- Crosby system
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The Crosby system was an FM stereophonic broadcasting standard, developed by Murray G. Crosby, that used an FM subcarrier for higher fidelity. It competed with the Zenith/GE system that used an AM subcarrier. Many audiophiles were disappointed when the Zenith/GE was chosen as the nationwide standard, since the Crosby system had many advantages.
The Crosby system's signal was less noisy, offering the best frequency response in stereo with less degradation under weak signal conditions. It utilized the matrix principle, transmitting the sum signal L+R as the main channel modulation and the difference signal L-R as a 50 kHz subcarrier. The Crosby system's main advantage over the Zenith/GE system was its use of an FM subcarrier. FM is less susceptible to interference than AM. In addition, most AM radio stations do not reproduce faithfully sounds below 100 Hz or above 5 kHz; FM stations generally have a frequency range or 50 Hz to 15 kHz and the upper limit may be even higher.
However, the Crosby system was incompatible with existing subsidiary communications authorizations (SCAs) which used various subcarrier frequencies including 41 and 67 kHz. Many revenue-starved FM stations used SCAs for "storecasting" and other non-broadcast purposes. 1960 FCC tests confirmed that the GE/Zenith stereo multiplexing technique was compatible with 67 kHz SCA operation. Several FM stations relying on SCA revenue urged the FCC to adopt the GE/Zenith standard. Another (albeit relatively minor) factor was that radios used vacuum tubes in those days; the additional tubes for an all-FM system would have increased the size, weight, cost of and heat generated by each tuner or receiver.[1]
According to Jack Hannold:[2]
“ Crosby used a wideband FM subcarrier, providing a better signal-to-noise (S/N) ratio in stereo from all but the weakest RF signals. FM is not entirely free of noise. While AM has a rectangular noise spectrum, with the amplitude of random noise in the demodulated signal constant across the audio spectrum, FM has a triangular noise spectrum, with the amplitude of noise increasing with frequency, i.e., it rises at a rate of 6 dB per octave, or 20 dB per decade. Thus at 4 kHz, the noise level is 20 dB higher than at 400 Hz, and at 40 kHz it is 40 dB higher. So the L-R subcarrier, whether DSB or FM, is accompanied by a high level of noise. FM uses pre-emphasis to overcome high frequency noise. Sounds above 2 kHz are boosted 6 dB per octave at the transmitter by a pre-emphasis circuit. A complementary de-emphasis in receivers rolls off treble response at the same rate, restoring highs to their proper level and simultaneously reducing high-frequency noise. But it only works in mono, because the DSB subcarrier is not boosted by pre-emphasis, and is thus subject to AM noise when the main carrier signal is weak. But an FM subcarrier, like the main carrier, benefits from limiting (the elimination of AM noise imposed by interference), as long as its amplitude - which remains constant in FM - is slightly above the peak amplitude of the noise within the subcarrier channel, yielding a much better stereo S/N than Zenith on all but the weakest of weak signals. ” On April 19, 1961, the FCC released its Final Order selecting the Zenith/GE system as the FM stereophonic broadcasting standard. That system continues to be used today for stereo FM broadcasting, which accounts for the poor quality of stereo signals in comparison with monaural signals. When reception is poor, the stereo AM GE/Zenith signal will typically fade in and out, while the monaural FM signal remains relatively strong. On most car radios, this manifests itself as a "STEREO" light flickering on and off.
At 9:59 A.M. on the day that the FCC's rule came out, Crosby-Teletronics stock was worth $15 a share; by 2:00 P.M. it was down to less than $2.50.[3]
References
- Nichols, Roger: I Can't Keep Up With All The Formats, 2003.
- Schoenherr, Steven E.: Stereophonic Sound, 2001.
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