II. Basic black and white television
In a basic black and white TV, a single electron beam is used to scan a phosphor screen. The scan is interlaced, that is -- it scans twice per photographed frame.
The information is always displayed from left to right. After each line is written, when the beam returns back to the left, the signal is blanked (remember, the standard was invented before digital video!). When the signal reached the bottom it is blanked until it returns to the top to write the next line[6].
Conventional NTSC has 525 vertical lines. However lines number 248 to 263 and 511 to 525 are typically blanked to provide time for the beam to return to the upper left hand corner for the next scan. Notice that the beam does not return directly to the top, but zig-zags a bit[7]
The vertical scanning signal for conventional black and white NTSC is quite straightforward. It is simply a positive ramp until it is time for the beam to return to the upper left hand corner. Then it is a negative ramp during the blanked scan lines.
The horizontal scan signal is very much the same. The horizontal scan rate is 525*29.97 or 15,734 Hz. Therefore, 63.6 uS are allocated per line. Typically about 10 uS of this is devoted to the blanking line on the horizontal scan. There are 427 pixels per horizontal scan line[8] and so each pixel is scanned for approximately 125 nS. (The 427 pixel number comes from taking 1/4MHz, assuming 53.3 uS per active scan, and noting that two pixels encompass each wavelength.)
The electron beam is analog modulated across the horizontal line. The modulation then translates into intensity changes in electron beam and thus gray scale levels on the picture screen.
The combination horizontal blanking signal and synchronization pulse is quite well defined. For black and white TV, the "front porch" is 0.02 times the distance between pulses, and the "back porch" is 0.06 times the distance between pulses[9].
For color signals, a color synchronization signal called the color burst is inserted into the "back porch" of the horizontal blanking signal. Not only does this set the phase of the color signal (more on this later), it also provides a really easy way to tell if you have a black-and-white signal (no color burst) or a color signal (with a 8-10 cycle color burst)[10].
The vertical blanking signal also has a number of synchronization pulses included in it. These are illustrated below[11].
The television bandwidth is 6 MHz. The sub-carrier for the color is 3.58 MHz off the carrier for the monochrome information. The sound carrier is 4.5 MHz off the carrier for the monochrome information. There is a gap of 1.25 MHz on the low end, and 0.25 MHz on the high end to avoid cross-talk with other channels[12].
Television has a maximum frequency bandwidth of 6 MHz. This says that the highest resolution signal is something like 1/6MHz or 166.7 nS. This is consistent with a 330 element scan line with a 8.7 uS blanking time.