2000 May 19

4

Philips Semiconductors

Objective specification

ATSC Digital Terrestrial TV

demodulator/decoder

TDA8961

The incoming data has a sample rate of 36 MHz. This is

internally converted to a sample rate of 21.52 MHz which

is twice the 8-VSB symbol rate. This patented Philips

Semiconductors’ technology eliminates the need for

external symbol timing recovery loop components. When

the TDA8980 is decoding analog NTSC, the TDA8961 can

be set to a ITU656 bypass mode which allows the 8-bit

ITU656 data output from the TDA8980 to appear at the

TDA8961 Moving Pictures Expert Group (MPEG-2)

transport stream output, from where it can be fed to the

main graphics display device in the system.

The recovery of the carrier is performed entirely within the

TDA8961. This function consists of a digital frequency and

Frequency Phase-Locked Loop (FPLL).

Data shaping is performed by a square-root raised-cosine

(half Nyquist) filter having a roll-off factor of 11.5%.

After the TDA8961 has performed carrier recovery,

Nyquist filtering and symbol timing recovery, it then

performs adaptive equalization. The adaptive equalizer

uses a DFE structure with equalization based on the ATSC

field sync (trained equalization) and/or the 8-VSB data

itself (blind equalization).

The equalizer is followed by a patented NTSC co-channel

interference filter which removes any unwanted NTSC

signal interference from the 8-VSB terrestrial DTV signal.

After trellis decoding, the stream is de-interleaved to a

depth of 52 by a convolutional de-interleaver whose

memory is provided on-chip. The Reed Solomon decoder

is ATSC-compliant, with a length of 207, and able to

correct up to 10 bytes. The decoded stream is then

de-randomized using a pseudo-random binary sequence

(PRBS) and the data passed to a FIFO which prevents the

appearance of irregular gaps in the output data.

The output of the TDA8961 is a clock signal and an

ATSC-compliant MPEG-2 packetized data stream. Signal

flag outputs are provided to indicate the occurrence of

sync bytes, valid data bytes and uncorrected Reed

Solomon blocks. The packetized data stream is available

in either an 8-bit parallel, or a 1-bit serial format for

connection to an MPEG-2 transport stream demultiplexer.

An application using the TDA8961 and a stand-alone

TDA9829 Downconverter for DVB (Digital Video

Broadcast) with an A/D converter is shown in Fig.2. A tuner

converts the incoming RF signal to a fixed IF centered at

44 MHz. The output signal from the tuner is filtered using

two Surface Acoustical Wave (SAW) filters and then down

converted to an IF of 4 MHz by the TDA9829. The signal

is then digitized by an A/D converter at a sample rate of

36 MHz using the clock signal output from the TDA8961.

The full input range of the A/D converter is utilized by

placing it within what is effectively a fine-AGC loop

integrator circuit which has a variable gain stage at the

output of the IF downmixer section. However, it is also

possible to apply the AGC control output of the TDA8961

to the tuner via the integrator. The peak level of the input

signals to the TDA8961 is determined by the AGC output

detector which is located just after the A/D.

handbook, full pagewidth

TDA9829

IF DOWNMIXER

TDA8961

VoDVB

RF

ViIF

Vref

CLK36

ADCLK

ADIN9 to

ADIN0

MPEG-2

transport

stream

PDO7 to

PDO0

VAGC

integrator

AGCOUT

AGC

44 MHz

96 MHz

TUNER RF

VCO

12 MHz

XTALI

SAW

FILTER

OSCILLATOR

A

D

MGU086

SAW

FILTER

Fig.2 Front-end design for the TDA8961 using a stand-alone IF down converter (TDA9829) and A/D converter.