FM pre-emphasis boosts high audio frequencies before transmission; a receiver’s matching de-emphasis attenuates them after demodulation. The pair restores the program’s intended tonal balance while making high-frequency noise less audible. If their time constants differ, the recovered audio is left with a treble tilt instead.
How the paired curves reduce audible noise
In FM reception, noise in the demodulated audio tends to become more troublesome at higher frequencies. The transmitter compensates by boosting the high-frequency part of the program before it modulates the carrier. After demodulation, the receiver applies the corresponding high-frequency reduction.
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For the program audio, the transmitter’s rising response and the receiver’s falling response cancel when they have the same time constant. For noise, the receiver’s high-frequency reduction also lowers noise in that part of the recovered audio. This shaping can reduce the audibility of high-frequency noise; it does not remove all noise or guarantee a fixed amount of noise reduction.
What 50 μs and 75 μs mean
The time constant determines the shape of the pre-emphasis and de-emphasis curves. Recommendation ITU-R BS.450-4, listed as in force and approved on 14 October 2019, identifies 50 μs and 75 μs systems, noting 50 μs for Europe and 75 μs for the United States. In the United States, 47 CFR § 73.317(e) limits FM pre-emphasis to no greater than the characteristic of a network with a 75 μs time constant.
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These are regional conventions, not interchangeable receiver settings. The historical FCC measurement document describes a 75 μs de-emphasis convention for relevant station-response, distortion and noise measurements; the current US pre-emphasis limit is stated in the eCFR rule.
What happens when the settings do not match
The transmitter and receiver curves no longer cancel when their time constants differ. The combined response leaves either excess treble or reduced treble in the recovered audio:
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| Transmitter pre-emphasis | Receiver de-emphasis | Resulting tonal balance |
|---|---|---|
| 75 μs | 50 μs | Residual treble lift |
| 50 μs | 75 μs | Residual treble reduction |
This direction follows from the single-pole RC curves: the longer time constant produces a stronger rise on the transmit side and a stronger fall on the receive side over the audio band. The mismatch changes tonal balance; the standards cited here do not establish a universal decibel figure for the noise reduction or the size of an audible mismatch.
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How to check a receiver that sounds too bright or dull
- Identify the station’s standard. Check reliable station or broadcast-system information for its pre-emphasis time constant; the regional conventions above are useful context, not a complete country-by-country list.
- Check the receiver’s de-emphasis setting. If it is selectable, compare the setting with the station’s time constant. The transmitter and receiver should use the same value.
- Consider the receiver’s intended market. A fixed setting may reflect the convention for the market where the receiver was designed to be used. Do not assume that every receiver exposes a user-adjustable option.
- Reassess the sound after matching. A 75 μs station heard through 50 μs de-emphasis can sound too bright; a 50 μs station heard through 75 μs de-emphasis can sound too subdued. Other causes of tonal imbalance may still need separate diagnosis.
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