Figure 1. Details of NTSC FDM spectrum around the chrominance subcarrier. Click here to see an enlarged diagram.
In 1941, after many experiments with television, the Federal Communications Commission approved the NTSC standard. This was a revolutionary approach to television. It offered a high resolution of 525 interlaced scanning lines, negative video modulation with a 4.2MHz full upper sideband and a vestigial lower sideband, and FM sound — all transmitted in a 6MHz-wide channel. It also specified a VHF transmission channel allocation in preparation for the expected rapid development of television. By 1948, the VHF channel allocation had undergone several changes, including the disappearance of channel 1 and the introduction of UHF channels 14 to 89.
In 1950, following several years of experimenting with color television, the FCC approved a CBS-proposed sequential R,G,B color television system. This system used a rotating R,G,B segmented color wheel concept, which was incompatible with the NTSC all-electronic black-and-white standard.
The compatibility constraints are:
- Monochrome compatibility: A monochrome receiver must reproduce the brightness content of a color signal correctly in black and white, with no visible interference from the color information.
- Reverse compatibility: A color television receiver must reproduce a monochrome signal correctly, with no spurious color components.
- Scanning compatibility: The scanning system used for color transmissions must be identical to the one used by the existing monochrome service.
- Transmission channel compatibility: The color system must fit into the existing monochrome television channel and use the same spacing between the vision and sound carriers.
NTSC composite video concept
Following several years of experimenting with NTSC monochrome-compatible color television concepts, the FCC approved in 1953 the current analog color television standard. The NTSC system is based on the concept of composite video. Composite video describes a signal in which luminance, chrominance and synchronization information are multiplexed in the frequency, time and amplitude domain for single-wire distribution. The contemporary characteristics are defined in the SMPTE 170M standard.
• The luminance information: Monochrome compatibility requires the generation and transmission of a full bandwidth (4.2MHz) luminance electrical signal (E'Y) representing the brightness component of the image. This signal is obtained by a linear combination of three electrical signals (E'G, E'B and E'R) representing the three primary colors green, blue and red. The prime sign indicates that the respective signal is gamma corrected. The luminance signal is represented by the mathematical expression:
E'Y = 0.587 E'G + 0.114 E'B + 0.299 E'R
• The chrominance information: The chrominance information is transmitted by two of the primary colors (blue and red), less the brightness component, and are known as the color-difference signals. These signals have a reduced bandwidth, of the order of 500kHz, reflecting the reduced eye resolution. The mathematical expressions for these signals are:
E'B - E'Y = - 0.587 E'G + 0.886 E'B - 0.299 E'R
and
E'R - E'Y = - 0.587 E'G - 0.114 E'B + 0.701 E'R
The E'G - E'Y signal can be recreated in the receiver by a suitable combination of the three signals.
The color-difference is scaled in amplitude by suitable multiplication factors to avoid transmitter overmodulation. The scaled color-difference signals are:
E'B-Y = 0.493 (E'B - E'Y) and E'R-Y = 0.877 (E'R - E'Y)
Each color-difference signal modulates an assigned subcarrier. The two subcarriers are of identical frequency (about 3.58MHz) but of different phase. The phase difference between the two subcarriers is 90 degrees, so the original signals modulating the two subcarriers can be demodulated without crosstalk. The two subcarriers are obtained from a common crystal-controlled oscillator. The type of modulation used is suppressed-carrier amplitude modulation. Because the subcarrier is suppressed, only the sidebands are obtained at the output of the modulators. This results in complete cancellation of the chrominance signal when no colors are present.
Figure 2. Simplified block diagram of an NTSC B-Y/R-Y encoder. Click here to see an enlarged diagram.
The system takes into consideration the fact that in an analog video signal's spectrum is not continuous but features clusters of spectral components at the horizontal scanning frequency Fh and its multiples. So the subcarrier frequency (Fsc) is chosen to be an odd multiple of half of Fh, resulting in an interleaved chrominance/luminance spectrum known as half-line offset.
Figure 1 on shows the simplified representation of the NTSC interleaved spectrum around the chrominance subcarrier. It is to be noted that the original monochrome scanning frequencies of 30Hz and 15,750Hz have been altered to, respectively, 29.97Hz and 15,734.25Hz. This was aimed at reducing the visibility of the potential 920kHz beat between the color subcarrier and the audio subcarrier (4.5MHz). Therefore, the subcarrier frequency is:
Fsc = 455Fh /2 = 3,579,545 ±10Hz
• The chrominance synchronization: A burst of nine cycles subcarrier is transmitted during the backporch horizontal blanking interval to synchronize the receiver's local crystal oscillator.
Figure 2 shows a conceptual block diagram of an NTSC encoder using B-Y/R-Y color-difference signals. Green, blue and red signals are fed to a resistive matrix that algebraically combines percentages of the three primary color signals to form the luminance and the two color-difference signals. Each color-difference signal is band-limited before being fed to the respective balanced modulator.
A 3.58MHz subcarrier feeds the B-Y balanced and, through a 90-degree phase-shift network, the R-Y modulator. The adder combines the luminance, chrominance sidebands, composite (vertical and horizontal) sync, and a 180-degree phase-shifted gated subcarrier burst into a composite color signal.
Figure 3. The instantaneous amplitudes of the subcarrier result in a vector whose amplitude represents saturation and phase represents hue. Click here to see an enlarged diagram.
Figure 3 shows a vector representation of the chrominance subcarrier modulation process. A given color, described by a given set of E'B-Y and E'R-Y signal values, is represented by two amplitude-modulated subcarriers in phase quadrature. The instantaneous values of the two modulated subcarriers result in a vector described by its amplitude and phase angle with respect to the B-Y subcarrier reference phase. The vector amplitude represents the color saturation, and its phase angle represents the hue.
Problems with NTSC
NTSC was a compromise aimed at squeezing color information into a 6MHz monochrome television channel. The solution was brilliant, but the electronic tube technologies of the time were at best inadequate and at worst unacceptable. Solid-state technologies have removed many of the early problems. Here are a few of the problems:
• Transmitter overmodulation: The color-difference signals are scaled to avoid transmitter overmodulation. The video transmitter is modulated in amplitude and uses negative modulation. In a negative modulation system, increasing the brightness of the transmitted picture produces a decrease of the modulation envelope amplitude.
Figure 4. Significant video signal levels shown as a percentage of carrier amplitude in negative amplitude modulated systems. Click here to see an enlarged diagram.
The picture modulation envelope has four reference levels: peak white level, black level, blanking level and sync level. Figure 4 shows the NTSC reference levels as used in North America. White level (100 IRE) reduces the carrier to 12.5 percent. The carrier cancels at 120 IRE. Saturated yellow and cyan colors can produce video signal levels of 130.8 IRE, resulting in carrier cancellation and inter-carrier “buzz.” Because saturated yellow and cyan colors do not normally exist in nature, camera-generated video signals would not create problems. However, synthetically generated signals (100 percent color bars or character generators) could create problems.
• Video signal distortions: Analog composite video signals are subjected to various types of cumulative distortions and noise. Separate distortions of the luminance and the chrominance components as well as inter-modulation between them are likely to occur. PAL is more tolerant of luminance/chrominance inter-modulation. Such distortions can be reduced by performing all, or at least a major part of, production operations using component video signals.
Michael Robin, a fellow of the SMPTE and former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal, Canada. He is co-author of Digital Television Fundamentals, published by McGraw-Hill, and recently translated into Chinese and Japanese.
Send questions and comments to:michael_robin@primediabusiness.com
