A well-known problem in FM broadcasting is the excessive high-frequency preemphasis introduced by the 50- or 75-µs time constants now used. With modern program material the preemphasis results in possible overmodulation and the need for high-frequency limiting. If limiting is avoided in order to maintain signal integrity, then operation at an overall decreased level is required. The use of Dolby B-type compression during transmission provides an opportunity to reduce the preemphasis time constant to 25 µs, which permits the midband modulation level to be raised significantly with many types of program material. Listeners with conventional receivers are provided with a compatible signal but with the reception advantages of an increased modulation level. Listeners with 25-µs deemphasis time constants and B-type expanders have the dual advantages of increased level and lower noise.
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https://aes2.org/publications/elibrary-page/?id=1968
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