Digital Audio On Analog Television
We are now broadcasting digital video and audio routinely on DTV stations, but some might not be aware that in some countries of the world, digital audio has for some time been broadcast on analog television stations.
In the 1980s, we in the U.S. developed a method to transmit stereo and additional audio signals on NTSC television that ultimately became the BTSC multichannel television audio system. The U.S. was not alone, and not even first, in adding stereo audio to television, although it was the first with a system that accommodated audio signals in addition to stereo, that is, SAP and the Pro Channel.
The first TV stereo system to be implemented was developed in Germany, and the mechanism was the straightforward addition of a second aural carrier. This system was subsequently adopted by other countries, including Australia, which also preceded the U.S. in TV stereo broadcasting. At the early experimental stage in the U.S., around 1982, the Japanese were developing a TV stereo audio system, EIA-J, that was examined here in the U.S. but rejected. The BTSC stereo system is an adaptation with embellishment of the multiplex stereo system used on FM radio in the U.S., in which the main audio channel is modulated with the sum of the left and right stereo channels, and the difference between the left and right channels amplitude modulates a double-sideband suppressed carrier subchannel or subcarrier. A significant enhancement offered by the BTSC system is the use of a companding noise reduction system on the left minus right signal. This addition effectively cancels the signal-to-noise penalty contributed by the L-R subchannel. Parenthetically, the same noise reduction system is used on the SAP channel. The Japanese system also uses a difference-channel subcarrier, but the L-R signal frequency-modulates the subcarrier instead of amplitude-modulating it.
Europe and England pioneered the use of digital audio (and subsequently digital video) in professional applications about a decade before we began using such digital approaches in the U.S. It is thus not entirely surprising that a system for transmitting digital audio on analog television channels was developed by the BBC in the mid-1980s. The system is called NICAM 728, for Near Instantaneous Companding and Multiplexing, at a data rate of 728 kbps. In England, analog television is transmitted on 8-MHz-wide UHF PAL System I channels. NICAM 728 is transmitted on a second aural carrier located 6.552 MHz above the visual carrier, or 52 kHz above the mono aural carrier. The NICAM carrier is 20 dB weaker than the peak visual power and 10 dB weaker than the primary (mono analog) aural carrier. The digital data modulates the carrier using differential quasi phase-shift keying (DQPSK); the data capacity provides the capability to carry two audio signals plus a small amount of ancillary data.
To reduce the occupied bandwidth of the digital carrier to an acceptable level, the digital audio data is compressed using the BBC-developed NICAM-3 system. This system has an initial resolution of 14 bits, with digital compression to 10 bits in one-millisecond blocks. The sample rate of the audio is 32 kHz, which yields approximately 15 kHz frequency response. The distortion and noise performance of this system, good by analog broadcast standards, is subjectively about equivalent to the performance realized using the BTSC stereo system. It does, however, offer the very high channel separation and interchannel phase integrity that are characteristic of digital audio systems. The channel separation is in fact good enough that the system may be used to transmit two different audio programs instead of stereo. The BTSC stereo system does not have this capability, but it does have the SAP channel.
NICAM 728 was adopted by other countries, including New Zealand. This led to an incompatibility between New Zealand and her closest neighbor, Australia, which uses the German stereo system. Although the two countries use the same PAL channel system, a stereo TV set bought in Australia won't receive stereo in New Zealand, and vice versa.
It is interesting to note that digital audio and video were used in broadcasting and other professional applications far earlier in Europe than in the U.S., and that the broadcasting of digital audio had its beginnings on television, not radio.
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