TDA9875 Просмотр технического описания (PDF) - Philips Electronics
Номер в каталоге
Компоненты Описание
производитель
TDA9875 Datasheet PDF : 92 Pages
| |||
Philips Semiconductors
Digital TV Sound Processor (DTVSP)
Preliminary specification
TDA9875
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 circuit and then digitized by an 8-bit ADC
operating 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
Table 12 (subaddress 0).
The AGC can be controlled via the I2C-bus. Details can be
found in the I2C-bus register definitions (see Chapter 10).
6.1.3 MIXER
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
I2C-bus. 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 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.
6.1.5 FM IDENTIFICATION
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 I2C-bus interface. A selection
can be made via the I2C-bus for B/G, D/K and M standard
and for three different modes that represent different
trade-offs between speed and reliability of identification.
6.1.6 NICAM DEMODULATION
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 PCLK (pin 1) and NICAM (pin 2).
A timing loop controls the frequency of the crystal oscillator
to lock the sampling rate 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.
6.1.7 NICAM DECODER
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;
the device will then synchronize 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 the I2C-bus
register description in Section 10.4.2). 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) can be calculated using the following equation;
BER = b-t--o-i-t-t--a-e--l-r--rb--o--i-tr--ss-- ≈ error byte × 1.74 × 10–5
1998 Feb 13
10
Share Link: 