Video Standards

RS-170 video signal

RS-170 is the standard black and white video format used in the United States.

The EIA (Electronic Industry Association) is the standards body that originally defined the 525 line 30 frame per second TV standard used in North America, Japan, and a few other parts of the world. The EIA standard, also defined under US standard RS-170A, defines only the monochrome picture component but is mainly used with the NTSC colour encoding standard, although a version which uses the PAL colour encoding standard does also exist.

Introduction to RS-170

The EIA (Electronics Industries Association) standard that defines the timing of broadcast video in the United States, Japan, and several other markets. It specifies a 15.75-KHz horizontal and a 60-Hz vertical interlaced scan frequency as well as other aspects of the composite signal such as voltage, sync levels and timing of blanking.

An RS-170 video frame contains 525 lines and is displayed 60 times per second for a total of 15,750 lines, or 15.75 KHz. Of these lines, only the odd or even lines are displayed with each frame. A total of 60 frames per second allows 30 frames per second, or a 30-Hz update of each line.

The video signal encodes light intensity as a function of position. Position is represented by time and light intensity by voltage. This signal also included horizontal and vertical synchronization information.

RS-170 was the original "black-and-white" television signal definition, per EIA. The original standard defined a 75 ohm system and a 1.4 volt (peak-to-peak, including sync) signal. Signal level specifications form RS-170 were:

White:  +1.000 V
Black:  +0.075 V
Blank:  (0V reference)
Sync:   - 0.400 V

Nowadays RS-170 details are quite much in use, although the nominal signal level used nowadays is generally 1.0V (peak to peak). This 1.0V level was adopted from RS-343 standard to video industry.

To minimize flicker apparent at 30 Hz, the video signal is "interlaced", so that for each frame, first odd and then even scan lines are displayed. Each frame is composed of two fields, one consisting of 242.5 odd scan lines and the other 242.5 even scan lines, with a field produced each 1/60 second (60 Hz). The 60-Hz field rate was chosen to avoid interference from ac current fields.

Original RS-170 is strictly a timing specification for monochrome video signals.

Monochrome RS-170 Video

Black and white (monochrome) cameras are the simplest. They have single output cable which carries an RS-170 video signal. RS-170 signals are usually transferred using coaxial cable connected to BNC or RCA connectors.

RS-170 signal contains both image and timing information. The image is sent one line at a time, encoded using analog variation. The timing information consists of horizontal synch signals at the end of every line, and vertical synch pulses at the end of each field. There are also so-called horizontal and vertical blanking periods at the end of each line and field respectively, during which no image information is sent. The non-displayable lines in each field constitute the vertical blanking period.

The RS-170 standard specifies an image with 512 lines, of which the first 485 are displayable. The image information is actually sent in what is known as "interlaced" mode: The odd lines (1, 3, 5, ..., 485) are sent first, followed by the even lines (2, 4, 6, ..., 484). Each set of lines constitutes a "field". Fields are sent at a rate of 60 per second, which means that the entire image frame is refreshed 30 times a second. The reason for the interlaced format was to reduce perceptual flicker in the image displayed on a TV.

Horizontal resolution depends on the camera and other video system components. Since it is an analog signal, the exact number is not critical; it just limits the detail that can be resolved. The horizontal resolution of an analog video image is limited by the signal quality, as determined by all hardware--the video camera, storage medium (if used), intervening cables and circuitry, and display technology. Black and white cameras and CRT display tubes can resolve detail approaching or exceeding 1000 video lines. Typical resolution specs are on the order of 400-700 elements per line. This empirically defined quantity is the number of pairs of black and white parallel lines that could be counted across the display monitor at the limit of detection by a human observer.

The intensity information available in the video signal is limited by the dynamic range (the range of light intensities that can be detected) and the signal-to-noise ratio. The dynamic range is generally determined by the video camera.

The aspect (width to height) ratio for typical RS-170 signal rectangle is 4:3. The vertical resolution of video is limited to 485 pixels, as determined by the number of scan lines. The RS-170 standard specifies the aspect ratio (ratio of vertical/horizontal dimensions) of the video display as 3:4.

Digitizing RS-170 video

A typical digital image produced by video digitization would have a resolution of 512 (horizontal) X 480 (vertical) pixel resolution and would have individual pixels with a 5:6 aspect ratio. If you want square pixels, you have to digitize 646 pixels for each of the 485 lines. A fairly standard policy is to digitize 480 lines at 640 pixels per line (just leave out few scan lines and pixels from horizontal lines). Square pixels can simplify image spatial analysis and editing operations.

Color RS-170 video

Color video signals are a little more complicated than monochrome RS-170. Colors are produced on a CRT display monitor by mixing different intensities of red, green, and blue light.

There are three versions related to the RS-170 monochrome standard running over 1, 2, and 4 wires. Cameras and digitizers may use one or more of these.

The one-wire format is known as "composite video" or NTSC standard. Basically it contains intensity, color, and timing information on the same line. The intensity and timing information is basically consistent with the RS-170 monochrome signal. The color information is combined with the analog intensity signal using some sort of mixture encoding. Resolution of the color signal is considerably less than that of the intensity signal. The mixture encoding was designed so that in general, a monochrome RS-170 device, when fed a composite signal, will produce an acceptable monochrome output, although there are some exceptions to this (usually color mix produces some high-frequency noise in the digitized signal which looks like a very fine checkerboard overlaid on the picture). This picture format is called RS-170A according the standard.

The two-wire version is known as "S-video". In this format, one coax pair of wires, the Y channel, carries combined intensity and timing signals consistent with RS-170 mono. A second pair, the C channel, carries a separate color signal. S-video is usually carried on a single bundled cable with 4-pin connectors on either end.

The four-wire format is known as RGB or RGBS for Red, Green, Blue, Sync. In this case the color signal is broken into three separate and equal channels, each carrying high-resolution information. RGB video is a "component video" format, meaning the various components of information required to reproduce a video display are enclosed by separate signals. Timing information is provided on a separate wire - the sync channel. Timing information is sometimes also present on the green channel, but more often not. This video format is called RS-170 RGB.

RS-170A

Twenty years after the drafting of RS-170, the EIA video signal standard proposal RS-170A evolved into what is known today as the NTSC composite video signal. RS-170A specifies timing of scans (essentially the same as RS-170: 15.732 KHz horizontal and 59.94 Hz vertical) and blanking as well as the 3.58 MHz burst required to decode the colour signals. As adopted by the FCC for broadcast use, the standards are precisely adhered to and carry the force of law. For non-broadcast use, EIA standards are merely recommendations and are not enforced. Specifications referring to RS-170A do not necessarily mean the signals are broadcast standard.

RS-170A or National Television System Committee (NTSC) standard color is composite video; all of the information required to reproduce the display is enclosed on a single channel. The NTSC signal is used for television in the United States and Japan. When color television was introduced, video formats were constrained by the Federal Communications Commission (FCC) to be compatible with the installed base of RS-170 black and white sets, and available electronics technology limited the band width usable for signal encoding. Consequently, NTSC video incorporates a "sub carrier" for encoding color; color information is phase encoded by a lower frequency chrominance signal superimposed on the luminance signal.

RS-170 RGB

Refers to RGB signals timed to RS-170 specifications. By combining three such signals to control individual red, green, and blue sweep circuits, you can create a full-colour system. Since the red, green and blue signals are actually individual monochrome signals representing their respective colours, RS-170 RGB merely refers to three black and white signals sharing one sync signal which is either provided separately as a fourth signal or (usually) combined with the green signal.

RGB display system can have display resolution and timing is compatible with RS-170 (though this may not be the case with higher resolution RGB systems). Although video intensity information and blanking intervals are present in the red and blue signals, the synchronization pulses typically are not.

Some display system can be configured to produce composite synchronization on the green channel (which can be used to drive a black and white display). Alternatively, a fourth signal containing only the composite synchronization information may be used to drive an external synchronization input of an RGB display. Electronics Institute of America (EIA) composite synchronization consists of a negative synchronization pulse that is compatible with (and may be driven by) an RS-170 composite video signal.

Some computer graphic displays and some cameras require separate horizontal and vertical drive inputs and may require Transistor/Transistor Logic (TTL) voltage pulses (3-5 V).

PAL video timing specification

General timing

Line period 64 us (Micro-seconds)
Line blanking 12.05 +- 0.25 us
Line sync 4.7 +- 0.1 us
Front porch: 1.65 +- 0.1 us
Burst start 5.6 +- 0.1 us after sync start.
Burst 10 +- 1 cycles

Video signal

a = line sync
b = back porch

Rec.601 digital video:

720 samples per line, at 13.5 MHz sampling (with digital blanking)
or 702 samples per line, analogue blanking.
Note: Pixels are not square.

PAL and NTSC timing information

Horizonal Timing

                         PAL         NTSC 
A (us)                   64
B (us)                   4.7+-0.2    4.7+-0.1
C (us)                   ?           1.4
D (us)                   52
E (us)                   12

Horiz. Frequency (Hz)    15625       15734.2
Subcarr. frequency (MHz) 4.43        3.579545  
Subcarr. cycles per line ?           227.5



XXX        XXXXXXXXXXXXXXXXXXXXXX        XXXXX
XXX_   _W__XXXXXXXXXXXXXXXXXXXXXX_    _W_XXXXX
    |_|        VIDEO              |_|
    |B|    |---------D----------|C|
                                |----E---|
    |--------------A--------------|

Vertical Timing

                       PAL
Vertical frequency(Hz) 50      59.94
Blanking (lines)       25-37
Active lines                   482-486
Total lines            625     525

Equalization pulses            3+3+3
 
O (scanlines)           312.5    262.5
P (ms)
Q (ms)
R (scanlines)                  241-243
S (ms)


XXX       XXXXXXXXXXXXXXXXXXXXXX      XXXXX
XXX_   ___XXXXXXXXXXXXXXXXXXXXXX_   __XXXXX
    |_|       VIDEO              |_|
    |P|-Q-|---------R----------|S|
                               |---T--|
    |--------------O-------------|

Video signal levels

Video signals used in TV production

For all standard TV video signal systems the standard video level was 1 V p-p in 75 ohms, for video + sync. However, waveform monitors were marked in 140 "IRE units", 40 for sync, and 100 for video.

Under the original RS-170 standard, video was 1.4 Vp-p, with the white level 1.0V up from blank (which is the reference) and the sync tip 0.4V down (negative) with respect to the same reference level. And from blank to white was 100 IRE units.

The RS-170 levels have pretty much been replaced in most applications by the later RS-343 standard levels. Under this standard, the entire signal (including the sync pulses) was reduced to 1.0 Vp-p across 75 ohms. However, the same 100/40 division above and below the blank level was retained. This results in the reference white level being 0.714 V positive from blank, and the sync tips 0.286 V negative from blank. The BLACK level, under this standard, is slightly up from blank, having an 0.054V "setup" or "pedestal".

Standard practice in Europe has also been to use a 1.0V p-p signal, but they simplified life and set white to +0.700V and sync to -0.300V, again with respect to the blanking level, and did away with "setup" (i.e., black and blank are the same level).

PC video levels

From European 1 Vpp system we got one common PC video standard, which uses 0.700V p-p video (with separate syncs). Unfortunately, we also saw the development of ANOTHER common PC video practice, which was to simply use RS-343 with the setup chopped out - resulting in 0.660V p-p for the video.

Compatability issues

All of these are close enough to work about equally well on most equipment and monitors, but there are several "standards" for video floating around.

TABLE 1: RS-170A AND CCIR TIMING

	RS-170A	CCIR
# of raster lines/frame	525	625	Lines
# of raster lines/field	262.5	312.5	Lines
V total displayed lines/frame	485	575	Lines
V total displayed lines/field	242.5	282.5	Lines
V front porch/field	3.0	2.5	Lines
V sync/field	3.0	2.5	Lines
equalization pulse width	2.3± 1	2.35± 1	µs
V back porch/field	14	20	Lines
V blanking/field	20	25	Lines
line frequency	15.734	15.625	KHz
line duration	63.556	64.000	µs
line blanking	10.9 ± 0.2	12.00 ± 0.3	µs
front porch	1.5 ± 0.1	1.5 ± 0.3	µs
H sync pulse width	4.7 ± 0.1	4.7 ± 0.2	µs
back porch	4.7	5.8	µs
active horizontal	52.66	52	µs
nominal bandwidth	4.2	5.0, 5.5, 6.0	MHz
effective horizontal resolution	640	768	Pixels
output voltage	1.0	1.0	Vp-p
video voltage	0.7	0.7	Vp-p
sync voltage	0.3	0.3	Vp-p
impedance	75	75	ohm

STANAG is the NATO abbreviation for Standardization Agreement, which set up processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO state ratifies a STANAG and implements it within their own military. The purpose is to provide common operational and administrative procedures and logistics, so one member nation's military may use the stores and support of another member's military.

STANAGs are published in English and French, the two official languages of NATO, by the NATO Standardization Agency in Brussels.

Examples of international component video standards are:

RS-170 RGB (525 lines, based on NTSC timings, now EIA/TIA-343)

RS-343 RGB (525, 625 or 825 lines)

STANAG 3350 Analogue Video Standard (NATO military version of RS-343 RGB)

A standard defining the NTSC system was published by the International Telecommunication Union in 1998 under the title "Recommendation ITU-R BT.470-6, Conventional Television Systems." It isn't publicly available on the Internet, but it can be purchased from the ITU.

ITU

The International Telecommunication Union (ITU) is an international organization established to standardize and regulate international radio and telecommunications. It was founded as the International Telegraph Union in Paris in May 17, 1865, and is today the world's oldest international organization. Its main tasks include standardization, allocation of the radio spectrum, and organizing interconnection arrangements between different countries to allow international phone calls. (In which regard it performs for telecommunications a similar function to what the UPU performs for postal services.) It is one of the specialized agencies of the United Nations, and has its headquarters in Geneva, Switzerland, next to the main United Nations campus.

The international standards that are produced by the ITU are referred to as "Recommendations" (with the word ordinarily capitalized to distinguish its meaning from the ordinary sense of the word "recommendation"). Due to its longevity as an international organization and its status as a specialized agency of the United Nations, standards promulgated by the ITU carry a higher degree of formal international recognition than those of most other organizations that publish technical specifications of a similar form.

The work of the ITU is conducted by its members. As part of the United Nations structure, a country can be a member, in which case it is referred to as a Member State. Companies and other such organizations can hold other classes of membership referred to as Sector Member or Associate status. Sector and Associate memberships enable direct participation by a company in the development of standards (something not allowed in some other standards bodies such as ISO, where formal ballots are processed by a single entity per country and companies participate only indirectly through national delegations). Various parts of the ITU also maintaion liaison relationships with other organizations.

EIA

The Electronic Industries Alliance (EIA, until 1997 Electronic Industries Association) is a trade organization for electronics manufacturers in the United States. EIA is accredited by ANSI to help develop standards on electronic components, consumer electronics, electronic information, telecommunications, and Internet security.

Well known EIA standards include RS-170 for video signal and RS-232, EIA-422, RS-449, EIA-485 for serial data communications.

World Analogue TV transmission standards:

Yellow PAL
Green NTSC
Orange SECAM

**Television resolution**
LDTV	240i60, 288i50 (CIF)
SDTV	480i60, 480p30 (NTSC), 576i50, 576p25 (PAL, SÉCAM)
EDTV	480p60, 576p50
HDTV	720i50, 720i60, 720p24, 720p25, 720p30, 720p50, 720p60, 1080i50, 1080i60, 1080p24, 1080p25, 1080p30