The Signal

The broadcast signal is picked up by the TV receiver.  It is a modulated 6 MHz signal that contains the luma, chroma, several types of synch pulses, and FM sound.  

Composite Video Signal

Because it contains a number of signals frequency division multiplexed together, the signal is called a "composite" video signal.  The signal requires only 2 wires, which can be seen when looking at a standard cable TV connector - you will see the center pin, and the outer shielding (ground).

NOTE:  Component video, on the other hand, contains 3 signals - normally Red, Green, and Blue - but it can also be a combination of chroma and luma signals subtracted from one another.

The amplitude of the composite video signal is divided into two sections.  The lower 75% is for the camera signal (the actual picture and sound information) and the upper 25% is used for synchronization pulses and retrace.  During the time that the beam is retracing - either back to the left for horizontal or back to the top for vertical - thvideo signal is cut off by "blanking" pulses, which are square blocks at 75% amplitude.

Reverse Effect of Signal Amplitude

Since high amplitudes of the blanking pulses result in no video, the effect of the signal amplitude (voltage) is reversed.  The lower the signal level, the brighter the beam becomes.  Whenever there is a difference in voltages - electrical current flows from the lower voltage to the higher voltage.The faceplate is charged with approximately 30,000 volts (positive), and this draws electrons from the guns, which have much lower voltages.   The video signal is sent to the guns, therefore:

• when the video signal amplitude is low, the gun has a lower voltage, the voltage difference between the gun and the faceplate is increased, more current flows through the beam, and this increases the brightness of the screen
• when the signal amplitude is increased, the guns has a higher voltage, the voltage difference between the gun and the faceplate is decreased, less current flows through the beam, and this reduces the brightness of the screen

The luma (luminance) Signal

This is the monochrome information (brighteness).  It defines the brightness of each horizontal scan line, and is analog.  Therefore it cannot duplicate sudden changes, such as a checkerboard precisely - there must be a gradual change from white to black.  However, the term gradual is relative, and it will appear as if the brightness changes almost instantly on the screen.  Here is a typical luma signal, complete with horizontal retrace and synch pulses.  Note that there is no luma info during the 10 uSec retrace :

Each scan line is lighted by 53 uS of a composite video signal that contains a section of waveforms with a shorter rectangular sync pulse lasting 10 uS, during which time the beam retraces back to the left side.   Also, for every 525 of these - 40 of them have no information - while the beam retraces back from the bottom to top of the screen - this is called vertical retrace or flyback.  This actually occurs with each field of 262.5 lines - so the vert retrace consist of 20 invisible scan lines per field.  

The chroma (chrominance or color) Signal

The area before the sync pulse is the front porch and the area after it is the back porch. When NTSC engineers added the color burst reference to the back porch, it left an area which they called the breezeway, between the sync pulse and the color burst. The actual luminance information follows the back porch and continues until the next front porch :.

The color burst is a reference signal consisting of eight cycles of a sine wave. It recovers the color, or chrominance, information encoded in another sine wave, which itself is combined with the NTSC signal by means of a process called amplitude modulation. This modulated color signal.

A disadvantage of NTSC technology is that when all of these signals are carried on the same wire, they tend to interfere with each other. Y/C cables allow the S-VHS and Hi8 formats to achieve an increase in picture quality by simply separating the luminance and chrominance signals, but the NTSC monitor's limited resolution (480 lines) is another disadvantage. Finally, NTSC video relies on an analog signal that degrades with each copy made. All too often, the result is a frustrating lack of detail

Synch Pulses

It is very important that the high-speed scanning of lines is tightly controlled . . . . i.e. synchronized.  This is accomplished by synch pulses that are placed within the signal.  They are not the same signal as the sawtooth scanning and retrace signal - however, the two signals are synchronized with one another.