Silicon Laboratories SI4734/35-B20 Stereo Audio Processing, De-emphasis, Stereo DAC, Soft Mute

Mono Audio Left + Right

Modulation Level

	Si4734/35-B20
4.8. Stereo Audio Processing	4.9. De-emphasis
The output of the FM demodulator is a stereo	Pre-emphasis and de-emphasis is a technique used by
multiplexed (MPX) signal. The MPX standard was	FM broadcasters to improve the signal-to-noise ratio of
developed in 1961, and is used worldwide. Today's	FM receivers by reducing the effects of high-frequency
MPX signal format consists of left + right (L+R) audio,	interference and noise. When the FM signal is
left – right (L–R) audio, a 19 kHz pilot tone, and	transmitted, a pre-emphasis filter is applied to
RDS/RBDS data as shown in Figure 13.	accentuate the high audio frequencies. The Si4734/35
	incorporates a de-emphasis filter which attenuates high
	frequencies to restore a flat frequency response. Two
	time constants are used in various regions. The de-
	emphasis time constant is programmable to 50 or 75 µs
	and is set by the FM_DEEMPHASIS property.

	Stereo			Stereo Audio		RDS/
		Pilot
				Left - Right		RBDS
0	15	19	23	38	53	57

Frequency (kHz)

4.10. Stereo DAC

High-fidelity stereo digital-to-analog converters (DACs) drive analog audio signals onto the LOUT and ROUT pins. The audio output may be muted. Volume is adjusted digitally with the RX_VOLUME property.

Figure 13. MPX Signal Spectrum

4.11. Soft Mute

4.8.1. Stereo Decoder

The soft mute feature is available to attenuate the audio

The Si4734/35's integrated stereo decoder

outputs and minimize audible noise in very weak signal

automatically decodes the MPX signal using DSP

conditions. The softmute attenuation level is adjustable

techniques. The 0 to 15 kHz (L+R) signal is the mono

using the FM_SOFT_MUTE_MAX_ATTENUATION and

output of the FM tuner. Stereo is generated from the

AM_SOFT_MUTE_MAX_ATTENUATION properties.

(L+R), (L–R), and a 19 kHz pilot tone. The pilot tone is

4.12. RDS/RBDS Processor (Si4735 Only)

used as a reference to recover the (L–R) signal. Output

The Si4735 implements an RDS/RBDS* processor for

left and

right channels are obtained

by adding

and

symbol

decoding,

block

synchronization,

error

subtracting the (L+R) and (L–R) signals respectively.

detection, and error correction.

The Si4735 uses frequency information from the 19 kHz

stereo pilot to recover the 57 kHz RDS/RBDS signal.

The Si4735 device is user configurable and provides an

4.8.2. Stereo-Mono Blending

optional

interrupt

when

RDS

is

synchronized,

loses

synchronization, and/or the user configurable RDS

Adaptive noise suppression is employed to gradually

FIFO threshold has been met.

combine the stereo left and right audio channels to a

The Si4735 reports RDS decoder synchronization

mono (L+R) audio signal as the signal quality degrades

status, and detailed bit errors in the information word for

to maintain optimum

sound

fidelity

under

varying

each RDS block with the FM_RDS_STATUS command.

reception

conditions.

Stereo/mono

status

can

be

The range of reportable block errors is 0, 1–2, 3–5, or

monitored with the FM_RSQ_STATUS command. Mono

6+.

More than

six

errors

indicates

that

the

operation

can

be

forced

with

the

corresponding block

information word contains six or

FM_BLEND_MONO_THRESHOLD property.

more

non-correctable

errors,

or that

the

block

checkword contains errors.

*Note: RDS/RBDS is referred to only as RDS throughout the remainder of this document.

Rev. 1.0 23