TDA9875A Philips Semiconductors, TDA9875A Datasheet - Page 11

• Manufacturer Part Number: TDA9875A
• Description: Digital TV Sound Processor DTVSP
• Manufacturer: Philips Semiconductors

Philips Semiconductors
6
6.1
6.1.1
Two input pins are provided: SIF1 e.g. for terrestrial TV
and SIF2 e.g. for a satellite tuner. For higher SIF signal
levels the SIF input can be attenuated with an internal
switchable 10 dB resistor divider. As no specific filters are
integrated, both inputs have the same specification giving
flexibility in application. The selected signal is passed
through an AGC circuit and then digitized by an 8-bit ADC
operating at 24.576 MHz.
6.1.2
The gain of the AGC amplifier is controlled from the ADC
output by means of a digital control loop employing
hysteresis. The AGC has a fast attack behaviour to
prevent ADC overloads and a slow decay behaviour to
prevent AGC oscillations. For AM demodulation the AGC
must be switched off. When switched off, the control loop
is reset and fixed gain settings can be chosen
(see Table 15).
The AGC can be controlled via the I
found in the I
6.1.3
The digitized input signal is fed to the mixers, which mix
one or both input sound carriers down to zero IF. A 24-bit
control word for each carrier sets the required frequency.
Access to the mixer control word registers is via the
I
signal is added to the control word of the second carrier
mixer to establish a carrier-frequency loop.
6.1.4
An FM or AM input signal is fed via a band-limiting filter to
a demodulator that can be used for either FM or AM
demodulation. Apart from the standard (fixed)
de-emphasis characteristic, an adaptive de-emphasis is
available for encoded satellite programs. A stereo decoder
recovers the left and right signal channels from the
demodulated sound carriers. Both the European and
Korean stereo systems are supported.
1999 Dec 20
2
C-bus. When receiving NICAM programs, a feedback
Digital TV Sound Processor (DTVSP)
FUNCTIONAL DESCRIPTION
Demodulator and decoder section
SIF
AGC
M
FM
IXER
AND
2
INPUT
C-bus register definitions (see Chapter 10).
AM
DEMODULATION
2
C-bus. Details can be
11
6.1.5
The identification of the FM sound mode is performed by
AM synchronous demodulation of the pilot signal and
narrow-band detection of the identification frequencies.
The result is available via the I
can be made via the I
and for three different modes that represent different
trade-offs between speed and reliability of identification.
6.1.6
The NICAM signal is transmitted in a DQPSK code at a bit
rate of 728 kbit/s. The NICAM demodulator performs
DQPSK demodulation and feeds the resulting bitstream
and clock signal onto the NICAM decoder and, for
evaluation purposes, to pins PCLK and NICAM.
A timing loop controls the frequency of the crystal oscillator
to lock the sampling rate to the symbol timing of the
NICAM data.
6.1.7
The device performs all decoding functions in accordance
with the “EBU NICAM 728 specification” . After locking to
the frame alignment word, the data is descrambled by
applying the defined pseudo-random binary sequence and
the device will then synchronize to the periodic frame flag
bit C0.
Bit VDSP (see Section 10.4.1) indicates that the decoder
has locked to the NICAM data and that the data is valid
sound data.
The status of the NICAM decoder can be read out from the
NICAM status register by the user (see Section 10.4.2).
Bit OSB indicates that the decoder has locked to the
NICAM data. Bit C4 indicates that the sound conveyed by
the FM mono channel is identical to the sound signal
conveyed by the NICAM channel.
The error byte contains the number of sound sample
errors, resulting from parity checking, that occurred in the
past 128 ms period. The Bit Error Rate (BER) can be
calculated using the following equation:
BER
=
bit errors
---------------------- -
total bits
FM
NICAM
NICAM
IDENTIFICATION
DEMODULATION
DECODER
error byte 1.74 10
2
C-bus for B/G, D/K and M standard
2
C-bus interface. A selection
Product specification
TDA9875A
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5