The BBC has unique experience in the benefits and problems of digital audio, having used digital systems in the normal distribution of sound signals from London to various transmitters around the British Isles continuously for a period of some ten years. The BBC also evolved a digital stereo tape recorder in 1971, followed by a multitrack recorder and a working digital sound control desk in the late seventies. Experiments in how the broadcast transmission of digital audio is affected by difficult reception conditions have also been conducted. At a time when the BBC as well as other users and manufacturers of audio equipment are contemplating lager scale excursions into the use of digital techniques, it is appropriate to discuss our experience in the subjective and objective evaluation of such systems during their design, acceptance, and continuing use. Some of the impairments which may arise are not disclosed by conventional distortion measuring techniques and although they may only be detected subjectively on a limited variety of program material under ideal listening conditions, if uncorrected they may lead to the generalized criticism of digital sound which exists in some areas. It is also important to consider the repercussions on associated analog components of the introduction of digital processes into parts of the audio chain, such as the performance of analog limiters, the limitations of existing level indicating meters, and even the criteria for acoustic noise levels in studios.
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