Composite Video Signals (CVBS)

Composite Video Signals (CVBS)

TV broadcasting originally began with "monochrome" signals, those containing just sync and luminance (brightness) information. This was superseded by the "composite color" signal in the 1950s, a combination of luminance and color signals, which remains today as the dominant TV broadcast format.

The composite signal is also referred to as CVBS (Colour, Video, Blank, and Sync) or sometimes CCVS (Composite Colour Video Signal). Composite color video signals are comprised of three components:

• The luminance (luma), Y, signal which contains the intensity information.
• The chrominance (chroma), C, signal which contains the color information.
• The synchronization (sync), S, signal which controls the horizontal line frequency.

The definition of the composite video signal differs around the world according to the combination of three fundamental types of standards.

Possible Signal Distortions

In composite video, interference between the chrominance and luminance information is inevitable, and tends to be worst when the signal is weak.

Television sets are actually designed to display luminance and chrominance signals separately. Composite signals must be separated before they can be displayed. When the signals are sent as a composite, they overlap at a frequency range above 2.1 megahertz (MHz). The overlapping areas are difficult to separate entirely, and the remnants of either signal within the other create video errors.

Minor elements of chrominance data remaining in the luminance data cause a cross-luminance effect that creates a dot structure pattern (sometimes referred to as "dot crawl"). Likewise, minor elements of luminance data remaining in the chrominance data create "rainbow" effects in detailed patterns called "cross-color".

Colour Encoding Standards

There are three color encoding standards that define how color information is modulated and combined with luminance:

1. NTSC (National Television Standard Committee) – used in North America, Central America, some parts of South America, Japan
2. PAL (Phase Alternating Line) – used in Europe, Asia, Pacific, Africa, South America
3. SECAM (SÉquence de Colour À Mémoire) – used in France, Russia, Africa

PAL and NTSC use Quadrature Amplitude Modulation (QAM) to generate a signal carrying component color difference signals. The two color difference signals, R’-Y’ and B’-Y’, are modulated onto separate high-frequency sine wave carriers having the same frequency but a 90° phase difference. The two modulated signals are then added together prior to transmission. The color subcarrier burst on active lines is used by decoders to demodulate and extract the original color differences, and when combined with the luminance, Y, can be used to generate an RGB color picture. SECAM on the other hand, uses frequency modulation of two different sub-carriers on alternate lines.

Scanning Standards

There are two scanning standards that define how a picture is arranged in space and time to form a single time-varying voltage signal. These standards are specified by three parameters: lines per frame, field rate (in Hz), and interlace ratio. If the video picture is interlaced, the frame is broken into fields that are transmitted alternately. The frame rate is obtained by dividing the field rate by the interlace ratio.

1. 525 / 59.94 / 2:1
2. 625 / 50 / 2:1

Transmission Standards

There are a number of television transmission standards that differ in video and synchronization levels and timing, and in the characteristics of their radiated signals, such as RF carrier polarity, distance between sound carrier and vision carrier. They are denoted by one of the following letters hyphenated to the name of the color TV system:

Common TV Systems

With the possible combinations of color and transmission systems, there are seven main television systems in use:

1. NTSC-M
2. NTSC-4.43
3. PAL-M
4. PAL-N
5. Combination PAL-N
6. PAL-B/D/G/H/I
7. SECAM-B/D/G/K/K1/L

All television systems listed in this document use an aspect ratio of 4:3 (width:height), a scanning sequence from left to right and from top to bottom, and an interlace ratio of 2/1, resulting in a picture (frame) frequency of half the field frequency.

NTSC uses only M. NTSC-4.43 is a slight variation of NTSC-M that has a subcarrier frequency of 4.43361875 MHz, and is used for playback of NTSC material on modified PAL TV sets. NTSC-M in Japan is different from standard NTSC-M in that it does not have a Setup level. Some standards have only very limited geographic use—for example, Combination PAL-N is only used in Argentina.

Common Characteristics

The most common formats are NTSC-M and PAL-B/D/G/H/I (simplified in this document to PAL) and their fundamental characteristics are listed in the table below. NTSC uses the IRE unit of amplitude, where 1 IRE = 1/140 Volt or 7.1428 mV. * denotes a half-line.