The successful generation, transmission and reproduction of a televised scene requires a tightly controlled synchronization between the studio color camera and the TV receiver triple gun picture tube.
Figure 1. Simplified diagram of a sync separator
In order to synchronize the receiver scanning circuits the video signal contains synchronizing information. Since the early days of television, the picture electrical signal and the synchronizing signal have been combined into what came to be called a composite video signal. The composite video signal is bipolar with a normalized signal amplitude of 1 V p-p. The 700mV (714.3mV on the North American continent) positive part of the signal conveys picture information. The negative 300mV part of the signal (285.7 mV on the North American continent) conveys synchronizing information.
Synchronizing the receiver
The horizontal sync information is transmitted as a short (4.7 µsec duration) pulse located in the horizontal blanking interval. The vertical sync signal is more complex. Essentially it occupies a space equivalent to nine lines in the vertical blanking interval (7.5 lines in the 625/50 format). The vertical sync signal is composed of six (five in the 625/50 format) short, 2.35 µsec duration pre-equalizing pulses, followed by six (or five) serrated vertical sync pulses (approximately 27 µsec duration), followed by six (or five) short, 2.35 µsec duration post-equalizing pulses. The serrations are needed to ensure continuing horizontal synchronization during the vertical blanking period. The pre- and post-equalizing pulses serve to unambiguously synchronize the vertical scanning in the receiver in order to maintain the interlaced scanning. The relative complexity of the vertical synchronization is the price to pay for the reduced bandpass afforded by interlace.
In addition to the scanning synchronizing information, chrominance subcarrier frequency and phase information is also transmitted as a burst of 9±1 cycles (at 3.58MHz in NTSC) or 10±1 cycles (at 4.43MHz in PAL). This information is required as a reference for the regeneration of the suppressed chrominance subcarrier used by the synchronous detector of the chrominance decoder.
The receiver extracts the horizontal and vertical scanning information through the use of a clipper with a threshold of 0V. Picture information is thus removed and only the sync information is passed. The horizontal sync pulses pass through a differentiating circuit that generates short pulses coincident with the horizontal sync leading edge. These pulses feed a PLL-controlled VCO to generate the horizontal scanning waveform. The vertical sync pulses pass through an integrating circuit that practically removes the short duration horizontal sync and pre-equalizing pulses. The serrated pulses charge a capacitor to the value required to synchronize the vertical scanning generator. The bursty chrominance sync information is keyed out of the horizontal blanking interval and passed through a bandpass filter centered on the subcarrier frequency (3.58MHz or 4.43MHz). The filtered bursts feed the PLL, crystal-controlled oscillator part of the chrominance decoder. Figure 1 shows a simplified diagram of a sync separator.
Synchronizing the signal sources
The process of combining video signals originating from different local sources requires perfect synchronism and relative timing of all the signals present at the input of a production switcher. The synchronization is obtained by locking all video signal sources to a common reference black burst signal generated by the master sync generator. Modern equipment provides adjustments to meet the specs.
Figure 2. Simplified block diagram of genlocking to an external source
External, incoming video feeds are non-synchronous. They can be recorded without difficulty as the VTR locks onto the incoming video signal. On-air switching of the non-synchronous external sources, such as may occur in regional affiliated stations, does not require synchronous network feeds if a temporary loss of vertical sync when switching from a network feed to a local program is acceptable. Some simple on-air switchers use “V-fade” switching, fading the signal to black before and during switching, thus masking the switching transient. Mixing the incoming feeds with the locally generated signals requires that the external and internal video signal sources be synchronous and meet the timing requirements.
Early operational methods used the concept of genlocking. This required that all local signals be locked to the incoming external feed. To achieve this the master sync generator used to be locked to the external feed. Alternately, various operational configurations, consisting of a studio, several cameras, VTRs and production switchers, were genlocked to specific external sources. This method is awkward but possible when dealing with a single external source. When dealing with simultaneous, multiple non-synchronous external sources this method simply does not work. Figure 2 shows a simplified diagram of genlocking.
Figure 3. Simplified block diagram of the use of a frame synchronizer
Handling external video sources became simpler when digital frame synchronizers made their appearance in the mid-1970s. The frame synchronizer samples the incoming signal, stores it in a digital memory and reads it out at a rate controlled by the local master sync generator. Early frame synchronizers sampled the composite video signal at 3Fsc (10.7MHz in NTSC) with a resolution of eight bits per sample. Current technologies use digital adaptive comb filters to decode the composite video signal into Y and multiplexed CB/CR digital components with a resolution of 10 bits per sample. The Y and CB/CR signals are stored in separate frame memories that together store a full video frame. The two memories are read out at a rate controlled by the station master sync generator. The synchronized digital components are subsequently encoded into an analog composite video signal. The frame synchronizer usually provides for timing adjustments of the synchronized video signal to match the local requirements. In addition to synchronizing the external feed, frame synchronizers include additional features such as adjustments of video frequency response, composite signal gain, setup level, chrominance gain and chrominance phase with respect to burst. Figure 3 shows a simplified diagram of the use of a frame synchronizer.
Michael Robin, a fellow of the SMPTE, former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal, Canada. He is co-author of Digital Television Fundamentals, published by McGraw-Hill.
Send questions and comments to:michael_robin@primediabusiness.com
