si4704 Silicon Laboratories, si4704 Datasheet - Page 21

• Manufacturer Part Number: si4704
• Description: Broadcast Fm Radio Receiver For Consumer Electronics
• Manufacturer: Silicon Laboratories

4.5. Stereo Audio Processing
The output of the FM demodulator is a stereo
multiplexed (MPX) signal. The MPX standard was
developed in 1961, and is used worldwide. Today's
MPX signal format consists of left + right (L+R) audio,
left – right (L–R) audio, a 19 kHz pilot tone, and
RDS/RBDS data as shown in Figure 13 below.
4.5.1. Stereo Decoder
The
automatically decodes the MPX signal using DSP
techniques. The 0 to 15 kHz (L+R) signal is the mono
output of the FM tuner. Stereo is generated from the
(L+R), (L–R), and a 19 kHz pilot tone. The pilot tone is
used as a reference to recover the (L–R) signal. Output
left and right channels are obtained by adding and
subtracting the (L+R) and (L–R) signals respectively.
The Si4705 uses frequency information from the 19 kHz
stereo pilot to recover the 57 kHz RDS/RBDS signal.
4.5.2. Stereo-Mono Blending
Adaptive noise suppression is employed to gradually
combine the stereo left and right audio channels to a
mono (L+R) audio signal as the signal quality degrades
to maintain optimum sound fidelity under varying
reception conditions. Stereo/mono status can be
monitored with the FM_RSQ_STATUS command. Mono
operation
FM_BLEND_MONO_THRESHOLD property.
0
Mono Audio
Left + Right
Si4704/05's
Figure 13. MPX Signal Spectrum
can
15
Stereo
Pilot
19
Frequency (kHz)
23
integrated
be
Stereo Audio
Left - Right
forced
38
stereo
with
53
decoder
RBDS
RDS/
57
the
Rev. 1.0
4.6. De-emphasis
Pre-emphasis and de-emphasis is a technique used by
FM broadcasters to improve the signal-to-noise ratio of
FM receivers by reducing the effects of high-frequency
interference and noise. When the FM signal is
transmitted, a pre-emphasis filter is applied to
accentuate the high audio frequencies. The Si4704/05
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.
4.7. 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.
4.8. Soft Mute
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in very weak signal
conditions. The softmute attenuation level is adjustable
using
property.
4.9. RDS/RBDS Processor (Si4705 Only)
The Si4705 implements an RDS/RBDS* processor for
symbol
detection, and error correction.
The Si4705 device is user configurable and provides an
optional interrupt when RDS is synchronized, loses
synchronization, and/or the user configurable RDS
FIFO threshold has been met.
The Si4705 reports RDS decoder synchronization
status and detailed bit errors in the information word for
each RDS block with the FM_RDS_STATUS command.
The range of reportable block errors is 0, 1–2, 3–5, or
6+.
corresponding block information word contains six or
more non-correctable errors or that the block checkword
contains errors.
*Note: RDS/RBDS is referred to only as RDS throughout the
More
remainder of this document.
the
decoding,
than
FM_SOFT_MUTE_MAX_ATTENUATION
six
Si4704/05-C40
block
errors
synchronization,
indicates
that
error
the
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