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FM pre-emphasis boosts high audio frequencies before transmission; a receiver’s matching de-emphasis reduces them after demodulation. Together, the two restore the intended tonal balance while making high-frequency transmission noise less audible. If their time constants differ, the audio is left with a treble tilt instead of the intended response.
How the paired filters work
In FM broadcasting, high-frequency noise becomes increasingly troublesome in the recovered audio. The transmitter counters this by applying pre-emphasis: it boosts the upper frequencies of the program before the signal modulates the carrier. After demodulating the signal, the receiver applies de-emphasis, reducing those frequencies.
In the idealized response, pre-emphasis and de-emphasis are complementary RC curves. The first rises with frequency and the second falls. When both use the same time constant, their effects on the program audio cancel, restoring its intended tonal balance. De-emphasis also reduces high-frequency noise along with the boosted audio, but it does not remove all noise; the result depends on transmission and reception conditions. The curve definitions are described in Recommendation ITU-R BS.450, and the FCC specifies a de-emphasis convention for relevant measurement procedures in its historical Federal Register document.
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50 μs and 75 μs: which standard applies?
The time constant determines how strongly the response changes with frequency. Recommendation ITU-R BS.450-4, approved on October 14, 2019 and listed as in force, identifies 50 μs for Europe and 75 μs for the United States. In the US, the current rule limits FM pre-emphasis to the characteristic of a network with a 75 μs time constant: 47 CFR § 73.317(e).
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These regional conventions are a useful starting point, not a complete current country-by-country setting list. The historical ITU Library scan also reports 50 μs and 75 μs usage, including 75 μs in Canada and the United States, but it should not be treated as a current allocation source: ITU Library scan.
What a mismatched setting sounds like
If the transmitter and receiver use different time constants, their curves do not cancel. The longer time constant produces the stronger rise or fall across the audio band, leaving a residual tonal change after the two responses are combined.
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| Transmitter pre-emphasis | Receiver de-emphasis | Residual audio response |
|---|---|---|
| 75 μs | 50 μs | Excess treble emphasis |
| 50 μs | 75 μs | Excess treble attenuation |
These directions follow from the standard single-pole RC curves. They describe the tonal mismatch, not a fixed amount of noise reduction or a guaranteed number of decibels; the cited standards do not establish a universal noise-reduction figure.
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How to check a receiver with the wrong tonal balance
- Identify the station’s broadcast standard, if it is published. For a US station, the applicable pre-emphasis limit is 75 μs under 47 CFR § 73.317(e); ITU-R BS.450-4 identifies 50 μs for Europe.
- Check the receiver’s de-emphasis setting in its manual or audio settings. Confirm that it matches the station’s time constant, rather than assuming that a setting is correct solely because it matches the receiver’s intended market.
- If the receiver offers only one setting, compare its specified standard with the station’s. A mismatch can leave the audio too bright or too subdued; the table above shows which way each mismatch tilts the response.
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