NTSC
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NTSC is the analog television system in use in Canada, Japan, South Korea, the Philippines, the United States, and some other countries, mostly in the Americas (see map). It is named for the National Television Standards Committee, the U.S. standardization body that adopted it.
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[edit] History
The National Television Standards Committee was established in 1940 by the Federal Communications Commission (FCC), in the United States (US), to resolve the conflicts which had arisen between companies over the introduction of a nationwide analog television system in the U.S. The committee in March 1941 issued a technical standard for black and white television. This built upon a 1936 recommendation made by the Radio Manufacturers Association (RMA) that used 441 lines. With the advancement of the vestigial sideband technique for broadcasting that increased available bandwidth, there was an opportunity to increase the image resolution. The NTSC compromised between RCA's desire to keep a 441-line standard (their NBC TV network was already using it) and Philco's desire to increase it to between 605 and 800, settling on a 525-line transmission. Other technical standards in the final recommendation were a frame rate (image rate) of 30 frames per second consisting of 2 interlaced fields per frame (2:1 interlacing) at 262½ lines per field or 60 fields per second along with an aspect ratio of 4:3, and frequency modulation for the sound signal.
In January 1950 the Committee was reconstituted, this time to decide about color television. In March 1953 it unanimously approved what is now called simply the NTSC color television standard, later defined as RS-170a. The updated standard retained full backwards compatibility ('compatible color') with older black and white television sets. Color information was added to the black and white image by adding a color subcarrier of 39375000/11 Hz (approximately 3.58 MHz) to the video signal. For technical reasons the addition of the color subcarrier also required a slight reduction of the frame rate from 30 frames per second to 30/1.001 (very close to 29.97) frames per second, or 227.5 subcarrier cycles per scanline.
The FCC had briefly approved a different color television system, starting in 1950. It was developed by CBS and was incompatible with black and white broadcasts. It used a rotating color wheel, reduced the number of scanlines from 525 to 405, and increased the field rate from 60 to 144 (but had an effective frame rate of 24 frames a second). Delay tactics by rival RCA kept the system off the air until mid-1951, and regular broadcasts only lasted a few months before manufacture of CBS-compatible systems was banned by the National Production Authority (NPA). Most of the existing devices were soon destroyed and only two receivers are known to exist today. The CBS system was rescinded by the FCC in 1953 and was replaced later that year by the NTSC color standard, which had been developed with the cooperation of several companies including RCA and Philco. A variant of the CBS system was later used by NASA to broadcast pictures of astronauts from space.
A third "line sequential" system from Color Television Inc. (CTI) was also considered. The CBS and final NTSC systems were called "field sequential" and "dot sequential" systems, respectively.
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 TV camera. This was replaced later that year by an improved version, the TK-41, which 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, with NTSC broadcasts scheduled to end in the United States in early 2009.
[edit] Technical details
[edit] Lines and refresh rate
The NTSC format is used with the M format (see broadcast television systems), which consists of 29.97 interlaced frames of video per second. Each frame consists of 484 lines out of a total of 525 (the rest are used for sync, vertical retrace, and other data such as captioning). PAL uses 625 lines, and so has a better picture quality. The NTSC system interlaces its scanlines, drawing odd-numbered scanlines in odd-numbered fields and even-numbered scanlines in even-numbered fields, yielding a nearly flicker-free image at its approximately 59.94 hertz (nominally 60 Hz/100.1%) refresh frequency. The refresh compares favorably to the 50 Hz refresh rate of the PAL and SECAM video formats used in Europe, where 50 Hz alternating current is the standard; flicker was more likely to be noticed when using these standards until modern PAL TV sets began using 100 Hz refresh rate to eliminate flicker. This produces a far more stable picture than native NTSC and PAL had, effectively displaying each frame twice. This did, at first, cause some motion problems, so it was not universally adopted until a few years ago. Interlacing the picture does complicate editing video, but this is true of all interlaced video formats, including PAL and SECAM.
The NTSC refresh frequency was originally exactly 60 Hz in the black and white system, chosen because it matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the screen refresh rate to the power source avoided wave interference that produces rolling bars on the screen. Synchronization of the refresh rate to the power cycle also 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 as a shutter trigger.
The 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.
The only practical method of frequency division available at the time was the use of multivibrators, which could only divide by small numbers. For interlaced scanning an odd number of lines per frame was required, and so a chain of multivibrators was needed, each of which had to divide by a small, odd number. (Note that an odd number is never divisible by any even number). The closest practical sequence to 500 was 3 × 5 × 5 × 7 = 525. Similarly, the British 405-line system used 3 × 9 × 9 × 5. Although other values were theoretically possible, all of them involved division by unacceptably large numbers like 13 or 17, which produced reliability problems. Modern systems derive all their frequencies from the color subcarrier frequency (see below).
In the color system the refresh frequency was shifted slightly downward to 59.94 Hz to eliminate stationary dot patterns in the color carrier, as explained below in "Color encoding".
There are a lot of possible timings behind an NTSC signal (much more than behind a PAL signal). An NTSC signal can be actually a 60i signal, it can be a 30p signal after a 2:2 pullup, it can be a 24p signal after a 3:2 pullup, a bobbed PAL signal after a 3:2 pullup, to mention some legal examples. A lot of versions due to mastering errors of DVDs follow. For further information see http://www.hometheaterhifi.com/volume_7_4/dvd-benchmark-part-5-progressive-10-2000.html .
===Color encoding===There are 3 main standards in use around the world.
P A L N T S C S E C A M
Each one is incompatible with the other.
For example, a recording made in the France could not be played on an American VCR or DVD player.
If you or your clients view video tapes or DVD's that are from outside of the United States, or send videos abroad, you must first convert the tape or disc to or from the foreign television standard.
The system used in America & Canada is called "NTSC". Western Europe and Australia use a system called "PAL", and Eastern Europe and France use "SECAM". Without standards conversion, it is impossible to view a video program that is recorded in a foreign country without first converting it..
Here are some charts illustrating the differences
N T S C National Television System Committee Lines/Field 525/60 Horizontal Frequency 15.734 kHz Vertical Frequency 60 Hz Color Subcarrier Frequency 3.579545 MHz Video Bandwidth 4.2 MHz Sound Carrier 4.5 MHz
P A L Phase Alternating Line SYSTEM PAL PAL N PAL M Line/Field 625/50 625/50 525/60 Horizontal Freq. 15.625 kHz 15.625 kHz 15.750 kHz Vertical Freq. 50 Hz 50 Hz 60 Hz Color Sub Carrier 4.433618 MHz 3.582056 MHz 3.575611 MHz Video Bandwidth 5.0 MHz 4.2 MHz 4.2 MHz Sound Carrier 5.5 MHz 4.5 MHz 4.5 MHz
SECAM Sequential Couleur Avec Memoire or Sequential Color with Memory SYSTEM SECAM B,G,H SECAM D,K,K1,L Line/Field 625/50 625/50 Horizontal Frequency 15.625 kHz 15.625 kHz Vertical Frequency 50 Hz 50 Hz Video Bandwidth 5.0 MHz 6.0 MHz Sound Carrier 5.5 MHz 6.5 MHz
For backward compatibility with black and white television, NTSC uses a luminance-chrominance encoding system invented in 1938 by Georges Valensi. Luminance (derived mathematically from the composite color signal) takes the place of the original monochrome signal. Chrominance carries color information. This allows black and white receivers to display NTSC signals simply by ignoring the chrominance. In NTSC, chrominance is encoded using two 3.579545 MHz signals that are 90 degrees out of phase, known as I (in-phase) and Q (quadrature) QAM. Mathematically, the combination of two sine waves 90 degrees out of phase with each other, with varying respective amplitudes, can be viewed as a single sine wave with varying phase relative to a reference, and varying amplitude. In essence, the phase represents the instantaneous color hue captured by a TV camera and the amplitude represents the color saturation.
For a TV or a display to recover hue information from the I/Q phase as just described, it must know the reference for it (i.e. what phase is zero). It also needs a reference against which to compare the amplitude to make saturation sense out of it. So the NTSC signal includes a short sample of this reference signal, known as the color burst, located on the 'back porch' of each horizontal line (the time between the end of the horizontal synchronization pulse and of the blanking pulse on each line). The color burst consists of a minimum of eight cycles of the unmodulated (fixed phase and amplitude) color subcarrier. By comparing the reference signal derived from color burst to the chrominance signal's amplitude and phase at a particular point in the scan, the device knows what chrominance to assign to the pixel then being displayed. Combining that with the amplitude of the luminance signal, the receiver knows exactly what color to make the pixel.
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 3.58 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 60 Hz field rate down by a factor of 1000/1001, to approximately 59.94 fields per second.
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. They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize interference between the audio signal and the chrominance signal. The original black and white standard, with its 15750 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 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 slop in the line frequency. So that is the route the color standard took. In the black and white standard, the ratio of audio subcarrier frequency to line frequency is 4.5 MHz / 15750 = 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 ~ 15734 lines per second. Dividing by 262.5 lines per field, this gives approximately 59.94 fields per second.
[edit] Transmission modulation scheme
An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz. A guard band, which does not carry any signals, occupies the lowest 250 kHz of the channel to avoid interference between the video signal of one channel and the audio signals of the next channel down. 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 any modulated signal, 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 750 kHz 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 suppressed carrier. The highest 25 kHz of each channel contains the audio signal, which is frequency-modulated, making it compatible with the audio signals broadcast by FM radio stations in the 88-108 MHz band. The main audio carrier is 4.5 MHz above the video carrier. Sometimes a channel may contain an MTS signal, which is simply more than one audio signal. This is normally the case when stereo audio and/or second audio program signals are used.
- One odd thing about NTSC is the Cvbs (Composite vertical blanking signal) is something called "setup." This is a voltage offset between the "black" and "blanking" levels. Cvbs is unique to NTSC.
- Cvbs has one defect: it makes NTSC more easily separated from its primary sync signals, but Cvbs has a smaller dynamic range when compared with PAL or SECAM.
[edit] Framerate conversion
There is a large difference in framerate between NTSC and film, the latter consisting of 24.0 frames per second whereas NTSC is displayed at approximately 29.97 frames per second. Unlike the two other video formats, PAL and SECAM, this difference cannot be overcome by a simple speed-up. A complex process called "3:2 pulldown" is needed, which duplicates parts of frames. This induces noticeable judder during slow pans of the camera. See telecine for more details.
For viewing native PAL or SECAM material (such as European television series and some European movies) on NTSC equipment, a standards conversion has to take place. There are basically two ways to accomplish this.
- The framerate can be slowed from 25 to 23.976 frames per second (a slowdown of about 4%) to subsequently apply 3:2 pulldown.
- Interpolation of the contents of adjacent frames in order to produce new intermediate frames; this introduces artifacts, and even the most modestly trained of eyes can quickly spot video that has been converted between formats. (See also stutter frame)
[edit] Use with Progressive Sources
When NTSC is used to transmit content which was originally composed of 29.97 progressive full frames per second, the even field of the frame is transmitted first. This is opposite to PAL, and opposite to what would be expected ('Even first' means the frame starts being drawn on the second line). Systems which recover progressive frames or transcode video should ensure that this '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.
[edit] Comparative quality
Video professionals and television engineers jokingly referred to NTSC as "Never The Same Color" or "Never Twice the Same Color". Reception problems can degrade an NTSC picture by changing the phase of the color signal, so the color balance of the picture will be altered unless a compensation is made in the receiver. This necessitates the inclusion of a tint control on NTSC sets, which is not necessary on PAL or SECAM systems.
However, the mismatch between NTSC's 30 frames per second and film's 24 frames is well overcome by an ingenious process that capitalizes on the field rate of the interlaced NTSC signal, thus avoiding the film playback speedup that is used for PAL and SECAM at 25 frames per second (which results in audio distortion). See Framerate conversion.
There is no question the NTSC system reflects the technology of its originating era, but its compatibility and flexibility has been the key to its longevity over seven decades. The coming of digital television and high-definition television may end the need for analog television systems.
[edit] Variants of NTSC
[edit] NTSC-M
Unlike PAL, with its many varied underlying broadcast television systems in use throughout the world, NTSC color encoding is invariably used with broadcast system M, giving NTSC-M.
[edit] NTSC-J
Only Japan's variant "NTSC-J" is slightly different: in Japan, black level and blanking level of the signal are identical (at 0 IRE), as they are in PAL, while in American NTSC, black level is slightly higher (7.5 IRE) than blanking level. Since the difference is quite small, a slight turn of the brightness knob is all that is required to enjoy the "other" variant of NTSC on any set as it is supposed to be; most watchers might not even notice the difference in the first place.
[edit] PAL-M
The Brazilian PAL-M system uses the same broadcast bandwidth, frame rate, and number of lines as NTSC, but using PAL encoding. It is therefore NTSC-compatible in sources such as video cassettes and DVDs, but its color picture cannot be received on a standard NTSC television set.
[edit] NTSC 4.43
In what can be considered an opposite of PAL-60, NTSC 4.43 is a pseudo color system that transmits NTSC encoding (525/29.97) in a color subcarrier of 4.43 MHz instead of 3.58MHz. The resulting output is only viewable by TVs that support the resulting pseudo-system (usually multi-standard TVs). Using a native NTSC TV to decode the signal yields no color, while using a PAL TV to decode the system yields erratic colors (observed to be lacking red and flickering randomly). The format is apparently limited to few early laserdisc players and some game consoles sold in markets where the PAL system is used.
[edit] Evolution of the NTSC signal
[edit] NTSC I
NTSC I is the original monochromatic 525/60 signal that first became standard in the U.S. in 1941 and later in Canada.
[edit] NTSC II
NTSC II is the color system with some but not all aspects of the signal rigorously defined. NTSC II has a minor change in its temporal structure, becoming a 525/59.94 system. From this point 525/60 [RGB] becomes a separate production standard that interoperates with NTSC via a 1/100.1% drop frame solution.
[edit] NTSC III
NTSC III came about due to digital television routing during the 1980s*; all aspects of NTSC III are rigidly mathematically defined.
[edit] The current state of NTSC III
The North American analog transmission chain is strictly NTSC III now. Many NTSC II devices feed into existing transmission chains, with NTSC III compatibility being achieved by signal processing in the digital domain.
Typical terrestrial TV transmitters or cable company distribution units send out NTSC III signals, especially if the originating signal comes from a TVRO or ATSC source. All free-to-air analog satcom transmissions are NTSC III. Video scrambling systems such as VideoCipher cannot achieve full NTSC III compatibility due to end-to-end analog processing issues.
There are no known compatibility problems between NTSC II and NTSC III. Older NTSC II sets should handle NTSC III signals without any problems, even with respect to minor frequency variances of the color sync subcarrier that exist in NTSC II.
[edit] Vertical Interval Reference
The standard NTSC video image contains some lines (lines 1–21 of each field) which are not visible; all are beyond the edge of the viewable image, but only lines 1–9 are used for the vertical-sync and equalizing pulses. The remaining lines were deliberately blanked in the original NTSC specification to provide time for the electron beam in CRT-based screens to return to the top of the display.
VIR (or Vertical interval reference), widely adopted in the 1980s, attempts to correct some of the color problems with NTSC video by adding studio-inserted reference data for luminance and chrominance levels on line 19. [1] Suitably-equipped television sets could then employ this data in order to adjust the display to a closer match of the original studio image. The actual VIR signal contains three sections, the first having 70 percent luminance and the same chrominance as the color burst signal, and the other two having 50 percent and 7.5 percent luminance respectively. [2]
A less-used successor to VIR, GCR, also added ghost (multipath interference) removal capabilities.
The remaining vertical blanking interval lines are typically used for datacasting or ancillary data such as video editing timestamps (vertical interval timecodes or SMPTE timecodes on lines 12–14 [3] [4]), test data on lines 17–18, a network source code on line 20 and closed captioning, XDS and V-chip data on line 21. Early teletext applications also used vertical blanking interval lines 14–18 and 20, but teletext over NTSC was never widely adopted by viewers [5].
[edit] Countries and territories that use NTSC
[edit] North America
[edit] Central America and the Caribbean
[edit] South America
[edit] Asia
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[edit] The Pacific
US Territories
American Samoa
Guam
Saipan also known as Northern Mariana Islands- Midway Atoll (a US military base)
Other Pacific island nations
Marshall Islands (in Compact of Free Association with US; US aid funded NTSC adoption)
Micronesia (in Compact of Free Association with US)
Palau (in Compact of Free Association with US; adopted NTSC before independence)
Samoa (closely tied to American Samoa; US aid funded NTSC adoption)
Tonga (US aid funded NTSC adoption)
Historic (used NTSC experimentally before adopting PAL)
Fiji (Historic; used before 1989, Fiji has used PAL since 1990)
Australia (Historic; Australia now uses PAL)
[edit] Indian Ocean
[edit] Middle East
South Yemen (Historic; all of Yemen now uses PAL)
[edit] Europe
United Kingdom (Experimented with a 405-line variant of NTSC in the 1950s and 1960s; dropped in favor of PAL)
[edit] See also
[edit] References
- A standard defining the NTSC system was published by the International Telecommunication Union in 1998 under the title "Recommendation ITU-R BT.470-7, Conventional Analog Television Systems." It isn't publicly available on the Internet, but it can be purchased from the ITU.
- Ed Reitan (1997). CBS Field Sequential Color System.
[edit] External links
- Representation of the NTSC refresh rate on a television and on a DVD
- A thorough explanation of the reasons why the field rate was changed to 59.94 Hz when color was introduced to the NTSC standard, including all the magic numbers and mathematics
| Designation | Usage examples | Definition (lines) | Rate (Hz) | |
|---|---|---|---|---|
| Interlaced (fields) | Progressive (frames) | |||
| Low; MP@LL | LDTV, VCD | 240; 288 (SIF) | 24, 30; 25 | |
| Standard; MP@ML | SDTV, SVCD, DVD, DV | 480 (NTSC, PAL-M) | 60 | 24, 30 |
| 576 (PAL, SECAM) | 50 | 25 | ||
| Enhanced | EDTV | 480; 576 | 60; 50 | |
| High; MP@HL | HDTV, HD DVD, Blu-ray Disc, HDV | 720 | 24, 30, 60; 25, 50 | |
| 1080 | 50, 60 | 24, 30; 25 | ||
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| This table illustrates total horizontal and vertical pixel resolution via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is either 4:3 or 16:9. | ||||
| edit Video formats |
|---|
| Defunct analog systems: 405 lines (1985) | 819 lines (1986) | PALplus (2007) |
| Defunct analog systems (cable, TVRO): PALplus (2009) | MUSE (2008) | MAC (2010s) |
| Analog terrestrial broadcast: NTSC | PAL | PAL-M (525 lines) | PAL-N (3.58 mhz colour) | SECAM |
| Digital (interlaced): SDTV (480i, 576i) | HDTV 1080i |
| Digital progressive: LDTV (240p, 288p) | EDTV (480p, 576p) | HDTV (720p | 960p | 1080p) |
| HDTV broadcast formats: ATSC | DVB | ISDB (ATSC & ISDB are part of the DVB standard) |
| Ultra High Definition TV (UHDV): Resolutions 2540p & 4320p; Audio 22.2 |
