Analog Television

Please note that this chapter is in outline form and will be fleshed out later. Use it to guide you through your own research.

Analog television is now obsolete, but until recently, it was the apex of modern electronics. This chapter is included so that you can study the marvel of sending moving pictures over an analog electronic circuit, before the medium was taken over by computers.

Invention

Philo T. Farnsworth, at about 12 years old (1918), lived and worked on a farm in Idaho.

The farm was electrified (unusual for the year) and he made a hobby of repairing electrical equipment (motors, etc.) around the farm. He found a large stash of technology magazines in the attic of his home. He excelled in science classes in high school.

He came up with the idea for electronic television while plowing a field. The back and forth track of the plow gave him the idea of scanning an electron beam across a photosensitive screen to create a television image. In a later patent interference lawsuit between Farnsworth and RCA, his high school chemistry teacher recalled Farnsworth made a detailed drawing the concept on the blackboard. The courts eventually found that Farnsworth was the inventor of electronic television. Many sources credit Vladimir Zworykin as the inventor of electronic television. However, it is now known that Zworykin copied Farnsworth's system.

Basic Electronic Video

An image is focused on a photoconductive plate inside a specially-shaped vacuum tube

Where light hits the plate, it is conductive. Where the plate is darker, it is less conductive.

An electron beam is scanned over the plate in a rectangular pattern.

When the beam hits a brighter part of the image, electrical current (supplied by electrons hitting the plate) flows from that spot through the plate to an electrode on the edge of the plate. The current then flows through a wire to the back of the tube. When the beam hits a darker part of the plate, less current flows. The current creates a voltage at a terminal that is proportional to the amount light striking the screen where the electron beam is also hitting the screen.

As the beam scans the plate, the tube produces a varying voltage. At any moment in time, this voltage is proportional to the amount of light striking the plate where the electron beam is also striking the plate.

The voltage produced by one scan line of video.

 The illustration above shows the signal produced by the video camera system for a single scan line of composite video (video signal plus synch pulses). It starts with a short period at 0.285 volts called the "front porch." This voltage is slightly below the voltage produced when the electron beam strikes a portion of the screen that is not illuminated and is called the blanking level^[1]. The voltage then drops to 0 volts for about 5 mS. This is the horizontal synch pulse. It is used by the monitor to synchronize the horizontal oscillator in the rmonitor with the horizontal oscillator in the camera. The voltage then returns to the video blanking level for approximately 10 mS. This period is called the "back porch." All of this takes place while the electron beam is returning from the right side of the screen to the left side after scanning the previous line of video.

When the electron beam has returned to the left side of the screen and starts scanning from left to right, the system starts producing a voltage that is proportional to the amount of light falling on the screen where the electron beam is striking the screen. As the electron beam scans from left to right, this voltage changes as the beam scans across brighter and darker areas of the picture. This will have a minimum of the black level voltage (0.339 volts), produced when the electron beam is striking the darkest parts of the picture and a maximum of the white level voltage (1 volt), when the electron beam is striking the brightest parts of the picture. This is represented by the squiggly line in the gray area in the illustration above. When the electron beam reaches the right side of the screen, it returns to the left side while another synch pulse sequence is produced.

This repeats 243 times for each field of video. While scanning the picture, only the odd-numbered lines are scanned on the first pass. The electron beam returns to the top of the screen and scans the 243 odd-numbered lines to complete one frame of video (each frame consists of two fields, see Interlacing below). This produces the final 486 lines of video. While the beam is returning to the top of the screen horizontal synch pulses continue to be sent although the pulse width is changed three times, indicating that this is the vertical sync pulse stream. This is used by the monitor to synchronize its vertical oscillator with the camera's vertical oscillator. The time taken to return to the top of the screen is that same time taken to scan either 19.5 lines of video. The electron beam starts scanning the second field of a frame in the middle (left to right) of the screen.

The monitor has a similar tube to the camera's imaging tube, except its electron beam is focused on a screen that lights up when electrons strike it. As the receiver beam scans the screen, the signal from the transmitter tells the receiver how strong to make its electron beam in order to reproduce the original image.

Circuitry

The electron beam is moved by two methods

In the transmitter, it is usually moved by electrostatic plates. The electron beam passes between these plates that have high voltages on them that move the beam back and forth and up and down.

In the receiver, the electron beam is usually moved by electromagnets wrapped around the outside of the tube.

The voltage that moves the beam horizontally is produced by an oscillator called the horizontal oscillator

The horizontal oscillator produces a sawtooth wave. The voltage from this oscillator is amplified and sent to the deflection plates (transmitter) or deflection yoke (coils of wire surrounding the receiver's picture tube). The wave rises relatively slowly causing the beam to move from left to right across the screen, then the wave drops quickly to move the beam back to the left of the screen. In the U.S. (NTSC standard) the horizontal oscillator had a frequency of 15.750 kHz.

The voltage that moves the beam vertically is produced by an oscillator called the vertical oscillator

A sawtooth, like the horizontal oscillator. Moves the beam slowly (compared to the horizontal frequency) from top to bottom as the faster horizontal oscillator moves the beam from side to side.

60 Hz in the U.S.

Most sources say that NTSC video (U.S. standard) produced 525 lines of video. Actually produces 486 lines. Beam turned off for 39 lines while the beam returns to the top and synchronizes with the transmitter.

Interlacing

System scanned odd-numbered lines, then even numbered lines.

This caused the beam to scan the screen 60 times per second while only sending 30 pictures per second.

30 pictures per second is all that is needed to produce a viewable moving image.

Scanning from top to bottom only 30 times per second would produce a horrible flicker in the image (hello seizures).

Interlacing causes the screen to be scanned from top to bottom twice for each image or 60 times per second (50 Hz in Europe [PAL standard]). This reduces flicker to a tolerable level.

While the beam is returning from right to left, the transmitter sends a series of pulses that synchronize the beams (horizontal sync).

While the beam is returning from the bottom to the top the transmitter sends a longer series of pulses to synchronize the system (vertical synch)

Closed captioning data is hacked into the vertical sync signal

Broadcast Television

Audio is sent by a separate transmitter and received by a separate receiver

Video is AM, sound is FM

Television Receiver

Block diagram of a television receiver from Wikipedia (slightly incorrect in that it doesn't show separate audio and video receivers.

Two radio receivers

One AM for video

The signal is a standard AM signal except about half of the lower sideband is filtered out. The upper side band is 4 MHz wide and the truncated lower sideband is 2 MHz wide for a total bandwidth of the television signal of 6 MHz.

One FM for audio

Audio receiver is similar to FM radio receiver, including stereo and subcarrier programs.

Less bandwidth and stereo subcarrier at lower frequency

Horizontal oscillator

Vertical oscillator

Circuitry to separate synch signals from video signal

Amplifiers and circuitry to send oscillator outputs to deflection yoke.

High voltage DC power supply

To supply high voltage to anode of the picture tube

Dangerous voltages - televisions can and have killed

Video amplified and sent to electron gun in picture tube to control intensity of electron beam as it scans picture

Color Television

Color signal hacked into monochrome signal.

The color signal is a 3.579545 Mhz sine wave centered on the monochrome signal. As the monochrome signal varies up and down in voltage, the color signal moves with it.

The color signal is a sine wave superimposed on the monochrome signal. The block in the back porch area is 8 to 10 cycles of the color information at a phase of 0 degrees, sent as a reference (the color burst).

A closeup of the signal shows that the color signal is a 3.579545 MHz sine wave. This is phase modulated to convey color information.

The phase of the color signal, compared to a reference (the color burst) sent by the transmitter, tells the receiver what color to display.

In reality, the monochrome signal carries the blue information and the color signal carries the difference between the red and green signals. The original red, green and blue information is extracted by differential amplifiers.

In real reality, the colors are mathematically split up where the monochrome signal is mostly blue information with some red and green and the color signal has the difference between two other signals where one is mostly green and the other is mostly red. Ideally, a black and white TV should display an equal mix of the red, green and blue signals. However, a black and white TV can only see the center voltage of the color signal. It cannot extract any of the color information. The signals are designed such that a black and white TV gives a reasonable rendition of a mix of the original colors.

 The color signal has less bandwidth so the color resolution is less than the monochrome resolution.

The Color Picture Tube

The color picture tube has three electrons guns. One for red information, one fore blue information and one fore green information.

The electron guns shoot colorless electrons. Colored filters on the front of the screen give the picture its color.

The electron guns are aimed at a shadow mask. The shadow mask has a hole in it for each color spot (triad) on the screen. On the viewing side of the screen, each shadow mask hole lines up with a group of three color spots. The blue gun is aimed so that it only hits blue spots, etc.

Video