SI4720-B20-GMR Datasheet, PDF (32/48 Pages) Silicon Laboratories – BROADCAST FM RADIO TRANSCEIVER FOR PORTABLE APPLICATIONS

Si4720/21-B20
LIATTEN[1:0] code still needs to satisfy the condition of
being just greater than the attenuated voltage. In this
example, a line attenuation code of LIATTEN[1:0] = 11
has a Peak Input Voltage of 636 mV, which is just
greater than the expected peak attenuated voltage of
600 mV. Also, the expected peak attenuated voltage is
entered into the LILEVEL[9:0] parameter. Again, in this
example, 600 mV is entered into LILVEVEL[9:0]. This
example shows one possible solution, but many other
solutions exist. The optimal solution is to apply the
largest possible voltage to the LIN and RIN pins for
signal-to-noise considerations; however, practical
resistor values may limit the choices.
Note that the TX_LINE_INPUT_LEVEL parameter will
affect the high-pass filter characteristics of the ac-
coupling capacitors and the resistance of the audio
inputs.
The Si4720/21 has a programmable low audio level and
high audio level indicators that allows the user to
selectively enable and disable the carrier based on the
presence of audio content. The TX_ASQ_LEVEL_LOW
and TX_ASQ_LEVEL_HIGH parameters set the low
level and high level thresholds in dBFS, respectively.
The time required for the audio level to be below the low
threshold is set with the TX_ASQ_DURATION_LOW
parameter, and similarly, the time required for the audio
level to be above the high threshold is set with the
TX_ASQ_DURATION_HIGH parameter.
5.18. Audio Dynamic Range Control
The Si4720/21 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.
The audio dynamic range control can be used to reduce
the dynamic range of the audio signal, which improves
the listening experience on the FM receiver. 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 a
threshold by a fixed gain and compresses audio signals
above a threshold by the ratio of
Threshold/(Gain + Threshold). Figure 24 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.
Input [dBFS]
–90 –80 –70 –60 –50 –40 –30 –20 –10
0
Compression
2:1 dB
–10
Threshold
= –40 dB
–20
M=1
No
–30
Compression
–40
–50
M=1
–60
Gain
= 20 dB
–70
–80
–90
Figure 24. 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.
Figure 25 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.
Threshold
Audio
Input
Audio
Output
Attack
time
Release
time
Figure 25. Time Domain Characteristics of the
Audio Dynamic Range Controller
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Rev. 1.0