -5V at the control input would produce black while \u002B5V would yield a white pedestal level, with values in between producing various grey levels. You will find that the useful range will lie somewhat within the -5V to \u002B5V standard. So it will take less than 10V P-p (peak to peak) to sweep the effective range. Since the Analog Synthesizer puts out 10V P-p, it is a simple matter to reduce this voltage to the desired level. Most of the control knobs on the video modules have associated jacks for taking in control voltages. A changing control voltage applied to one of these jacks has the effect of \u0022automatically\u0022 turning the knob. Advantages of using a control voltage instead of just turning the knob include (1) variations which are smoother or quicker than would be possible by hand, (2) increased precision and (3) step-like movement. Manual control is useful for complex changes. There are three important aspects of a control voltage: 1. GAIN the total voltage excursion, expressed in volts peak to peak 2. BIAS the relative placement of the waveform relative to ground 3. WAVESHAPE the nature of change with respect to time We represent these aspects graphically in the example below by plotting voltage (expressed along the Y axis) with respect to time (expressed along the X axis). In this example, the GAIN is 10V P-p, the BIAS is centered at ground or OV, and the WAVESHAPE is called triangle. The rate or FREQUENCY of this signal is 1 Hertz expressed as 1 Hz. This means that the waveshape takes one second of time to complete its form and begin to repeat itself. A triangle waveform is called PERIODIC because it repeats itself on a regular basis within a given period of time. Hertz (Hz) refers to the number of cycles the waveform goes through in one second. To illustrate how such a control signal can be used in the video synthesizer, let\u0027s take this example signal and look at how the patch would be set up. The attenuator, which means reducer, is simply a potentiometer (pot) or knob that enables us to lower the gain of a signal with respect to ground. An ordinary volume control is an application of this type of device. In the patch, Point A is the example signal from the Analog Synthesizer. This is patched to the input of the attenuator. Depending on the knob setting, we can obtain any peak to peak value less than 10 V P-p at the output of the attenuator. The graph \u0022Signal at Point B\u0022 shows the attenuator output, arbitrarily selected to be 2V P-p. This is the signal we will apply to a control input on a video module. Here we must examine how the control knob on the video module interacts with the incoming control signal. The first point to understand is that the knob is also a source of a control voltage. If there is no input to the VC input jack, then the knob voltage is the only one used. When a voltage is applied to the VC input, it is mixed with the knob voltage. Since it is the nature of our mixers to invert (change the sign) of the incoming voltage, the knobs have been made so that counter-clockwise represents \u002B5V and clockwise represents -5V. This is so that after the mixer, the voltage will be positive for clockwise-of-center rotations. Thus turning the knob clockwise results in black to white, low to high and so on. This also means that, with the knob centered at 0V, a control input of \u002B5V is the same as having the knob all the way counter-clockwise with no control input. You must think of a falling (negative going) control signal as producing the effect of turning the knob clockwise. Below is a graph of the voltage output from a knob. The following graph represents an inverted mix of this knob voltage and our attenuated control signal. You can see how the relative placement of the control signal is moved by the knob. This is called BIASING the signal. Let\u0027s continue to analyze how the attenuator and bias knobs allow us to get the effect we want. As an example, let\u0027s control the clip of a keyer. With no control voltage input, the extremes that we wish are at these knob settings: 11 o\u0027clock and 2 o\u0027clock which are about one-third of the total knob rotation. From this we can guess that we will need about 3 or 4 volts P-pk to get the entire sweep we require. As long as we start with a larger signal, the attenuator will allow us to achieve the proper gain. With the signal applied to the VC input, the control knob can now be used to bias the signal so that the start point and end point of the effect are where we want them. At this point it is a matter of adjusting the knobs while observing the image in order to set up the desired effect. Now we will look at the different types of signals in the Analog Synthesizer. A signal can most generally be defined as a voltage that changes with respect to time. In the Analog Synthesizer we can think of a signal as the changing voltage of an oscillator that we are going to process and use as an input to video image processing modules. The terms signal and control signal are relative and refer only to use. In general, a control is lower in frequency than the signal it is modifying. In the Analog Synthesizer signals are classified according to bias, function and waveshape. BIAS 1) \u002B- signal This type of signal usually remains within \u002B-5V. However if several of these are added in a mixer, the total possible range is about \u002B-12V. This is the maximum voltage excursion in the Analog Synthesizer. 2) \u002B signal This type never goes negative and usually stays in the 0 to \u002B5V range. Certain modules output this type of signal and if a negative signal is presented at their inputs it will not pass through. The output will hold at 0 V. FUNCTION 1) PULSE This is sometimes called TRIGGER. It is a short positive voltage excursion from 0 to \u002B10V and back. It is the point in time that the voltage jumps high that is significant. This point may be used to determine the start of other events in the system. 2) GATE A signal which is either 0V or \u002B5V. 0V signifies OFF while \u002B5V signifies ON. This acts like a switch to control the duration of an event, or how long it goes on. WAVESHAPE Signals classified according to waveshape are named for their graphic representations. See the following page. These periodic waveforms repeat. The number of periods in one second is the frequency in Hertz. The terms positive and negative ramp refer to direction of change, not to bias. Note that inverting (turning upside-down) a ramp or pulse changes the shape, while inverting a triangle, sine or square does not. These are commonly used waveforms. The Y axis represents voltage. The X axis represents time. DETAILED MODULE DESCRIPTION The Analog Synthesizer has two boxes each with its own set of modules. The panel controls are listed first and then the functions are described. Module panels have the following features: 1.Signal inputs 2. Control inputs 3. one or more outputs 4. knobs for setting initial points (bias knobs) 5. knobs for inverting or attenuating control voltages and 6. special devices such as switches or lights. Inputs are generally to the left and top while outputs are to the right and bottom. Black lines between jacks indicate internal electrical connections. NEGATIVE RAMP RATE Bias pot setting initial rate of fall VC Attenuator/inverter pot with control voltage input IN Main signal input OUT Main signal output P Pulse Output LAMP Output level indicator The function of this module is to limit the rate at which the output can follow as the input voltage falls. This rate is initially set by the RATE bias control, with more clockwise settings corresponding to higher rates. The rate may also be varied by a control voltage applied at VC. The knob above VC attenuates and/or inverts the control voltage in this manner. At the center of its rotation, an incoming control voltage will be most greatly attenuated, that is will have the least effect. Turning the knob to the right, clockwise, will cause a positive voltage being presented at the VC input to effect a higher rate. Counter-clockwise rotation with a positive VC input will lower the rate. The farther the knob is turned in either direction, the greater the effect. Negative voltages may be applied to the VC input. The negative ramp module puts out only positive voltages, thus the input must be positive to pass through. There are two outputs. The main signal output will accurately follow a rising voltage at the input, while limiting the rate of a falling input. The pulse output remains at \u002B10V while the main output is close to 0V. If the main output rises above about .2V, the pulse output will go 0V. The lamp indicates main output voltage, getting brighter as the main output increases in voltage. If the output frequency is rapid, the light will appear to remain constant at a medium brightness. What happens if the pulse output is patched to the main signal input? First, the output has been at 0V, so the pulse output is high, \u002B10V. When the connection is made the main output rises quickly following the rising input voltage, but as soon as the main output rises this causes the pulse output to go to 0V. Since the main output can\u0027t fall any faster than the control will allow, we will get a falling ramp at the main output. When the ramp approaches 0 V the pulse output jumps high and the process is repeated. Thus we have an oscillator with ramp and pulse outputs. When patched as above, the two negative ramp modules on the Analog Synthesizer each have different frequency ranges. These are shown below. MODULE RANGE with CONTROL VOLTAGE Top Negative Ramp 38 sec/cycle to 500 Hz up to 1200 Hz ","offset":0,"nextOffset":12000,"totalCharacters":32307,"complete":false},"rightsUrl":"https://www.videohistoryproject.org/terms-service"}