Hawes Mechanical Television Archive by James T. Hawes, AA9DT

Animation: Mechanical TV receiver with magnifier

Mechanical TV Block Diagram Theory

About Mechanical Television Receiver Blocks

  1. The Line-Level Source provides the input signal from the audio-frequency source. The source can be a preamplified radio, tape, CD output, etc. These devices can put out a volt or more of signal. Smaller input signals require more extensive line preamplifiers. Voltage gain is a maximum of 30 times per stage. Typically, the output level is at least in the millivolt range.

Block diagram of mechanical TV receiver (mechanisches fernsehen)

  1. The Line-Level Preamp accepts the input signal (sine wave) from an audio-frequency source. Bandwidth varies. Pictures with a small number of lines (24) need only a narrow band of frequencies. Pictures with a large number of lines (60 and up) need a broad band that extends into the ultrasonic frequencies. The line-level preamp is a class-A voltage amplifier. This amplifier provides an appropriate level for the display driver. From peak to peak, the AC input signal amplitude might be 700 millivolts to about a volt. From the CD player speaker, the necessary preamplifier is usually only one stage (one transistor). From the source's line output, the necessary preamplifier is two to three stages. A simple Darlington preamplifier often works splendidly.

  2. The Display Driver accepts the output signal from the line-level preamplifier. The display driver provides sufficient current to drive the display. The driver stage is a power amplifier. The DC restorer (if necessary) will be at or very near the driver stage. The same goes for the optional gamma corrector. (The DC restorer allows large detail to display in its natural contrast range. This circuit reverses pictorial damage from AC coupling. The gamma corrector alters the brightness contour or slope.) With LEDs, the driver usually direct-couples to the display. The driver should probably include some sort of current-limiting circuit. Except in the simplest applications, a current limiter protects the driver and display from shorts.

  3. The Display. Original displays were neon glow tubes, crater neon tubes, or modulated arcs. Experiments with color mechanical TV added other types of glow lamps. (For instance, mercury vapor and argon.) Some displays used light valves with rear illumination from incandescent or arc sources. (With laser illumination, we still use light valves today.) Today, hobbyists use super LEDs and direct-drive them with transistors, Darlingtons or MOSFETs. The display driver amplifier can drive LEDs in only one phase: That is, the driver turns on, or turns off the display. Biphase displays are possible, but only with multiple LED colors. (Otherwise dark and light picture areas would cancel. That is, they'd appear equally bright.) Like normal diodes, the LEDs rectify. That's why LED displays only use one phase of the amplified video signal. To use more of the signal, a designer can apply DC bias to the LED. The bias voltage turns the LED on halfway. With zero signal, then, the LED produces a mid-gray tone. The video signal then adds to or subtracts from the bias voltage. Adding produces white, while subtracting produces black. (The same technique works for other nonlinear displays, such as neon tubes and CRTs.) The display driver amplifier can operate in series or in shunt with the display.

  4. The Sync Preamp. The sync photosensor output level is fairly high. (Volts: The photosensor has an LED shining straight at it.) The sync preamp amplifies and limits (clips) this level. The sync preamp also builds up the power (current) of the photosensor signal. The output is strong enough to operate the motor driver.

  5. The Motor Driver. With sufficient current from the sync preamp, the motor driver turns on all the way. The motor then operates at full speed. At this speed, a lagging disc soon comes into sync with the transmitter disc. When the transmitter and receiver discs are in sync, the motor driver receives medium current. This current maintains the present speed. If the receiver disc is fast, the motor driver receives low or no current. The motor soon slows down to sync speed.

  6. The Scanning Motor typically turns at synchronous speed. In the MTV era, usual speeds were 12.5, 15 or 20 frames per second Mfps. Today, mechanical TV devices such as micromirrors can scan much faster (180 fps). Engineers tried slower and faster speeds. Slower speeds didn't allow the eye to fuse the image. (Fusion requires about 15 fps.) The viewer perceives flicker or even lines scanning across, as with slow-scan television (SSTV). Flicker reduction requires 48 fps or a greater speed.

  7. The Scanning Disc (not in diagram) varies in size. Disc diameter varies according to picture dimensions and picture resolution. The number of disc holes determines picture resolution. Disc-based pictures are wedge-shaped. For a roughly square picture, the disc diameter is roughly this wide: [(Picture width • number of holes) / 2].

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