The first NTSC standard was developed in 1941 and had no provision for color. In 1953 a second NTSC standard was adopted, which allowed for color television broadcasting which was compatible with the existing stock of black-and-white receivers. NTSC was the first widely adopted broadcast color system and remained dominant until the 2000s, when it started to be replaced with different digital standards such as ATSC and others.
Most countries using the NTSC standard, as well as those using other analog television standards, have switched to, or are in process of switching to newer digital television standards, there being at least four different standards in use around the world. North America, parts of Central America, and South Korea are adopting or have adopted the ATSC standards, while other countries (such as Japan) are adopting or have adopted other standards instead of ATSC. After nearly 70 years, the majority of over-the-air NTSC transmissions in the United States ceased on January 1, 2010, and by August 31, 2011 in Canada and most other NTSC markets. The majority of NTSC transmissions ended in Japan on July 24, 2011, with the Japanese prefectures of Iwate, Miyagi, and Fukushima ending the next year. After a pilot program in 2013, most full-power analog stations in Mexico left the air on ten dates in 2015, with some 500 low-power and repeater stations allowed to remain in analog until the end of 2016. Digital broadcasting allows higher-resolution television, but digital standard definition television continues to use the frame rate and number of lines of resolution established by the analog NTSC standard.
History
The National Television System Committee was established in 1940 by the United States Federal Communications Commission (FCC) to resolve the conflicts that were made between companies over the introduction of a nationwide analog television system in the United States. In March 1941, the committee issued a technical standard for black-and-white television that built upon a 1936 recommendation made by the Radio Manufacturers Association (RMA). Technical advancements of the vestigial side band technique allowed for the opportunity to increase the image resolution. The NTSC selected 525 scan lines as a compromise between RCA's 441-scan line standard and Philco's and DuMont's desire to increase the number of scan lines to between 605 and 800. The standard recommended a frame rate of 30 frames (images) per second, consisting of two interlaced fields per frame at 262.5 lines per field and 60 fields per second. Other standards in the final recommendation were an aspect ratio of 4:3, and frequency modulation (FM) for the sound signal (which was quite new at the time).
In January 1950, the committee was reconstituted to standardize color television. The FCC had briefly approved a color television standard in October 1950 which was developed by CBS. The CBS system is incompatible with existing black-and-white receivers. It uses a rotating color wheel, reduced the number of scan lines from 525 to 405, and increases the field rate from 60 to 144, but has an effective frame rate of only 24 frames per second. Legal action by rival RCA kept commercial use of the system off the air until June 1951, and regular broadcasts only lasted a few months before manufacture of all color television sets was banned by the Office of Defense Mobilization in October, ostensibly due to the Korean War. CBS rescinded its system in March 1953, and the FCC replaced it on December 17, 1953, with the NTSC color standard, which was cooperatively developed by several companies, including RCA and Philco.
In December 1953 the FCC unanimously approved what is now called the NTSC color television standard (later defined as RS-170a). The compatible color standard retains full backward compatibility with existing black-and-white television sets. Color information was added to the black-and-white image by introducing a color subcarrier of precisely 315/88 MHz (usually described as 3.579545 MHz or 3.58 MHz). The precise frequency was chosen so that horizontal line-rate modulation components of the chrominance signal fall exactly in between the horizontal line-rate modulation components of the luminance signal, thereby enabling the chrominance signal to be filtered out of the luminance signal with minor degradation of the luminance signal. Due to limitations of frequency divider circuits at the time the color standard was promulgated, the color subcarrier frequency was constructed as composite frequency assembled from small integers, in this case 5×7×9/(8×11) MHz. The horizontal line rate was reduced to approximately 15,734 lines per second (3.579545×2/455 MHz = 9/572 MHz) from 15,750 lines per second, and the frame rate was reduced to 30/1.001 ≈ 29.970 frames per second (the horizontal line rate divided by 525 lines/frame) from 30 frames per second. These changes amount to 0.1 percent and were readily tolerated by existing television receivers.
The first publicly announced network television broadcast of a program using the NTSC "compatible color" system was an episode of NBC's Kukla, Fran and Ollie on August 30, 1953, although it was viewable in color only at the network's headquarters. The first nationwide viewing of NTSC color came on the following January 1 with the coast-to-coast broadcast of the Tournament of Roses Parade, viewable on prototype color receivers at special presentations across the country. The first color NTSC television camera was the RCA TK-40, used for experimental broadcasts in 1953; an improved version, the TK-40A, introduced in March 1954, was the first commercially available color television camera. Later that year, the improved TK-41 became the standard camera used throughout much of the 1960s.
The NTSC standard has been adopted by other countries, including most of the Americas and Japan.
With the advent of digital television, analog broadcasts are being phased out. Most US NTSC broadcasters were required by the FCC to shut down their analog transmitters in 2009. Low-power stations, Class A stations and translators were required to shut down by 2015.
Technical details
Lines and refresh rate
NTSC color encoding is used with the System M television signal, which consists of 30⁄1.001 (approximately 29.97) interlaced frames of video per second. Each frame is composed of two fields, each consisting of 262.5 scan lines, for a total of 525 scan lines. 483 scan lines make up the visible raster. The remainder (the vertical blanking interval) allow for vertical synchronization and retrace. This blanking interval was originally designed to simply blank the receiver's CRT to allow for the simple analog circuits and slow vertical retrace of early TV receivers. However, some of these lines may now contain other data such as closed captioning and vertical interval timecode (VITC). In the complete raster (disregarding half lines due to interlacing) the even-numbered scan lines (every other line that would be even if counted in the video signal, e.g. {2, 4, 6, ..., 524}) are drawn in the first field, and the odd-numbered (every other line that would be odd if counted in the video signal, e.g. {1, 3, 5, ..., 525}) are drawn in the second field, to yield a flicker-free image at the field refresh frequency of 60⁄1.001 Hz (approximately 59.94 Hz). For comparison, 576i systems such as PAL-B/G and SECAM use 625 lines (576 visible), and so have a higher vertical resolution, but a lower temporal resolution of 25 frames or 50 fields per second.
The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped kinescope cameras record early live television broadcasts, as it was very simple to synchronize a film camera to capture one frame of video on each film frame by using the alternating current frequency to set the speed of the synchronous AC motor-drive camera. When color was added to the system, the refresh frequency was shifted slightly downward by 0.1% to approximately 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers, as explained below in "Color encoding". By the time the frame rate changed to accommodate color, it was nearly as easy to trigger the camera shutter from the video signal itself.
The actual figure of 525 lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator. For interlaced scanning, an odd number of lines per frame was required in order to make the vertical retrace distance identical for the odd and even fields, which meant the master oscillator frequency had to be divided down by an odd number. At the time, the only practical method of frequency division was the use of a chain of vacuum tube multivibrators, the overall division ratio being the mathematical product of the division ratios of the chain. Since all the factors of an odd number also have to be odd numbers, it follows that all the dividers in the chain also had to divide by odd numbers, and these had to be relatively small due to the problems of thermal drift with vacuum tube devices. The closest practical sequence to 500 that meets these criteria was 3×5×5×7=525. (For the same reason, 625-line PAL-B/G and SECAM uses 5×5×5×5, the old British 405-line system used 3×3×3×3×5, the French 819-line system used 3×3×7×13 etc.)
Colorimetry
The original 1953 color NTSC specification, still part of the United States Code of Federal Regulations, defined the colorimetric values of the system as follows:
| Original NTSC colorimetry (1953) | CIE 1931 x | CIE 1931 y |
|---|---|---|
| primary red | 0.67 | 0.33 |
| primary green | 0.21 | 0.71 |
| primary blue | 0.14 | 0.08 |
| white point (CIE Standard illuminant C) 6774 K | 0.310 | 0.316 |
SMPTE C
To ensure more uniform color reproduction, receivers started to incorporate color correction circuits that converted the received signal — encoded for the colorimetric values listed above — into signals encoded for the phosphors actually used within the monitor. Since such color correction can not be performed accurately on the nonlinear gamma corrected signals transmitted, the adjustment can only be approximated, introducing both hue and luminance errors for highly saturated colors.
Similarly at the broadcaster stage, in 1968-69 the Conrac Corp., working with RCA, defined a set of controlled phosphors for use in broadcast color picture video monitors. This specification survives today as the SMPTE "C" phosphor specification:
| SMPTE "C" colorimetry | CIE 1931 x | CIE 1931 y |
|---|---|---|
| primary red | 0.630 | 0.340 |
| primary green | 0.310 | 0.595 |
| primary blue | 0.155 | 0.070 |
| white point (CIE illuminant D65) | 0.3127 | 0.3290 |
In 1987, the Society of Motion Picture and Television Engineers (SMPTE) Committee on Television Technology, Working Group on Studio Monitor Colorimetry, adopted the SMPTE C (Conrac) phosphors for general use in Recommended Practice 145, prompting many manufacturers to modify their camera designs to directly encode for SMPTE "C" colorimetry without color correction, as approved in SMPTE standard 170M, "Composite Analog Video Signal — NTSC for Studio Applications" (1994). As a consequence, the ATSC digital television standard states that for 480i signals, SMPTE "C" colorimetry should be assumed unless colorimetric data is included in the transport stream.
Japanese NTSC never changed primaries and whitepoint to SMPTE "C", continuing to use the 1953 NTSC primaries and whitepoint. Both the PAL and SECAM systems used the original 1953 NTSC colorimetry as well until 1970; unlike NTSC, however, the European Broadcasting Union (EBU) rejected color correction in receivers and studio monitors that year and instead explicitly called for all equipment to directly encode signals for the "EBU" colorimetric values, further improving the color fidelity of those systems.
Color encoding
For backward compatibility with black-and-white television, NTSC uses a luminance-chrominance encoding system invented in 1938 by Georges Valensi. The three color picture signals are divided into Luminance (derived mathematically from the three separate color signals (Red, Green and Blue)) which takes the place of the original monochrome signal and Chrominance which carries only the color information. This process is applied to each color source by its own Colorplexer, thereby allowing a compatible color source to be managed as if it was an ordinary monochrome source. This allows black-and-white receivers to display NTSC color signals by simply ignoring the chrominance signal. Some black-and-white TVs sold in the US after the introduction of color broadcasting in 1953 were designed to filter chroma out, but the early B&W sets did not do this and chrominance could be seen as a 'dot pattern' in highly colored areas of the picture.
In NTSC, chrominance is encoded using two color signals known as I (in-phase) and Q (in quadrature) in a process called QAM. The two signals each amplitude modulate 3.58 MHz carriers which are 90 degrees out of phase with each other and the result added together but with the carriers themselves being suppressed. The result can be viewed as a single sine wave with varying phase relative to a reference carrier and with varying amplitude. The varying phase represents the instantaneous color hue captured by a TV camera, and the amplitude represents the instantaneous color saturation. This 3.58 MHz subcarrier is then added to the Luminance to form the 'composite color signal' which modulates the video signal carrier just as in monochrome transmission.
For a color TV to recover hue information from the color subcarrier, it must have a zero phase reference to replace the previously suppressed carrier. The NTSC signal includes a short sample of this reference signal, known as the colorburst, located on the 'back porch' of each horizontal synchronization pulse. The color burst consists of a minimum of eight cycles of the unmodulated (fixed phase and amplitude) color subcarrier. The TV receiver has a "local oscillator", which is synchronized with these color bursts. Combining this reference phase signal derived from the color burst with the chrominance signal's amplitude and phase allows the recovery of the 'I' and 'Q' signals which when combined with the Luminance information allows the reconstruction of a color image on the screen. Color TV has been said to really be colored TV because of the total separation of the brightness part of the picture from the color portion. In CRT televisions, the NTSC signal is turned into three color signals called Red, Green and Blue, each controlling that color electron gun. TV sets with digital circuitry use sampling techniques to process the signals but the end result is the same. For both analog and digital sets processing an analog NTSC signal, the original three color signals (Red, Green and Blue) are transmitted using three discrete signals (Luminance, I and Q) and then recovered as three separate colors and combined as a color image.
When a transmitter broadcasts an NTSC signal, it amplitude-modulates a radio-frequency carrier with the NTSC signal just described, while it frequency-modulates a carrier 4.5 MHz higher with the audio signal. If non-linear distortion happens to the broadcast signal, the 3.579545 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. To make the resulting pattern less noticeable, designers adjusted the original 15,750 Hz scanline rate down by a factor of 1.001 (0.1%) to match the audio carrier frequency divided by the factor 286, resulting in a field rate of approximately 59.94 Hz. This adjustment ensures that the sums and differences of the sound carrier and the color subcarrier and their multiples (i.e., the intermodulation products of the two carriers) are not exact multiples of the frame rate, which is the necessary condition for the dots to remain stationary on the screen, making them most noticeable.
The 59.94 rate is derived from the following calculations. Designers chose to make the chrominance subcarrier frequency an n + 0.5 multiple of the line frequency to minimize interference between the luminance signal and the chrominance signal. (Another way this is often stated is that the color subcarrier frequency is an odd multiple of half the line frequency.) They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize visible (intermodulation) interference between the audio signal and the chrominance signal. The original black-and-white standard, with its 15,750 Hz line frequency and 4.5 MHz audio subcarrier, does not meet these requirements, so designers had either to raise the audio subcarrier frequency or lower the line frequency. Raising the audio subcarrier frequency would prevent existing (black and white) receivers from properly tuning in the audio signal. Lowering the line frequency is comparatively innocuous, because the horizontal and vertical synchronization information in the NTSC signal allows a receiver to tolerate a substantial amount of variation in the line frequency. So the engineers chose the line frequency to be changed for the color standard. In the black-and-white standard, the ratio of audio subcarrier frequency to line frequency is 4.5 MHz⁄15,750 Hz = 285.71. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is 4.5 MHz⁄286 ≈ 15,734 Hz. Maintaining the same number of scan lines per field (and frame), the lower line rate must yield a lower field rate. Dividing 4500000⁄286 lines per second by 262.5 lines per field gives approximately 59.94 fields per second.
Transmission modulation method
An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz. The actual video signal, which is amplitude-modulated, is transmitted between 500 kHz and 5.45 MHz above the lower bound of the channel. The video carrier is 1.25 MHz above the lower bound of the channel. Like most AM signals, the video carrier generates two sidebands, one above the carrier and one below. The sidebands are each 4.2 MHz wide. The entire upper sideband is transmitted, but only 1.25 MHz of the lower sideband, known as a vestigial sideband, is transmitted. The color subcarrier, as noted above, is 3.579545 MHz above the video carrier, and is quadrature-amplitude-modulated with a suppressed carrier. The audio signal is frequency-modulated, like the audio signals broadcast by FM radio stations in the 88–108 MHz band, but with a 25 kHz maximum frequency deviation, as opposed to 75 kHz as is used on the FM band, making analog television audio signals sound quieter than FM radio signals as received on a wideband receiver. The main audio carrier is 4.5 MHz above the video carrier, making it 250 kHz below the top of the channel. Sometimes a channel may contain an MTS signal, which offers more than one audio signal by adding one or two subcarriers on the audio signal, each synchronized to a multiple of the line frequency. This is normally the case when stereo audio and/or second audio program signals are used. The same extensions are used in ATSC, where the ATSC digital carrier is broadcast at 0.31 MHz above the lower bound of the channel.
"Setup" is a 54 mV(7.5 IRE) voltage offset between the "black" and "blanking" levels. It is unique to NTSC. CVBS stands for Color, Video, Blanking, and Sync.
Frame rate conversion
There is a large difference in frame rate between film, which runs at 24.0 frames per second, and the NTSC standard, which runs at approximately 29.97 (10 MHz×63/88/455/525) frames per second. In regions that use 25-fps television and video standards, this difference can be overcome by speed-up.
For 30-fps standards, a process called "3:2 pulldown" is used. One film frame is transmitted for three video fields (lasting 1½ video frames), and the next frame is transmitted for two video fields (lasting 1 video frame). Two film frames are thus transmitted in five video fields, for an average of 2½ video fields per film frame. The average frame rate is thus 60 ÷ 2.5 = 24 frames per second, so the average film speed is nominally exactly what it should be. (In reality, over the course of an hour of real time, 215,827.2 video fields are displayed, representing 86,330.88 frames of film, while in an hour of true 24-fps film projection, exactly 86,400 frames are shown: thus, 29.97-fps NTSC transmission of 24-fps film runs at 99.92% of the film's normal speed.) Still-framing on playback can display a video frame with fields from two different film frames, so any difference between the frames will appear as a rapid back-and-forth flicker. There can also be noticeable jitter/"stutter" during slow camera pans (telecine judder).
To avoid 3:2 pulldown, film shot specifically for NTSC television is often taken at 30 frame/s.
To show 25-fps material (such as European television series and some European movies) on NTSC equipment, every fifth frame is duplicated and then the resulting stream is interlaced.
Film shot for NTSC television at 24 frames per second has traditionally been accelerated by 1/24 (to about 104.17% of normal speed) for transmission in regions that use 25-fps television standards. This increase in picture speed has traditionally been accompanied by a similar increase in the pitch and tempo of the audio. More recently, frame-blending has been used to convert 24 FPS video to 25 FPS without altering its speed.
Film shot for television in regions that use 25-fps television standards can be handled in either of two ways:
The film can be shot at 24 frames per second. In this case, when transmitted in its native region, the film may be accelerated to 25 fps according to the analog technique described above, or kept at 24 fps by the digital technique described above. When the same film is transmitted in regions that use a nominal 30-fps television standard, there is no noticeable change in speed, tempo, and pitch.
The film can be shot at 25 frames per second. In this case, when transmitted in its native region, the film is shown at its normal speed, with no alteration of the accompanying soundtrack. When the same film is shown in regions that use a 30-fps nominal television standard, every fifth frame is duplicated, and there is still no noticeable change in speed, tempo, and pitch.
Because both film speeds have been used in 25-fps regions, viewers can face confusion about the true speed of video and audio, and the pitch of voices, sound effects, and musical performances, in television films from those regions. For example, they may wonder whether the Jeremy Brett series of Sherlock Holmes television films, made in the 1980s and early 1990s, was shot at 24 fps and then transmitted at an artificially fast speed in 25-fps regions, or whether it was shot at 25 fps natively and then slowed to 24 fps for NTSC exhibition.
These discrepancies exist not only in television broadcasts over the air and through cable, but also in the home-video market, on both tape and disc, including laser disc and DVD.
In digital television and video, which are replacing their analog predecessors, single standards that can accommodate a wider range of frame rates still show the limits of analog regional standards. The ATSC standard, for example, allows frame rates of 23.976, 24, 29.97, 30, 59.94, and 60 frames per second, but not 25 and 50.
Modulation for analog satellite transmission
Because satellite power is severely limited, analog video transmission through satellites differs from terrestrial TV transmission. AM is a linear modulation method, so a given demodulated signal-to-noise ratio (SNR) requires an equally high received RF SNR. The SNR of studio quality video is over 50 dB, so AM would require prohibitively high powers and/or large antennas.
Wideband FM is used instead to trade RF bandwidth for reduced power. Increasing the channel bandwidth from 6 to 36 MHz allows a RF SNR of only 10 dB or less. The wider noise bandwidth reduces this 40 dB power saving by 36 MHz / 6 MHz = 8 dB for a substantial net reduction of 32 dB.
Sound is on a FM subcarrier as in terrestrial transmission, but frequencies above 4.5 MHz are used to reduce aural/visual interference. 6.8, 5.8 and 6.2 MHz are commonly used. Stereo can be multiplex, discrete, or matrix and unrelated audio and data signals may be placed on additional subcarriers.
A triangular 60 Hz energy dispersal waveform is added to the composite baseband signal (video plus audio and data subcarriers) before modulation. This limits the satellite downlink power spectral density in case the video signal is lost. Otherwise the satellite might transmit all of its power on a single frequency, interfering with terrestrial microwave links in the same frequency band.
In half transponder mode, the frequency deviation of the composite baseband signal is reduced to 18 MHz to allow another signal in the other half of the 36 MHz transponder. This reduces the FM benefit somewhat, and the recovered SNRs are further reduced because the combined signal power must be "backed off" to avoid intermodulation distortion in the satellite transponder. A single FM signal is constant amplitude, so it can saturate a transponder without distortion.
Field order
An NTSC "frame" consists of an "even" field followed by an "odd" field. As far as the reception of an analog signal is concerned, this is purely a matter of convention and, it makes no difference. It's rather like the broken lines running down the middle of a road, it doesn't matter whether it is a line/space pair or a space/line pair; the effect to a driver is exactly the same.
The introduction of digital television formats has changed things somewhat. Most digital TV formats store and transmit fields in pairs as a single digital frame. Digital formats that match NTSC field rate, including the popular DVD format, record video with the even field first in the digital frame, while the formats that match field rate of the 625 line system often record video with odd frame first. This means that when reproducing many non-NTSC based digital formats it is necessary to reverse the field order, otherwise an unacceptable shuddering "comb" effect occurs on moving objects as they are shown ahead in one field and then jump back in the next.
This has also become a hazard where non NTSC progressive video is transcoded to interlaced and vice versa. Systems that recover progressive frames or transcode video should ensure that the "Field Order" is obeyed, otherwise the recovered frame will consist of a field from one frame and a field from an adjacent frame, resulting in "comb" interlacing artifacts. This can often be observed in PC based video playing utilities if an inappropriate choice of de-interlacing algorithm is made.
During the decades of high-power NTSC broadcasts in the United States, switching between the views from two cameras was accomplished according to two standards, the choice between the two being made by geography, East versus West. In one region, the switch was made between the odd field that finished one frame and the even field that began the next frame; in the other, the switch was made after an even field and before an odd field. Thus, for example, a home VHS recording made of a local television newscast in the East, when paused, would only ever show the view from one camera (unless a dissolve or other multicamera shot were intended), whereas VHS playback of a situation comedy taped and edited in Los Angeles and then transmitted nationwide could be paused at the moment of a switch between cameras with half the lines depicting the outgoing shot and the other half depicting the incoming shot.
Source From Wikipedia
没有评论:
发表评论