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 secondfor 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 lso included horizontal and vertical synchronization information.

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

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

Nowdays 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 vidoe 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 tranferred 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 constistute 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 scanlines and pixels from horizonal lines). Square pixels can simplify image spatial analysis and editing operations.

Color RS-170 video

Color video signals are a little more complicated than monocrome 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 ooks 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, Synch. 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 synch channel. Timing information is sometimes also present on the green channel, but more often not. This video format is called RS-170 RGB.


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 Commision (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 "subcarrier" 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).

Other somewhat related video formats

Monocrome NTSC

Color NTSC video format RS-170A has evolved from RS-170 standard and is nowadays more often seen term than RS-170. Sometimes term "monocrome NTSC" when referring to the old RS-170 specification.


A standard recommended by EIA for signals generated by closed-circuit TV cameras scanned at 525/60 and interlaced 2:1. The standard is more or less similar to RS-170, but H-sync pulses are absent during V-sync. Equalizing pulses are not required and may be added optionally during the V-blanking interval.

This standard is also used for color television studio electrical signals.


RS-343 is an EIA (Electronics Industries Association) standard for non-broadcast high resolution monochrome video. RS-343 was introduced later than RS-170 and intended, according to the title, as a signal standard for "high-definition closed-circuit television", which among other things reduced the signal amplitude to 1.00 Vp-p. The signal specifications according RS-343:

White: +0.714V
Black: +0.054V
Blank: (0V reference)
Sync: -0.286V       

RS-343 specifies a 60 Hz non-interlaced scan with a composite sync signal with timings that produce a non-interlace (progressive) scan at 675 to 1023 lines.

This standard is used by some computer systems and high resolution video cameras. Precision imaging systems, infrared targeting, low-light TV, night-vision and special military display systems, usually operate to high-resolution, RS-343 standards (875-line, 30-frame scan). They require specialized and costly recording and display equipment.


EIA standards for high resolution monochrome CCTV. Based on RS-343.


The CCIR is a standards body that originally defined the 625 line 25 frames per second TV standard used in many parts of the world. The CCIR standard defines only the monochrome picture component, and there are two major colour encoding techniques used with it, PAL and SECAM. The CCIR standard uses 1:2 interlace which means that each 625 line TV image is composed from 2 video fields, each consisting of 312 lines of picture information. The first video field contains just the odd numbered lines from the 625 line picture, the second field contains just the even ones.

CCIR video format is the format used is the European Broadcast Standard. The picture has 582 lines and uses interlacing. Horizonal sync rates of 15625 Hz and field rate of 50 Hz. CCIR electrical signal is 75 ohm system and 1.0V volt (peak-to-peak, including sync) signal.

Here is some data of CCIR video signal:

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
For color television (PAL standard) the following extra details were defined:
Color burst start   5.6 +- 0.1 us after sync start.
Color burst         10 +- 1 cycles
Color subcarrier    4.433 MHz


There never really WAS an official standard for VGA video, but it's more closely related to 343 than 170, since it uses a roughly 0.7 V signal (black/blank to white).

Tomi Engdahl <[email protected]>