MC92314 Datasheet, PDF (45/100 Pages) Motorola, Inc – DVB-T Single Chip Demodulator Application Note

Freescale Semiconductor, Inc.
Device Description
3.2.3.5.4 Reed-Solomon Bit Error and Bad Frame Monitor
There are two parameters accessible through the I²C Interface that the Reed-Solomon decoder
circuit uses to track error rates: BER_COUNT and BAD_FRAME.
BAD_FRAME
This parameter gives the number of bad frames that could not be decoded and corrected during
an interval of frames specified through TIME_COUNT (another I²C Interface parameter register).
BER_COUNT
BER_COUNT is the number of bit errors within the 188 information bytes during the same
interval of frames specified by TIME_COUNT. Hence, in order to determine a bit error rate, one
Codeword should be counted as 188 bytes instead of 204 bytes. If more than 8 byte errors occur
n _{in a frame, BER_COUNT cannot be updated since it is not possible to determine how many bits}
tio ^{were corrupted. To obtain a better estimate of the BER rate into the Reed-Solomon decoder}
block when more than 8 bytes are corrupted, BAD_FRAME and BER_COUNT should be
a _combined.
rm _{TIME_COUNT}
fo _{The parameter TIME_COUNT specifies the number of Codewords during which the bit errors}
In ^{and bad frames are counted (note that a frame is used here to denote a Codeword of 204 bytes).}
The number of Codewords is given by (TIME_COUNT * 4) + 2. In addition, BER_COUNT and
y _{BAD_FRAMES are updated every (TIME_COUNT * 4) + 2 Codewords only. Internally, the}
r _{corresponding counters are reset and immediately work on the following (TIME_COUNT * 4) + 2}
a _{window. Both counters have overflow protection; therefore, once the maximum value is reached,}
in _{it will remain stable throughout the entire period.}
lim ^{As an example, consider the calculation of the post-Viterbi BER using these registers. In the}
default configuration TIME_COUNT contains 255, resulting in a number of (255 * 4 + 2) = 1022
Pre ^{MPEG-2 packets for the update period of the BAD_FRAME and BER_COUNT registers.}
After reading both values immediately one after the other to ensure consistency of the results,
first check the BAD_FRAME. If it contains zero there were not more than 8 wrong bytes in all the
MPEG-2 packets watched in the update period completed before the read-out. The exact
number of bit errors detected and corrected by the Reed-Solomon decoder is therefore given in
the BER_COUNT register.
To calculate the BER after the Viterbi decoder use the formula
BER_COUNT / (188 * 8 (TIME_COUNT * 4 + 2))
with the number of wrong bits in the numerator and the total number of bits (188 bytes per
MPEG-2 packet) in the denominator.
If the BAD_FRAME is not zero there was at least one packet with more than 8 wrong bytes
leading to a not correctable packet. This prevents the BER_COUNT from being updated
Single Chip DVB-T Demodulator - Rev. 1.3 (11/30/98)
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