NTSC (or National Television System Committee according to abbreviationfinder) is an analog color television encoding and transmission system developed in the United States around 1940, and is currently used in the most of America and Japan, among other countries. A derivative of NTSC is the PAL system used in Europe and some South American countries.
The NTSC television system consists of an extension of the North American monochrome (black and white) system, its development was initiated by the Columbia Broadcasting System (CBS) at the end of the 1930s, but it was in the 1950s 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 are 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 video frames in interlaced mode with a total of 525 lines of resolution and an update rate of 30 video frames per second and 60 line alternating fields.
In order to correctly understand and evaluate the existing difficulties in an electronic Color-TV system such as NTSC, it is essential to consider the different inconveniences that were initially proposed to be overcome when creating this system, and therefore it is important to consider inverse compatibility and of all color TV system, which is summarized in the following points:
- Existing blackand white receivers must receive color signals and reproduce them in black and white.
- Color receivers must receive black and white signals and reproduce them correctly in black and white.
- Color receptors must receive 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 color and black and white images indistinctly.
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 channel RF carrier 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.
- Audiosignal (monaural or stereo).
It is also important to mention that for greater color fidelity, the chrominance signal is transmitted in two components, (RY) and (AY). Both are in quadrature, meaning they are transmitted 90 degrees out of phase and modulated on the same chrominance subcarrier. This is explained in more detail later.
All of 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 picture information. The lower sideband is 0.75 MHz wide and the upper sideband is 4 MHz wide. Consequently, low video frequencies (an overall picture profile) are emphasized relative to high video frequencies (more exact details of the image). 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 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 black-and-white signal as the luminance component of the color image, while the two chrominance components are modulated with amplitude modulation. in quadrature on a subcarrier of 3.579545 MHz. The demodulation of the chrominance components is necessarily synchronous, therefore a sinusoidal phase reference signal known as “color burst” is sent at the beginning of each line. or “colorburst”. This signal has a phase of 180 oand it is used by the chrominance demodulator to perform the demodulation correctly. Sometimes the burst level is used as a reference to correct chrominance amplitude variations in the same way that the sync level is used for gain correction of the entire video signal.
On the other hand, NTSC systems offer the advantage of tiring the eyes less because they do not have the classic flickering of the PAL system operating at 50Hz/25FPS, instead NTSC works at 60Hz, tiring the eyes less, this is like comparing the flickering of a lamp or fluorescent tube to an incandescent bulb at present this problem was overcome with televisions that updated the image at twice the standard, that is, at 100hz.
On digital devices, such as digital television, modern video game consoles, DVDs, etc., it doesn’t even matter what color encoding is used, and there is no longer a 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 the image of 29,970 images per second, or double in fields per second for interlaced images.
A television channel transmitted in the NTSC M system (US standard) uses 6 MHz of bandwidth (radioelectric space), to contain the video signal, the audio signal and some guard bands. The 6 Mhz of bandwidth are distributed as follows: at 1.25 Mhz from the lower limit is the main video carrier with two side bands, a vestigial one of 0.75 MHz and another complete one of 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 audio subcarrier of 4.5 Mhz transmitted on top of the main video signal and with a bandwidth of 25 Khz in stereo and frequency modulation.
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 picture loses its color balance when it is 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 a lower quality image than it is possible to send in the same bandwidth with other systems.