MT312 查看數據表（PDF） - Zarlink Semiconductor Inc

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MT312

Satellite Channel Decoder

Zarlink Semiconductor Inc 

Functional Overview MT312

1.4 Forward Error Correction

The MT312 contains FEC blocks to enable error

correction for DVB-S and DSS transmissions. The

Viterbi decoder block can decode the convolutional

code with rates 1/2, 2/3, 3/4, 5/6, 6/7 or 7/8. The

block features automatic synchronisation, automatic

IQ phase resolution and automatic code rate

detection. The trace back depth of 128 provides

better performance at high code rates and the built-in

synchronisation algorithm allows the Viterbi decoder

to lock onto signals with very poor signal-to-noise

ratios. Viterbi bit error rate monitor provides an

indication of the error rate at QPSK output.

The 24-bit error count register in the Viterbi decoder

allows the bit error rate at the output of the QPSK

demodulator to be monitored. The 24-bit bit error

count register in the Reed-Solomon decoder allows

the Viterbi output bit error rate to be monitored. The

16-bit uncorrectable packet counter yields

information about the output packet error rate. These

three monitors and the QPSK SNR register allows

the performance of the device and its individual

components, such as the QPSK demodulator and

the Viterbi decoder, to be monitored extensively by

the external microprocessor.

The frame/byte align block features a sophisticated

synchronisation algorithm to ensure reliable recovery

of DVB and DSS framed data streams under worst

case signal conditions. The de-interleaver uses on-

chip RAM and is compatible with the DVB and DSS

algorithms.

The Reed-Solomon decoder is a truncated version of

the (255, 239) code. The code block size is 204 for

DVB and 146 for DSS. The decoder provides a count

of the number of uncorrectable blocks as well as the

number of bit errors corrected. The latter gives an

indication of the bit error rate at the output of the

Viterbi decoder.

In DVB mode, spectrum de-scrambling is performed

compatible with the DVB specification. The final

output is a parallel or serial transport data stream;

packet sync; data clock; and a block error signal. The

data clock may be inverted under software control.

1.4.1.1 Viterbi Error Count Measurement

A method of estimating the bit error rate at the output

of the QPSK block has been provided in the Viterbi

decoder. The incoming data bit stream is delayed

and compared with the re-encoded and punctured

version of the decoded bit stream to obtain a count of

errors see Figure 4 - Viterbi block diagram.

The measurement system has a programmable

register to determine the number of data bits (the

error count period) over which the count is being

recorded. A read register indicates the error count

result and an interrupt can be generated to inform

the host microprocessor that a new count is

available.

The VIT ERRPER H-M-L group of three registers is

programmed with required number of data bits (the

error count period) (VIT ERRPER[23:0]). The actual

value is four times VIT ERRPER[23:0]. The count of

errors found during this period is loaded by the

MT312 into the VIT ERRCNT H-M-L trio of registers

when the bit count VIT ERRPER[23:0] is reached. At

the same time an interrupt is generated on the IRQ

line. Setting the IE FEC[2] bit in the IE FEC register

enables the interrupt, see page 32. Reading the

register does not clear VIT ERRCNT [23:0], it is only

loaded with the error count.

VITERBI

DECODER

DATA BIT STREAM

VITERBI

ENCODER

DELAY

COMP

ERROR COUNT

Figure 4 - Viterbi block diagram

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