Chrominance phase
Mr. Robin,
I am an intern working with the Naval Air Warfare Center. After reading your article entitled “Analog video synchronization” in the January 2003 issue, I have a few questions. I understand the specifications for the chrominance amplitude phase and luminance pedestal for red, green and blue fields. But, what exactly does the chrominance phase refer to, and what is it referenced against? Also, can you explain how the chrominance amplitude affects color saturation? I always assumed the scale was from 0V to 0.7V (as opposed to -313.33mV to 313.33mV), and 0V would have implied no color; thus, the positive and negative voltages have thrown me for a loop. Finally, I'm confused as to what exactly the luminance pedestal is. Any help you could provide would be much appreciated.
Megan C.
NAVAIR, Lakehurst
Michael Robin responds:
Television is an evolving technology. It is amazing how the 1941-vintage NTSC television standard withstood the test of time, while making way for monochrome-compatible color transmissions in 1954 — all this without altering the vestigial sideband negative modulation transmission standard.
Negative modulation means that the instantaneous video transmitter power is proportional to the black level of the transmitted picture. So, early decisions adopted a composite video signal, which contained brightness information as well as horizontal and vertical scanning information. In the 1940s, the brightness information in video signals varied from black (0V) to white (+1V). The synchronizing signal has a negative value of -400mV to allow for an easy recovery of the synchronizing information through a process of removing (clipping) all positive excursions. Difficulties with unstable vacuum-tube technologies of the late 1940s made it difficult to maintain a stable black level (0V). Therefore, the black level was shifted to a slightly positive value, which was called “setup” (also called the “luminance pedestal”).
Color television initially used three primary colors: red, blue and green. Early transmissions in 1950 used the CBS sequential-color system, which transmitted the three primary colors sequentially. This system was incompatible with the existing monochrome transmitters and receivers and eventually was abandoned.
The NTSC color television system, which went on the air in 1954, was fully compatible and still is being used today. Among the changes introduced was a redefined composite video signal. Now the composite video signal carried luminance information, chrominance information, scanning sync and chrominance sync.
The luminance information (Y) is the equivalent of the brightness information in monochrome, and it was obtained by adding red, green and blue signals according to a well-known and universally adopted mathematical formula. The chrominance information was carried by two “color-difference” signals, namely blue-minus-Y (B-Y) and red-minus-Y (R-Y). These signals are bipolar (equal in positive and negative excursions) and are reduced in amplitude to avoid transmitter overmodulation.
These signals amplitude-modulate two equal-frequency subcarriers (about 3.58MHz) in phase quadrature (their phase is offset by 90 degrees). The subcarriers are cancelled so the system transmits only the chrominance sidebands.
To recover the B-Y and R-Y signals, the receiver requires two demodulators: one for B-Y and the other for R-Y. The receiver must also regenerate the missing subcarrier. To this effect, it uses the transmitted color-sync signal, a burst of about 10 cycles of subcarrier phase and amplitude reference to control a crystal oscillator, which feeds properly phased subcarrier signals to the B-Y and R-Y detectors. The burst is the system phase reference, so any instantaneous or constant phase shift of the B-Y and R-Y results in a change of signal shape, which translates into a color change.
Existing (pre-color) monochrome television signal distribution elements had difficulties carrying the composite color video signals, so the signal amplitude was reduced from 1.4V p-p to 1V p-p while maintaining the 1:4 video-signal-to-sync-amplitude ratio. This resulted in the currently used signal with 714.3mV of video and 285.7mV of sync.
April Freezeframe:
Q. In 1992, Broadcast Engineering judges for the first time selected a handtool as a Pick Hit. While it was the least expensive item ever selected as a Pick Hit, it remains a common item in almost every video engineer's tool box. What was it?
A. Canare Cable, Coaxial cable stripper
Winner:
No correct answers were received.
