National Television Systems Committee
The National Television Systems Committee [ ˈnæʃənəl ˈtɛləvɪʒən ˈsɪstəmz kəˈmɪti ] ( NTSC, RS-170A ) is a US institution that established the first color transmission system for analog television signals, used in much of the Americas and some countries in East Asia. The abbreviation NTSC established itself as the name for this television system. NTSC was increasingly replaced in North America by digital television through the procedures of the Advanced Television Systems Committee (ATSC) in the 2000s . Since 2009, NTSC has been replaced by ATSC for terrestrial television broadcasts.
history
The National Television Systems Committee was founded in 1940 by Walter Ransom Gail Baker as part of the Radio Manufacturers Association (RMA) and in cooperation with the Federal Communications Commission (FCC), to resolve the conflict that has flared up between the companies over a national television standard to be introduced. In March 1941 the committee published a technical standard for black and white television. This was based on a proposal by the Radio Manufacturers Association (RMA) from 1936, which proposed a 441 line system. With the further development of the vestigial sideband for television transmission, which reduced the required bandwidth (so that more transmitters can be accommodated per frequency band), there was a possibility of improving the image resolution. The committee made a compromise between the desire of the Radio Corporation of America (RCA), which already used a 441-line standard with the NBC network, to keep the old standard and Philco's interest in a resolution of 600 to 800 lines by setting a 525-line standard.
In January 1950 the committee was convened again to make a decision on color television. In March 1953 the members unanimously approved the standard known today as the NTSC. However, this new standard had full backwards compatibility with the old black and white standard.
The FCC agreed to a different color television standard on short notice in early 1950. It was developed by Columbia Broadcasting Inc. (CBS) and was incompatible with the old black and white standard. It used changing color mixes, reduced the number of lines from 525 to 405 and increased the rate of the individual fields from normally 60 (in 2 fields at an effective 30 fps ) to 144 (which, however, with 6 different fields resulted in only 24 frames ). The RCA's delaying tactics prevented the use of this system until mid-1951 and the actual broadcast with this system only ran for a few months until the Office of Defense Mobilization (ODM) on the occasion of the Korean War Production of color TVs was banned. The standard was officially abolished in 1953 by the FCC and was replaced in the same year by the colored NTSC standard, which had been developed with the involvement of many companies, including RCA and Philco. Most of the receivers manufactured up to then were then scrapped, today only two devices exist. A variant of the CBS system was later used by NASA to transmit the astronauts' images from space.
A third line-changing system from Color Television Incorporated (CTI) was also considered. Accordingly, the CBS system was later called field changing and the NTSC format was called point changing.
The first commercially available NTSC color camera was the RCA TK-40A in March 1954, which was later replaced by the improved version TK-41. This became the most widely used television camera in the 1960s.
In spite of all efforts, the NTSC standard has only been adopted by a few industrialized countries besides the USA, especially Canada and Japan (see world map). The SECAM and PAL standards were introduced in Europe .
Technical details
The NTSC signal is used (unlike PAL ) in all countries that use it today or have used it until the respective analog switch-off , with identical transmission parameters: 525 lines (of which a maximum of 486 visible), interlace , 29.97 frames per second (originally 30 for black and white), color carrier approx. 3.58 MHz , sound carrier 4.5 MHz, channel spacing 6 MHz, negative amplitude modulation for the picture, frequency modulation for the sound, stereo sound (if available) encoded with the analog MTS process. All of these properties taken together are called NTSC-M. Only Japan uses a slightly different standard with a slightly different black level; By adjusting the brightness control on the receiver, this difference can be compensated for with all NTSC receivers.
In the digital sector, for example on DVDs , NTSC only refers to the image format of 720 × 480 pixels at 29.97 full frames per second. The other properties listed above are no longer relevant for digital signals. The aspect ratio is 4: 3, the resolution is 3: 2.
Refresh rate
The NTSC standard, or more correctly the M standard , consists of 29.97 frames per second. Each image consists of a maximum of 486 visible lines (of which only 480 are often used today; due to overscan , the viewer can usually actually see less than 480 lines on his screen anyway). The remaining 525 lines are used for synchronization, reconstruction of the image and other data such as subtitles. The NTSC system composes a picture ("frame") from two fields ("fields") by alternately transmitting only the odd and only the even lines of the picture. This creates an almost flicker-free image that vibrates at 59.94 Hz (60 Hz in the original black and white system). The 625-line formats PAL and SECAM , both of which only work at 50 Hz, are somewhat more prone to image flickering. Putting the images together makes video editing a bit more complicated, but this applies to all widely used analog video formats, including PAL and SECAM.
The NTSC frame rate initially ran at exactly 60 Hz, based on the AC power system customary in the USA. It was better to adapt the frame rate to the frequency of the energy source, otherwise bars would appear in the vicinity of strong power sources or in the light of fluorescent tubes would have been visible on the screen. Matching the repetition rate to the power supply was also helpful for the early live broadcasts: it made it very easy to get the camera to save an image using the AC voltage as the shutter release. In the color system, the repetition rate was then lowered slightly to 59.94 Hz (more precisely: 60,000 fields per 1001 s), as this reduces certain interference between the color carrier and the sound carrier (image frequency = 59.94005994 ... Hz, line frequency = 262.5 × frame frequency = 15,734.265734 ... Hz, color carrier frequency = 227.5 × line frequency = 3,579,545.454 ... Hz, sound carrier frequency = 286 × line frequency = 4,500,000,000 Hz: color carrier and sound carrier differ by a half-integer multiple of the line frequency).
The different frame rates between NTSC and the other two image formats PAL and SECAM pose a problem when converting video. The conversion is often done by omitting or repeating (half) images, which leads to jerking when the camera is moved. High quality and expensive converters rely on object tracking and interpolation of images from neighboring images. This technology was first used on a larger scale at the soccer World Cup in 1994 - at that time it was quite complex. Although this process is now state-of-the-art (every 100 Hz TFT television can use the process in a slightly simplified form), it is still common today to use the old jerky conversion method.
Film scanning
Since the 1920s, movies have been shot worldwide at 24 full frames per second, which corresponds to neither the PAL nor the NTSC standard. This poses a problem when such films are to be shown on television. While you can play the film 4% faster with PAL (25 frames per second) , which is not very noticeable, with NTSC (approx. 30 frames per second) you would have to play the film 25% faster, which would be very noticeable . Instead, 4 images are converted into 5 images each using the 3: 2 pull-down . Every second film frame is contained in three TV fields. This leads to tearing contours and strong jerking even when viewing on tube TV sets, for example when panning a film camera. In addition, various technical difficulties arise if it has to be reversed using inverse telecine (for recording or playback on a non-standard tube television such as LCD, plasma, 100 Hz tube, etc.).
Color coding
For downward compatibility with black and white television, the NTSC format uses a brightness and a color system that are transmitted separately. This composition was invented by Georges Valensi in 1938 . The brightness system is basically the old black and white system, whereas the color system contains the actual color information: It uses two color difference signals according to the YUV model for color transmission . This allows the old black and white receivers to display color broadcasts by simply ignoring the color information.
For transmission in the FBAS video signal , the NTSC format modulates the color information onto a carrier wave, also called color carrier or color subcarrier, with a frequency of 3.579545 MHz.
The type of modulation is a QAM residual sideband modulation (see SSB , PAL ), with which both color difference signals can be transmitted simultaneously.
The carrier wave itself is suppressed during transmission for reasons of efficiency and later reconstructed in the receiver during (QAM) demodulation . A short reference signal is used for this, a “package” of sinusoidal oscillations of the same frequency and phase , which is called a color burst or burst for short . It is located at the beginning of each image line in the actually unused space between the horizontal (horizontal) synchronization pulse and the start of the visible image content (i.e. in the back porch ). The color burst itself consists of eight to ten clocks of the unmodulated carrier wave with 180 ° phase position as reference. To reconstruct the color carrier, a ( quartz ) oscillator, which can be tuned within narrow limits, is synchronized with a phase-locked loop ( PLL ) to the color burst .
In order to obtain the two color signals U and V in the receiver, the QAM color signal is now demodulated with the reconstructed color (auxiliary) carrier.
On the radio transmission channel and in the receiver, phase shifts between the burst or color carrier and the QAM signal can easily occur. Because, in contrast to the alternative systems PAL and SECAM, NTSC did not originally have dynamic corrective measures against it, characteristic color falsifications such as green or violet falsified skin tones used to occur. See section “ Color Defects ” below .
Encryption systems
Analogue and digital NTSC have different access barriers ("access methods") to the system that can be used for Pay Per View (PPV) and other customer-related services.
Commonly used in North America Motorola (GI) systems:
- Videocipher I (analog, no longer used)
- Videocipher II (still in use)
- Digicipher II (digital, widest distribution)
No teletext (teletext)
Teletext (also known as videotext in Germany) has never been able to establish itself in the NTSC sector and is hardly broadcast any more today; and very few receivers have such decoders. Only a system called "closed captioning" for displaying subtitles for the deaf and hearing impaired is in use, which is similar to the teletext subtitles customary in the PAL sector. However, this can only be switched on or off, page numbers do not exist.
Color defects
If one imagines the modulation in the vector diagram , the color tone is in the phase of the respective pointer and the color saturation in the length of the pointer. Phase shifts that can occur in the entire transmission chain (especially with conventional terrestrial reception, less via cable or satellite) result in color errors. In principle, this cannot be corrected automatically. Televisions with NTSC color parts therefore traditionally have an additional setting option compared to PAL devices, the hue control ("hue control" or "tint control").
Most newer devices now have automatic hue control circuitry. The main aim is to achieve natural skin and face colors, since errors in this area are most noticeable. In order to achieve this, some reference values (black, 50% gray and “skin-colored”) are transferred to line 19 using the vertical interval reference method (VIR). Since these reference values are sent as normal picture content (but still invisible in the upper edge of the picture), compatibility with old receivers is guaranteed.
Due to the often noticeable color errors in the NTSC process, there is a joke interpretation of the abbreviation NTSC, which is known either as "Never The Same Color" or "Never twice the same color" is, which mockingly refers to the problem that even with the slightest transmission errors - especially in the color range of facial colors - visible to annoying discoloration occurs.
Countries and territories using NTSC
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North America: |
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Central America: |
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South America: |
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Asia: |
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Pacific: |
Remarks
- ↑ Today the YUV model is used, red minus lightness and blue minus lightness, in earlier times the YIQ color space, which is more efficient on channels with bandwidth restrictions ; I and Q are not to be confused with the two vector components of the QA-modulated color signal, which are also referred to as I and Q.
- ↑ Exactly 315/88 MHz, the reason for the number is a frequency interlocking with the brightness system in order to avoid moiré image disturbances due to interference and intermodulation . That was also the reason for the easy adjustment of the refresh rate. See PAL ; With SECAM , frequency interlocking is not possible due to the FM used.
- ↑ PAL -M system, NTSC-M standard plus PAL phase modulation for color values
Individual evidence
- ↑ ATSC Salutes The 'Passing' Of NTSC - Press Release. (No longer available online.) Archived from the original on May 24, 2010 ; Retrieved April 11, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
