Subject: Video Transmission and Receiving Systems Topic: CVBS (Color, Video, Blanking, Sync) Transmitter
Figure 1 Composite Video Signal (CVBS) Voltage and Time Intervals As we have been discussing during this series, video transmission and receiving systems are all about moving huge amounts of information between two points as quickly, reliably and as economically as possible. Consider the composite video signal (CVBS) of Figure 1. This is a highly complex signal with multiple levels of analog voltages, time intervals, edge rates, and differential gain and phase relationships. Of course contained within this signal are multiple frequency components that are highly susceptible to interference pickup. Portions of the waveform, particularly during the horizontal blanking interval demand exact voltage levels that are flat over the required time interval(s) as well as transition edges that must slew various voltage levels within given time limits. 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. Figure 1 also represents a typical waveform of an all-white NTSC composite video signal.
During the active video interval, the standard video color bar is transmitted that contains even higher frequency components that have given voltage gain and phase information that sets the color and intensity that is being transmitted and received (see Figure 2). This figure also depicts the portion of the signal that represents one horizontal scan line. Again, 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. As you can see (from both Figures 1 and 2) that the voltage during the active video portion would yield a bright-white picture for the high amplitude horizontal scan line, whereas the horizontal blanking portions would be displayed as black and therefore not seen on the screen. Please refer back to Figure 2 for a more 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. As complicated as a composite video signal is, the great benefit in using this video format, particularly in video transmission and receiving systems, is the fact that all the video information is contained within a single line.
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 also be seen in Figure 2, which shows a horizontal scan line of color bars.
Figure 2 A Single Line Composite Video Signal (CVBS) With Color Bar As you can see, there are many different design constraints that would impact any given video transmission and receiving system. Transmitting a composite video format signal (CVBS), although it requires only a single transmission line requires special attention due to high cable insertion loss over various frequencies. Currently, low cost CAT 5e cabling is the desired method to transmit the signal. Designers have two choices in transmitting the CVBS signal. There are passive transmitters (video baluns) and active transmitters (as shown in Figure 3 and we will go over the details of this design next week). Remember, signal loss, which also called attenuation, is measured in dB (decibels). The more attenuation there is, the poorer the signal will be at the receiver. As we have said, attenuation is a problem with UTP cable due to the cable's inherent lack of uniformity caused by differences in twist tension and rate, bends in the cable, and other inconsistencies, as well as lack of shielding and variances in insulation type/thickness—all problems that are not an issue with coaxial cable. The use of a passive balun transformer does not address these issues. By their very nature, passive transmitters and receivers can provide no help for signal loss. Passive devices actually further attenuate the video signal, making the problem more serious. Far more satisfactory are the superior results obtained by using an active transmitter (again see Figure 3), which does not share this limitation, and overcomes the problems of signal loss by providing amplification, complemented by variable level and peaking controls designed to optimize the signal.
Figure 3 CVBS Low Noise Video Transmitter
To illustrate the difference between passive and active transmission technology, actual oscilloscope waveforms (see Figure 4) show a comparison of rise times between a passive balun transformer and an active transmitter/receiver pair. The video test signal in this case has an approximate rise time of 1.5 ns. Scope measurements in Figure 4 for both passive and active transmitters were taken using the same 100 feet and 300 feet of UTP cable. In each case, the reference signal, provided by a video test generator connected directly to the scope, appears as a thin black line, and the test signal (Ch1) appears as a heavy yellow line. Note the inferior response of the passive technology (poor rise time and low amplitude, resulting in a soft, dim image) using either length of UTP cable. At the same time, in both of the active technology scope waveforms, the active transmitter and receiver pair were able to compensate for the distance by using the peaking adjustment to improve the rise time (sharpening the picture) and the level adjustment to boost the amplitude of the signal to acceptable levels (brightening the picture). We will go over the active transmitter shown in Figure 3 in detail next week.
Figure 4 Passive vs. Active Transmitters
When designing a high performance video transmission and receiving system, it is important to simply break down the system into the various functional blocks that make up the system and address each performance limiting factor. Depending on the overall system specification, such as the video data transmission rate and resolution, these numbers will determine many of the required analog performance specifications of the system including simple layout geometries, amplifier bandwidths, slew rates, and required gains. The number one passive component within a high performance video transmission and receiving system is the cable and understanding the positive and adverse effects of this single component will greatly enhance your ability to design the system.
Kai ge from CADEKA (www.cadeka.com)