Television Stereo Sound System Chosen by EIA Submitted to FCC.
The dbx TV system is an optimized compander (the signal is compressed before transmission, then expanded in the television set) designed specifically for television audio. No compression is applied to the left-and-right (mono) signal in order to ensure compatibility with the millions of monophonic TV sets now in use. Companding is applied to the left-right signal (the stereo difference) and to second audio program signals.
In essence, the stereo transmission is similar to that for FM radio. An AM subcarrier is used to transmit left-right signal, which will be combined with the left-and-right signal transmitted by the main FM aural carrier to produce left and right stereo signals.
Design considerations for the sound system were determined by the noise levels and their spectra, and by various types of interference involved in stereo TV transmission/reception.
Because the noise levels in all the proposed television transmission systems were potentially high, dbx designed a new process for the system, called "spectral compression." Specifically, the company elected to dynamically vary the preemphasis characteristics in order to match the input signal spectrum to the transmission channel. It's a Cost-Effective Technique
The TV noise-reduction system is designed as a cost-effective technique to deliver a clean, noise-free audio signal through a noisy channel to the home. The system is capable of delivering consistent high-quality audio in the face of a variety of possible channel degradations, including hum, buzz and hiss.
The system consists of an encoder at the TV station's transmitter and a matching decoder in the consumer's TV set. The two are complementary in their operation.
The decoding circuitry for television receivers is based on the 22-pin AN6291 integrated circuit (IC) jointly developed by dbx and Matsushita Electric. The IC has been in use in demanding noise-reduction applications for two years. It Keeps Signal Levels High
According to the psychoacoustics of noise reduction, the ear functions similarly as a spectrum analyzer does, effectively dividing sounds into frequency bands, called "critical bands." In order to simultaneously mask a variety of possible noises occurring in different parts of the spectrum, it is necessary to keep signal levels high at all times relative to the noise level across the entire audio spectrum.
Two elements of the dbx system are key to satisfying this constraint: wideband compansion and spectral compansion. The wideband compansion keeps overall signal levels high during transmission. The spectral compansion optimizes the spectrum that is transmitted. It acts like a sophisticated, signal-controlled treble control hinged around 1,000 Hz. The Range of Control Is Wide
The range of control is a wide [plus-or-minus] 25 dB at high frequencies, with accompanying peak slopes of 12 dB per octave. Combined with the wideband compander and other elements, the resulting system has a wide range of independent-gain/spectral-tilt combinations, and can therefore deal with the broadest range of signal/noise spectral combinations, without overload or overmodulation.
This stereo sound system, along with Zenith's transmission system, was selected by EIA's Multichannel Television Sound Subcommittee following more than four years of exhaustive testing and deliberation that culminated a broadly based industry-wide effort to settle on a single, national system.
Subcommittee Chairman Thomas Keller, of the National Association of Broadcasters, said that his group's unanimous vote in favor of the chosen systems "represents a major milestone toward the improvement of television in the US."
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|Date:||Apr 1, 1984|
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