NTSC Color Signal

Cuthbert Nyack

Frequency Spectrum of one NTSC channel

A system of transmitting black and white signals was already in place for several years when it became possible to transmit color signals. If black and white did not exist before then the logical approach to take would be to transmit red, green and blue signals. In order to make the color signal usable by black and white receivers a different approach was used and is illustrated above by the signals in one channel. The bandwidth for each channel remained at 6MHz as for black and white. Sound in black and white had been transmitted as an FM signal with 25kHz deviation and this also was left unchanged. In b/w a bandwidth of 4.2MHz was allowed for the luminance signal with only a portion of the lower sideband being transmitted(VSB). The transition region of the luminance spectrum(max to min) is ~ 0.5MHz wide. VSB was used instead of SSB because of the importance of the low frequencies in the luminance signal. This method of transmitting the luminance also remained unchanged for color signals. For the color system the luminance signal was derived as a combination of red, green and blue signals weighted according to the sensitivity of the eye.
Y = 0.30R + 0.59G + 0.11B
In order to recover the red, green and blue signals at the receiver at least 3 signals which can be linear combinations of red, green and blue must be transmitted. With the luminance already chosen for b/w compatibility two other signals must still be chosen and transmitted within the 6MHz bandwidth.
The 2 signals chosen were the I and the Q signals, both of which are linear combinations of red, green and blue.
I = 0.60R - 0.28G -0.32B
Q = 0.21R - 0.52G + 0.31B
Both are transmitted as DSB signals with a 3.58MHz(suppressed) carrier. The carrier for the I and Q are 90º out of phase. The upper sideband of the I signal is also truncated to fit within the limited bandwidth. In order for the receiver to recover the carrier, a "color burst" of 8 to 10 cycles of the carrier is transmitted on the back porch of the horizontal sync pulse during the blanking interval(see below).

Color Circle

The relative phase of the color burst, I and Q signals are shown above. I and Q are combined to produce a C(color) signal. The magnitude of C fixes the saturation of the color and the phase of C fixes the hue of the color.
When viewed in time one line of a video signal will have 3 characteristics, an average level which fixes the luminance(b/w) signal, an oscillation at 3.58MHz whose magnitude fixes the saturation and whose phase fixes the hue of the color.
The line frequency is 4.5MHz/286 = 15734.26Hz, the vertical scanning frequency is 15734.26/262.5 = 59.94Hz. The Color subcarrier frequency is the 455th harmonic of half the line frequency 455 X 15734.26 /2 = 3.579545MHz. This choice of numbers means that color information in successive fields will be out of phase, minimising interference with b/w receivers. The lines in the luminance spectrum occur at nf_H while those of the color spectrum occur at (n + ½)f_H resulting in interleaving of the 2 spectra.
In the original ntsc monochrome signal the line frequency was 15750Hz and the vertical scanning frequency was 60Hz. Like in the FM band the sound FM is preemphasised with a time constant of 75µs. The sound can be transmitted in stereo using a system similar to how it is done in the FM band. In this case the pilot carrier is at 15.734kHz.

Blanking Intervals

The image is produced by an electron beam scaning from left to right to produce a line of the image. While each line is scanned, the beam is also scanned vertically so that a complete field is produced. After the first field, a second is interlaced with the first to produce a complete frame. When each line is scanned, the beam must return to the left side of the screen without producing a trace. This is achieved during the line blanking interval (~10 microseconds) as indicated above. The signal is brought to a reference level considered as the blanking level. The vertical blanking interval enables the beam to return to the top of the screen after each field without producing a line on the screen. L refers to the Line interval of ~ 63.5 microseconds and V is 1/60 Seconds. Purple lines indicate the visible part of each line (~53.5 microseconds).

Synchronising signals

In order to produce a stable picture, the oscillator producing the vertical deflection of the beam must be synchronised with that used by the camera. Without this the picture drifts up or down. The oscillator deflecting the beam horizontally must also be synchronised with that used by the camera to avoid horizontal drift or diagonal breakup of the picture.
Fortunately because of the blanking intervals in the video information the synchronising information can be inserted during these intervals. The vertical blanking interval has equalising and synchronising pulses inserted into it to provide vertical sync and proper interlacing. Horizontal sync pulses are inserted during the line blanking interval as shown above in green to provide horizontal sync. The color burst is also inserted into the line blanking interval to enable the receiver to regenerate the color carrier with the correct frequency and phase. The amplitude of the color burst is half of the sync signal and its mean value is the blanking level. Frequency of the burst is 3.58MHz.
Diagram above also shows the 3 numbers used to regenerate the color signal. H is the phase difference between the line information and the color burst (shown in red) and determines the hue of the signal. L is the average level of the signal and determines the luminance. S is the peak to peak amplitude and determines the saturation of the color.

Channel Assignments
Each channel is 6 MHz wide
Ch 2 to 6, 54 to 88MHz
Ch 7 to 13, 174 to 216MHz
Ch 14 to 20, 470 to 512MHz
Ch 21 to 25, 512 to 542MHz
Ch 26 to 30, 542 to 572MHz
Ch 31 to 35, 572 to 602MHz
Ch 36 to 40, 602 to 632MHz
Ch 41 to 45, 632 to 662MHz
Ch 46 to 50, 662 to 692MHz
Ch 51 to 55, 692 to 722MHz
Ch 56 to 60, 722 to 752MHz
Ch 61 to 65, 752 to 782MHz
Ch 66 to 69, 782 to 806MHz
Ch 70 to 83 also assigned to 806 to 890MHz band, but is used by translator stations and mobile radio.

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