SAA7284GP Philips Semiconductors, SAA7284GP Datasheet - Page 9

• Manufacturer Part Number: SAA7284GP
• Description: Terrestrial digital sound decoder for conventional intercarrier PLL-IF systems
• Manufacturer: Philips Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
DQPSK demodulation
Q
GAIN CONTROL
The DQPSK signal is fed into two differential input mixers,
where it is mixed with quadrature phases generated by the
carrier-loop quadrature VCO. The quadrature signals
modulated onto the NICAM carrier are thus recovered.
The mixers can be driven by either a single-ended or
differential source. In single-ended mode, the device is
driven directly from the sound IF down-converter into the
DQPSK input pin, with the MIXREF pin decoupled.
In differential mode, the signal is applied between the
DQPSK and MIXREF pins.
The outputs from the mixers are then fed into a
pulse-shaping filter, and FM/vision filter stage with
improved colour rejection to allow suitable performance
with SAW filters. The signal from the filtering stages is then
fed into the AGC, which ensures that the phase
comparator gain remains constant, irrespective of the
input signal level. This is important to maintain the stability
of Costas loop PLL.
AGC
The AGC controller monitors the I and Q channel signals
at the input to the carrier loop-phase comparator and
generates a reference voltage to set the AGC output level.
E
A differential to the single-ended converter provides the
baseband signal as an output at the pins CEYE and SEYE
for the I and Q channels respectively for eye-height
monitoring.
B
The I and Q channels are processed using edge detectors
and monostables, which generate a signal with a coherent
component at the data symbol rate. The outputs from the
I and Q channel monostables are each compared with the
clock derived from PCLK (364 kHz nominal), the resultant
output is used to derive a 3-state control signal used to
control two current sources at the CLKLPF output.
This error signal is loop filtered and used to control the
master clock oscillator. The bit rate clock, PCLK, and
symbol clock are derived from the master clock.
1996 Oct 24
YE BUFFER
IT RATE CLOCK RECOVERY
UADRATURE MIXERS
Terrestrial digital sound decoder for
conventional intercarrier PLL-IF systems
CONTROLLER
(AGC)
,
BASEBAND FILTERS AND AUTOMATIC
9
NICAM 728 decoding
D
The device performs all decoding functions in accordance
with the EBU NICAM 728 specification. After locking to the
frame alignment word, the data is de-scrambled by
application of the defined pseudo random binary
sequence, and the device synchronizes to the periodic
frame flag bit C0.
The relevant control information and scale factor word is
extracted, and with the integrated RAM the data is
de-interleaved and the scale factor word is extracted, and
expanded to 14 bits. Parity checking on the eleventh bit of
each sample word is carried out to reveal any sound
sample errors, which if detected are flagged, with the last
good sample being held.
Automatic muting
Enable when AMDIS = LOW. The I
registers which define an upper and lower limit for the
automatic muting function. When the number of errors
within a 128 ms period exceeds the number stored in the
upper error limit register, then the automatic muting will
switch the device output to the FM input, (dependent on
the relevant control bits in the I
zero) the data input to the filter (in that order). When the
error count in a 128 ms period is less than the value stored
in the lower error limit register then the data into the filter
is uninterrupted, and the device output is switched back to
the DAC (dependent on the value of the relevant control
bits in the I
open-drain pin MUTE is pulled LOW and the AM bit in the
status-byte is set HIGH. Figure 4 shows the dependency
of the automatic muting function on error_count, RSSF,
C4OV, output state and application mode. The automatic
muting function, if enabled, will override user mute via the
MUTE pin/bit.
When the transmission is DATA format or currently
undefined format (C3 = logic 1) the device will
automatically switch to the FM inputs regardless of
RSSF/C4OV states, and whether the automatic muting
function AMDIS is enabled or disabled.
ECODING FUNCTIONS
2
C-bus). During the muting operation the
2
C-bus) and mute (set to
Preliminary specification
2
C-bus section has two
SAA7284