User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 24/76
The benefit of working with two orthogonal carriers is: the possibility to implement corrections on non-
linear distortions, or simply implement digital pre-distortion. The digital pre-correction is possible using
LUT (Look up Tables); refer to Section 9 for more details. This table synthesizes inverted responses
regarding the RF power amplifier transfer function, reducing the IMD products.
The digital processing generates I and Q distorted, and these two signals are then converted analog via a
16 bits DAC, with two balanced outputs, with a total of four output analog signals. These signals are
termed; I, I’, Q and Q’, and are respectively present at the CN9, CN8, CN6, and CN5 connectors on
CIM3444.
These four signals will become the UHF up-converter - Module 4453 - input signals. After the data
stream is processed to receive the channel coding. This process contains the following steps:
Frame synchronization
For each 188 bytes on the MPEG2 package, this circuit identifies and removes the 47th byte.
Randomizer
This circuit equally spreads the modulated signal’s energy over the channel band. The final energy
density is similar to an Additive White Gaussian Noise. ; This technique greatly improves bandwidth
usage efficiency.
Reed-Solomon
Reed-Solomon is a block coder (207,187) that adds 20 redundant bytes on each 187 bytes of the MPEG2
package. With this method it is possible to detect and correct data errors that occur during the
transmission process.
Interleaving
Data stream interleaving is utilized to lower the transmissions susceptibility to interference that causes “burst data
errors.”
Trellis Code
Trellis code is closely related to the channel modulation. It is a convolution coder utilizing 2/3 rate, i.e.
for each 2 bits at the input, there are 3 coded bits at the output, creating the 8 symbols used on the 8VSB
modulation process (-7, -5, -3, -1, +1, +3, +5, +7). The encoding effectively improves the ability of the
data stream to withstand degradation during transmission effectively increasing the signal-to-noise ratio.
Synchronism Insertion
The synchronism insertion builds the data stream’s overall structure, creating the fields and frames as
specified on the A/53E standard.
Pilot Insertion
The low-level pilot is created by adding a DC (+1.25) value to the baseband data (data and Sync) . After
modulation, the DC value causes an in-phase pilot to be added to the data spectrum for transmission. The low-
level pilot is a constant RF level below the average data signal power and has a frequency offset from lower band
edge. The low-level pilot aids carrier recovery and is independent of the data.
Master Clock – CIP 8358 board – (CIM 3445)
Introduction
The CIP8358 board embedded into the module 4549, uses a 10MHz internal/external reference signal, to
synthesize a 172.16MHz signal that is 16 times the symbol rate used in the modulation process.