TDA9874H データシートの表示（PDF） - Philips Electronics
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TDA9874H Datasheet PDF : 56 Pages
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Philips Semiconductors
Digital TV sound demodulator/decoder
Preliminary specification
TDA9874H
6 FUNCTIONAL DESCRIPTION
6.1 Description of the demodulator and decoder
section
6.1.1 SIF INPUT
Two input pins are provided. SIF1 e.g. for terrestrial TV
and SIF2 e.g. for a satellite tuner. As no specific filters are
integrated, both inputs have the same specification giving
flexibility in application. The selected signal is passed
through an AGC and then digitized by an 8-bit ADC
running at 24.576 MHz.
6.1.2 AGC
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 from a table.
The AGC can be controlled via the I2C-bus. Details can be
found in Sections 7.3.1, 7.3.2 and 7.4.6.
6.1.3 MIXER
The digitized input signal is passed on 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 I2C-bus (see Sections 7.3.4 and 7.3.5). When
receiving NICAM programs, a feedback signal is added to
the control word of the second carrier mixer to establish a
carrier-frequency loop.
6.1.4 FM AND AM DEMODULATION
An FM or AM input signal is passed through a band-limiting
filter onto 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 Wegener-Panda 1 encoded satellite
programs.
6.1.5 FM DECODING
A two-carrier stereo decoder recovers the left and right
signal channels from the demodulated sound carriers.
Both the European and Korean stereo systems are
supported.
6.1.6 FM IDENTIFICATION
The identification of the FM sound mode is performed by
AM synchronous demodulation of the pilot and
narrow-band detection of the identification frequencies.
The result is available via the I2C-bus interface. A selection
can be made via the I2C-bus for B/G, D/K and M standard,
and for three different time constants that represent
different trade-offs between speed and reliability of
identification.
6.1.7 NICAM DEMODULATION
The NICAM signal is transmitted in a DQPSK code at a bit
rate of 728 kbits/s. The NICAM demodulator performs
DQPSK demodulation and passes the resulting bitstream
and clock signal to the NICAM decoder and, for evaluation
purposes, to pins.
A timing loop controls the frequency of the crystal oscillator
to lock the sampling instants to the symbol timing of the
NICAM data. The polarity of the control signal is selectable
to support applications, in which external circuitry is used
to boost the tuning voltage of the oscillator.
6.1.8 NICAM DECODING
The device performs all decoding functions in accordance
with the “EBU NICAM 728 specification”. After locking to
the frame alignment word, the data are descrambled by
application of the defined pseudo-random binary
sequence, and the device synchronizes to the periodic
frame flag bit C0.
The status of the NICAM decoder can be read-out from the
NICAM Status Register by the user (see Section 7.4.2).
The OSB bit indicates that the decoder has locked to the
NICAM data. The VDSP bit indicates that the decoder has
locked to the NICAM data and that the data is valid sound
data. The C4 bit indicates that the sound conveyed by the
FM mono channel is identical to the sound 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) is approximately 0.0000174 times the contents of
the error byte.
BER = b-t--o-i-t-t--a-e--l-r--rb--o--i-tr--ss-- ≈ error byte × 1.74 × 10–5
1998 Apr 27
10
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