L64777 LSI Logic Corporation, L64777 Datasheet - Page 37

• Manufacturer Part Number: L64777
• Description: DVB Qam Modulator
• Manufacturer: LSI Logic Corporation

2.8 Convolutional Interleaver
Figure 2.15 is a block diagram of a convolutional interleaver system,
which rearranges the ordering of a sequence of symbols in a
deterministic manner. A (B, N) periodic interleaver has the following
characteristics:
The scheme is also referred to as a convolutional interleaver/
deinterleaver (based on the Forney approach).
Figure 2.15 Interleaver Block Diagram
Encoder
The L64777 interleaver performs periodic interleaving with two fixed
parameters: B, the desired interleaving depth, and M, defined as:
The values of the interleaver in the L64777 are: N = 204, B = 12, and
M = 17. You can switch off the interleaver. It is fully transparent with an
intrinsic delay of three clock cycles.
The main modules are a set of configured RAM-based delay lines to
implement the proper delay for individual data bytes, and a controller to
handle and generate the strobes needed by subsequent modules in the
data path.
Convolutional Interleaver
The minimum separation at the interleaver output is B symbols for
any two symbols that are separated by less than N symbols at the
interleaver input.
Any burst of b < B errors inserted by the channel results in single
errors at the deinterleaver output.
M
(B
2M
1)M
T
M
=
CHANNEL
N
--- -
B
(B
M
2M
1)M
Decoder
2-23
T