AN332
192
Confidential Rev. 0.2
Figure 20. Time Domain Characteristics of the Audio Dynamic Range Controller
11.2. Audio Pre-emphasis for FM Transmitter
Pre-emphasis and de-emphasis are techniques used to improve the signal-to-noise ratio of an FM stereo
broadcast by reducing the effects of high-frequency noise. A pre-emphasis filter is applied to the broadcast to
accentuate the high audio frequencies and a de-emphasis filter is used by the receiver to attenuate high
frequencies and restore a flat frequency response. Depending on the region, a time constant of either 50 or 75 µs
is used. The frequency response of both of these filters is shown in Figure 21. For a 75 µs filter, a 15 kHz tone is
amplified by ~17 dB. For a 50 µs filter, a 15 kHz tone is amplified by ~13.5 dB. The pre-emphasis time constant is
programmable to off, 50 or 75 µs and is setting the TX_PREEMPHASIS property. When using the pre-emphasis
filter, care must be taken to account for amplification at high frequencies as not to distort or overmodulate.
Figure 21. Pre-emphasis Filter Response
Attack
time
Threshold
Release
time
Audio
Input
Audio
Output
Preemphasis Filter Transfer Function
0
5
10
15
20
0
3
6
9
12
15
Frequency (kHz)
dB
75 us
50us
