FM broadcasting uses a matched pair of audio filters: the transmitter boosts higher audio frequencies before modulation, and the receiver reduces them after demodulation. When the two responses are complementary, the intended audio response is restored while high-frequency noise in the recovered signal is reduced. The required filter time constant depends on the applicable regional standard; 75 microseconds is not universal.
How the matched filters work
At the transmitter: pre-emphasis
Before FM modulation, the transmitter increases the level of higher audio frequencies relative to lower ones. This is called pre-emphasis. The boost shapes the signal so the receiver can later reduce the high-frequency portion along with noise that is more troublesome at those frequencies.
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At the receiver: de-emphasis
After demodulation, the receiver applies de-emphasis, a complementary high-frequency roll-off. The desired program audio is brought back toward its intended response, while high-frequency noise is attenuated. The benefit is improved high-frequency signal-to-noise performance in the recovered audio; it is not a single fixed number of decibels for every FM system. MathWorks’ FM broadcast modulator documentation describes this paired filtering and regional values.
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The transmitter’s boost and receiver’s roll-off are designed to complement one another. If they do not match, their combined response will not produce the intended audio response: some frequencies may be left too prominent or too weak. The mismatch also means the receiver’s noise-reduction roll-off no longer corresponds to the transmitter’s boost as designed. Check the applicable standard and both ends of the signal path rather than treating either filter in isolation. Canada’s BETS-6 requirements specify a transmitter pre-emphasis curve and a corresponding demodulator response in their stated test context.
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What the time constant means
A common first-order RC emphasis response is described by a time constant, τ. Its break frequency is f₀ = 1/(2πτ). For a 75 μs time constant, this gives a corner frequency of approximately 2.1 kHz. In the cited 75 μs example, the de-emphasis response is Hd(f) = 1/(1 + jf/f₀), where f is frequency and j is the imaginary unit. This equation describes the filter response; it does not by itself determine which time constant a broadcaster must use. The Indian Institute of Technology Madras communications systems textbook presents the response and corner-frequency relationship.
Regional time constants differ
FM broadcast emphasis is set by regional conventions or requirements, not one worldwide value. MathWorks documentation identifies 75 μs for the United States and 50 μs for Europe. Canada’s BETS-6 describes normal 75 μs transmitter pre-emphasis. These examples are not a substitute for checking the current rules that apply to a particular transmitter and jurisdiction.
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| Region or source context | Documented value | Source and qualification |
|---|---|---|
| United States | 75 μs | Identified as a regional FM broadcast value in MathWorks documentation; publication date not stated. |
| Europe | 50 μs | Identified as a regional FM broadcast value in MathWorks documentation; publication date not stated. |
| Canada | 75 μs | Described as normal FM transmitter pre-emphasis in ISED’s BETS-6. |
What the Canadian transmitter standard specifies
For its stated measurement context, ISED’s BETS-6 requires the demodulator response to remain within ±1 dB of the normal 75 μs de-emphasis curve from 50 to 15,000 Hz. The same standard sets a maximum transmitter harmonic distortion of 1% over 50–15,000 Hz under the specified test. These are Canadian regulatory test requirements, not universal characteristics of FM broadcasting.
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