L64704 LSI Logic Corporation, L64704 Datasheet - Page 151

• Manufacturer Part Number: L64704
• Description: Satellite Decoder Technical Manual 5/97
• Manufacturer: LSI Logic Corporation

Table 7.2
Viterbi Threshold
Values
depunctured channel stream. Any discrepancy between two respective
symbols indicates a corrected error (or with a much smaller probability,
an erroneous output bit produced by a failure of the Viterbi decoder to
decode correctly). For code rates other than rate equal to 1/2, the
decoder disregards input symbols marked as erasures when it detects
the error events.
The decoder processes the bit error events further to produce a measure
of the actual channel bit error rate. The register VMDC2 (Group 4, APR
5:7) specifies the number of channel bits (and therefore the time period),
divided by four, over which the decoder counts the number of occurring
channel bit errors. Refer to Group 4, APR 5:7 or
tion of VMDC2.
The channel data error counter accumulates the errors internally and
updates the Viterbi Bit Error Rate Count (VBERC, Group 3, APR 4:5)
once at the end of the period specified by VMDC2. You can read the
value of the VBERC using the microcontroller interface. VBERC contains
the number of errors divided by four. In addition, the L64704 sets the
VBER flag in the System Status Register (Group 2). This flag indicates
that the decoder has reached the period specified by VMDC2. The
decoder asserts INT if the corresponding interrupt enable bit, VBER_IE,
is set in the System Mode Register (Group 2). Refer to
“Group 2 Registers”
Typical threshold values for each puncturing rate in the case of a
standard DVB application are shown in
Puncturing
Rate
VMDC1
VMBEC
Ratio
Viterbi Decoder Module
1/2
0x40
0x1E
23.6%
on
2/3
0x40
0x0E
11.1%
page 3-11
3/4
0x40
0x0A
8.0%
for details.
Table
5/6
0x40
0x07
5.6%
7.2:
page 3-31
7/8
0x40
0x05
4.1%
Section 3.4,
for a descrip-
7-7