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11. FM Transmitter
The FM Transmitter audio signal chain involves Audio Dynamic Range Control, Pre-emphasis and Limiter function.
Understanding what these three function blocks do in the signal chain will help user in maximizing the volume out
of the FM Transmitter.
11.1. Audio Dynamic Range Control for FM Transmitter
The audio dynamic range control can be used to reduce the dynamic range of the audio signal. Audio dynamic
range reduction increases the transmit volume by decreasing the peak amplitudes of audio signals and increasing
the root mean square content of the audio signal. In other words, it amplifies signals below the threshold by a fixed
gain and compresses audio signals above the threshold by the ratio of Threshold/(Gain + Threshold). Figure 19
shows an example transfer function of an audio dynamic range controller with the threshold set at –40 dBFS and a
Gain = 20 dB relative to an uncompressed transfer function.
Figure 19. Audio Dynamic Range Transfer Function
For input signals below the threshold of –40 dBFS, the output signal is amplified or gained up by 20 dB relative to
an uncompressed signal. Audio inputs above the threshold are compressed by a 2 to 1 dB ratio, meaning that
every 2 dB increase in audio input level above the threshold results in an audio output increase of 1 dB. In this
example, the input dynamic range of 90 dB is reduced to an output dynamic range of 70 dB. The FM Transmitter
includes digital audio dynamic range control with programmable gain, threshold, attack rate, and release rate. The
total dynamic range reduction is set by the gain value and the audio output compression above the threshold is
equal to Threshold/(Gain + Threshold) in dB. The gain specified cannot be larger than the absolute value of the
threshold. This feature can also be disabled if audio compression is not desired. Figure 20 shows the time domain
characteristics of the audio dynamic range controller. The attack rate sets the speed with which the audio dynamic
range controller responds to changes in the input level, and the release rate sets the speed with which the audio
dynamic range controller returns to no compression once the audio input level drops below the threshold. When
using the audio dynamic range control, care must be taken to configure the device such that the sum of the
threshold and gain is zero, or less, as not to distort or overmodulate.
0
–10
–20
–30
–40
–50
–60
Input [dBFS]
–70
Ou
tp
ut
[dB
FS
]
Threshold
= –40 dB
M = 1
–80
–10
–20
–30
–40
–50
–60
–70
–80
–90
–90
M = 1
Gain
= 20 dB
Compression
2:1 dB
No
Compression
