NTSC (or National Television System Commission according to AbbreviationFinder) is a system of coding and transmission of television analog color developed in the United States around 1940, and is used today in the most of America and Japan, among other countries. A derivative of NTSC is the PAL system used in Europe and some countries in South America.
The NTSC television system consists of an extension of the North American monochrome (black and white) system, its development was started by the Columbia Broadcasting System (CBS) at the end of the 30s, but it was in the 50s when it was approved by the FCC. This system consists of the transmission of about 30 images per second formed by 486 (492) visible horizontal lines with up to 648 pixels each. To take better advantage of the bandwidth, video is used in interlaced mode divided into 60 fields per second, which is 30 frames with a total of 525 horizontal lines and a useful band of 4.25 MHz that translates into a resolution of about 270 vertical lines.
The NTSC format consists of the transmission of 29.97 frames of video in interlaced mode with a total of 525 lines of resolution and a refresh rate of 30 frames of video per second and 60 fields of alternation of lines.
In order to understand and correctly evaluate the difficulties existing in an electronic Color TV system such as NTSC, it is essential to consider the different drawbacks that in the beginning were proposed to be overcome when creating this system, and therefore it is important to consider the inverse compatibility and direct from any color TV system, which is summarized in the following points:
- Receptors white and black existing must receive the signals and reproduce color in black and white.
- Color receivers must receive the signals in black and white and reproduce them correctly in black and white.
- Color receptors must receive the color signals and reproduce them correctly in color.
- The above conditions imply that the bandwidth of the system must be identical in both versions and that the picture tube used must be able to reproduce images in color and in black and white interchangeably.
To achieve the long-awaited compatibility, it was necessary to resort to dividing the signals into their color components (the chrominance signal), and black and white (the luminance signal). Both components are modulated on the same RF carrier of the channel and the following components are then transmitted:
- Luminance signal
- Chrominance signal in a quadrature modulation process on a single suppressed Subcarrier.
- Reference signal to be able to reconstruct the suppressed Subcarrier, in phase and amplitude very rigorously controlled.
- Audio signal (monaural or stereo).
It is also important to mention that for the highest color fidelity, the chrominance signal is transmitted in two components, (RY) and (AY). Both are in quadrature, which means that they are transmitted with a phase shift of 90 degrees and modulated on the same chrominance subcarrier. This is explained in more detail later.
All these signals must be transmitted and received simultaneously on a single 6Mhz channel. The image carrier is spaced 1.25 MHz above the lower limit for the channel and the sound carrier 0.25 MHz below the upper limit. Therefore, the image and sound carriers are always 4.5 MHz apart. The color subcarrier is located 3.579545 MHz above the image carrier. Commercial television broadcasting uses a vestigial sideband transmission for image information. The lower sideband is 0.75 MHz wide and the upper sideband 4 MHz. Consequently, low video frequencies (a general profile of the image) are emphasized relative to high video frequencies (more exact details of the picture). The FM sound carrier has a bandwidth of approximately 75 kHz (± 25 kHz deviation for 100% modulation). Amplitude and phase modulation is used to encode the color information on the 3.579545 MHz color subcarrier.
To ensure compatibility with the NTSC black and white system, the NTSC color system maintains the monochrome signal in black and white as the luminance component of the color image, while the two chrominance components are modulated with an amplitude modulation. in quadrature on a 3.579545 MHz subcarrier. The demodulation of the chrominance components is necessarily synchronous, therefore a sinusoidal phase reference signal known as “color salvo”, “burst” is sent at the beginning of each line. or “colorburst”. This signal has a phase of 180 orand is used by the chrominance demodulator to correctly perform demodulation. Sometimes the burst level is used as a reference to correct for variations in chrominance amplitude in the same way that the sync level is used to correct the gain of the entire video signal.
On the other hand, NTSC systems offer the advantage of tiring the eyes less by not having the classic flickering of the PAL system operating at 50Hz / 25FPS, while NTSC works at 60Hz, making the eyes less tiring, this is like comparing the flickering of a lamp or fluorescent tube to an incandecent bulb at present this problem was overcome with televisions that updated the image to twice the standard or 100hz.
On digital devices, such as digital television, modern video game consoles, DVD, etc., the color coding used does not even matter, and there is no difference between systems, the meaning of NTSC being reduced to a number of lines equal to 480 horizontal lines (240 for half resolution, like VCD) with a refresh rate of 29,970 images per second, or double the fields per second for interlaced images.
A television channel transmitted in the NTSC M system (American standard) uses 6 MHz of bandwidth (radioelectric space), to contain the video signal, the audio signal and some guard bands. The 6 Mhz bandwidth is distributed as follows: 1.25Mhz from the lower top is the main video carrier with two side bands, a vestigial 0.75 MHz and a full 4.25 Mhz; the color components at 3.579545 Mhz on the main video carrier, modulated in quadrature phase and with a bandwidth of 1 MHz; the main 4.5 Mhz audio subcarrier transmitted over the main video signal and with a 25 Khz bandwidth in stereo and frequency modulated.
Transmission and interference problems tend to degrade the quality of the image in the NTSC system, altering the phase of the color signal, so that sometimes the frame loses its color balance at the moment of being received, this makes it necessary to include a tint control, which is not necessary in PAL or SECAM systems. That is why it is jokingly called “NTSC: Never The Same Color” (“NTSC: Never the same color”). Another of its disadvantages is its limited resolution, of only 525 lines of vertical resolution, the lowest among all television systems, which results in an image of lower quality than that which is possible to send in the same bandwidth with other systems. In addition, converting movie formats to NTSC requires an additional process.