Video Basics Continued

Composite/CVBS Interface

Composite signals are the most commonly used analog video interface. Composite video is also referred to as CVBS, which stands for color, video, blanking, and sync, or composite video baseband signal. It combines the brightness information (luma), the color information (chroma), and the synchronizing signals on just one cable. The connector is typically an RCA jack. This is the same connector as that used for standard line level audio connections. A typical waveform of an all-white NTSC composite video signal is shown in Figure 5.


Figure 5. NTSC composite video waveform.

This figure depicts the portion of the signal that represents one horizontal scan line. Each line is made up of the active video portion and the horizontal blanking portion. The active video portion contains the picture brightness (luma) and color (chroma) information. The brightness information is the instantaneous amplitude at any point in time. The unit of measure for the amplitude is in terms of an IRE unit. IRE is an arbitrary unit where 140 IRE = 1Vp-p. From the figure, you can see that the voltage during the active video portion would yield a bright-white picture for this horizontal scan line, whereas the horizontal blanking portion would be displayed as black and therefore not seen on the screen. Please refer back to Figure 1 for a pictorial explanation. Some video systems (NTSC only) use something called "setup," which places reference black at a point equal to 7.5 IRE or about 54mV above the blanking level. 

Color information is added on top of the luma signal and is a sine wave with the colors identified by a specific phase difference between it and the color-burst reference phase. This can be seen in Figure 6, which shows a horizontal scan line of color bars.


Figure 6. Composite video waveform: color bars.

The amplitude of the modulation is proportional to the amount of color (or saturation), and the phase information denotes the tint (or hue) of the color. The horizontal blanking portion contains the horizontal synchronizing pulse (sync pulse) as well as the color reference (color burst) located just after the rising edge of the sync pulse (called the "back porch"). It is important to note here that the horizontal blanking portion of the signal is positioned in time such that it is not visible on the display screen.

Y/C Interfaces

The Y/C signal is a video signal with less encoding. Brightness (luma), which is the Y signal, and the color (chroma), the C signal, are carried on two separate sets of wires. The connector is a mini-DIN type and resembles a small version of a keyboard connector.

Note: The term "S-video" stands for "separate video" and sometimes is used to refer to a Y/C signal, sometimes used in reference to recording formats. It was originally a recording format, as used for Sony Betamax, in which the luma was recorded separately from the chroma. The term is also commonly used to refer to the S-VHS (Super VHS) video recording format.

Component Interfaces

Component signal interfaces are the highest performance, because they have the least encoding. The signals exist in a nearly native format. They always utilize three pairs of wires that are typically in either a luma (Y) and two-color-difference-signals format or a red, green, blue (RGB) format. RGB formats are almost always used in computer applications, whereas color-difference formats are generally used in television applications. The Y signal contains the brightness (luma) and synchronizing information, and the color-difference signals contain the red (R) minus the Y signal and the blue (B) minus the Y signal. The theory behind this combination is that each of the base R, G, and B components can be derived from these difference signals. Common variations of these signals are as follows:

• Y, B-Y, R-Y: Luma and color-difference signals.
• Y, Pr, Pb: Pr and Pb are scaled versions of B-Y and R-Y. Commonly found in high-end consumer equipment.
• Y, Cr, Cb: Digital-signal equivalent to Y, Pr, Pb. Sometimes incorrectly used in place of Y, Pr, Pb.
• Y, U, V: Not an interface standard. These are intermediate, quadrature signals used in the formation of composite and Y/C signals. Sometimes incorrectly referred to as a "component interface."

Computer Signal Interfaces

Virtually all computer interfaces utilize RGB format signals. The picture information is carried separately by the three base components of red, green, and blue. Synchronizing information is typically carried as separate horizontal (H) and vertical (V) signals. The five signals, R, G, B, H, and V, are carried on one cable consisting of a shielded bundle of wires. The connector is almost always a 15-pin D-type connector. Sometimes the H and V sync information is merged with one of the RGB signals, typically the green component, but this is becoming less common. This is referred to as "sync on green." In rarer cases, the sync information is on the red or the blue signal.