Here we are less than a year from the shutoff date of NTSC terrestrial broadcast in the United States. Some will no doubt be happy to see it go, after all this time, and others will mourn its passing.
Along with broadcast NTSC, broadcast BTSC, also known as multichannel television sound, will also disappear. BTSC was a genuine breakthrough that literally revolutionized television audio in the 1980s. BTSC brought us television stereo and other sound services, and in the process, it spelled the end of the big TV set with the tiny little loudspeaker.
By the early 1980s, U.S. television networks were still distributing signals to their affiliates on telco audio/video circuits. Before the late 1970s, audio and video had traveled different paths in the telco networks, with all the problems that brought with it. With the new diplexed audio/video circuits that were put into service in the late 1970s, a single coaxial cable carried baseband video and audio modulated onto subcarriers. This not only greatly improved performance and signal reliability, it also made it relatively less difficult to carry multiple audio signals associated with a single video signal. This caused television engineers to start thinking about the possibility of transmitting stereophonic sound on the television airwaves. Multichannel television sound experimentation and development began in the United States in the early 1980s, but by the time multichannel sound became a reality, the networks had shifted satellite delivery to their affiliates.
The United States was not the first to develop TV stereo. The first TV stereo system to be implemented was the German system, which involved the addition of a second aural carrier to the television signal. This was not a stereo subchannel modulating the main aural carrier, as the U.S. FM stereo broadcasting system and the BTSC system use. It was a second aural carrier sitting beside the main aural carrier. The main aural carrier was modulated with the sum of left and right stereo channels (L+R), as it is in the BTSC system. The second aural carrier was modulated with left channel only signals. If we do the math, we discover that the L+R, R matrix may be decoded into discrete left and right channels just as the L+R, L–R matrix may be. The rationale for using left channel only in the second channel rather than the difference between left and right was that the left channel only signal would have a higher average amplitude than the difference signal would, and thereby more completely fill the channel more of the time than the difference signal would, giving a signal-to-noise advantage. The German system was also adopted by Australia, again before the United States began stereo television broadcasting.
The BTSC system, as developed in the United States, incorporated several clever technological innovations. It is a system that, like FM stereo, uses a stereo pilot and an amplitude modulated L–R difference subchannel that modulates the aural carrier at twice the pilot frequency. The FM system uses a 19 kHz pilot (in order to keep the pilot as far away as possible from the high-frequency content of the main channel), and a 38 kHz stereo subchannel. The BTSC system uses a pilot at the NTSC horizontal scan frequency of 15,734 Hz, and a subchannel operating at twice that frequency or 31,468 kHz. The rationale for using the pilot at the horizontal scan frequency was that the pilot could thereby be locked to the horizontal scan frequency, reducing the potential for beat frequency interference between pilot, subcarrier and horizontal scanning frequencies.
In order to provide frequency response to just under 15 kHz, a highly sophisticated multipole filter was developed, and in order to preserve the phase integrity between L+R and L–R channels, one of these filters is required in both the L+R and L–R channels.
There was sufficient spectrum space around the NTSC aural carrier to do a couple more tricks that made BTSC work better. First, the monophonic deviation of the television aural carrier is +/–25 kHz, as opposed to the +/–75 kHz deviation of the FM carrier, giving FM an inherent 10 dB signal-to-noise ratio advantage in strictly monophonic performance. However, when stereo was added to FM, the total deviation remained at +/–75 kHz, which means that some of the main channel or monophonic deviation had to be given up to accommodate the stereo subchannel, reducing the performance of the FM system when stereo was used. With BTSC, this didn’t have to happen. While, to maintain compatibility, the BTSC L+R or monophonic aural carrier deviation was maintained at +/–25 kHz, the L–R channel was permitted to deviate the aural carrier an additional +/–50 kHz. In addition to maintaining the monophonic signal strength, the L–R subchannel was doubled in amplitude, improving its signal-to-noise ratio. Additionally, noise reduction companding was added to the L–R signal, further improving its noise performance. The net result was good stereo sound, with no penalty to monophonic performance.
REVOLUTIONIZED TV AUDIO
The BTSC system offered some further enhancements as well. The SAP or second audio program channel is a subcarrier at a frequency above the L–R subchannel’s occupied bandwidth. It is frequency modulated, and uses the same companding noise reduction that is used in the L–R subchannel. The SAP channel is much used today to broadcast second language or descriptive video information. Finally there was available a higher-frequency, narrow bandwidth Pro Channel subcarrier, which could be used for nonpublic communications purposes.
The BTSC system, as mentioned, revolutionized television audio in general, and not just by bringing us stereo television broadcasting. It forced the upgrade of stations’ and networks’ audio infrastructures, as well as the upgrade of television set sound technologies. The difference is radical, as anyone who lived through that transition will attest. Television sound came of age. BTSC stereo became a reality in 1985 in broadcast television. Although it is being replaced by even more sophisticated and well-performing systems in ATSC digital broadcast, it is in some ways too bad that broadcast BTSC will have only had a lifetime of a little more than 20 years.
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