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Fiber-Optic Transmission and Professional Audio
The virtues of the fiber-optic medium permit maximum fidelity of audio transmission, whether analog or digital. The benefit of fiber optics for audio is as striking as that of digital audio itself. Unlike the properties of DAT or CD media (with the severe limitation that longest playing time occurs with minimum sampling rate), the fiber-optic medium has virtually unlimited bandwidth. This makes it questionable whether sampling-rate standards established for digital audio still need to apply. However, the optimization of the performance of fiber-optic components needs to be understood to avoid poor fiber loss budget, intersymbol interference, or chromatic dispersion. Drifts of optical power (in decibels) double after conversion to current by the photodiode. Analogy to radar square-law detection fails, since the square-law device in the optical case precedes the receiver amplifier noise, whereas in radar it follows the receiver noise sources. Circuit implementation of optical receivers is discussed, especially as it relates to obtaining wide bandwidth (high bit rates) and high sensitivity. Included are the implications of avalanche photodiode (APD) approaches. The requirement for code dc-content minimization is also discussed, as well as the effect of code noise contamination on the final audio, along with a method to minimize the subjective impact.
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