SI4713-B30-GMR【PDF 26/42 页】IC TX FM RADIO RPS/RDS 20UQFN

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Si4712/13-B30

Rev. 1.1

5.6. Audio Dynamic Range Control

The Si4712/13 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

equal

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 18 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 18. 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 19 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.

Figure 19. Time Domain Characteristics of the

Audio Dynamic Range Controller

5.7. Audio Limiter

The 4712/13 also includes a digital audio limiter. The

audio limiter prevents over-modulation of the FM

transmit output by dynamically attenuating peaks in the

audio input signal that exceed a programmable

threshold. The limiter threshold is set to the

programmed audio deviation + ten percent. The

threshold ensures that the output signal audio deviation

does not exceed the programmed levels, avoiding

audible artifacts or distortion in the target FM receiver,

and complying with FCC or ETSI regulatory standards.

The limiter performs as a peak detector with an attack

rate set to one audio sample, resulting in an almost

immediate attenuation of the input peak. The recover

rate is programmable to the customers preference, and

is set by default to 5 ms. This is the recommended

setting to avoid audible pumping or popping. Refer to

AN332: Universal Programming Guide.

Input [dBFS]

Threshold

= 40 dB

M = 1

Gain

= 20 dB

Compression

2:1 dB

Compression

Attack

time

Threshold

Release

time

Audio

Input

Audio

Output

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