DSP Slide 1

1
Graph theory
identity assignment

• DSP graphs are made up of
• points
• directed lines
• special symbols
• points signals
• all the rest signal processing systems

y x
a
y a x
gain
y x and z x
adder
z x y
splitter tee connector
unit delay
y z-1 x
z x - y
2
Why is graph theory useful ?

• DSP graphs capture both
• algorithms and
• data structures
• Their meaning is purely topological
• Graphical mechanisms for simplifying (lowering
  MIPS or memory)
• Four basic transformations
• Topological (move points around)
• Commutation of filters (any two filters commute!)
• Unification of identical signals (points) and
  removal of redundant branches
• Transposition theorem

3
Basic blocks
yn xn - xn-1
yn a0 xn a1 xn-1
Explicitly draw point only when need to store
value (memory point)
4
Basic MA blocks
yn a0 xn a1 xn-1
5
General MA

• we would like to build
• but we only have 2-input adders !

tapped delay line FIFO
6
General MA (cont.)

• Instead we can build
• We still have tapped delay line FIFO (data
  structure)
• But now iteratively use basic block D (algorithm)

MACs
7
General MA (cont.)

• There are other ways to implement the same MA
• still have same FIFO (data structure)
• but now basic block is A (algorithm)
• Computation is performed in reverse
• There are yet other ways (based on other blocks)

FIFO
MACs
8
Basic AR block

• One way to implement
• Note the feedback
• Whenever there is a loop, there is recursion (AR)
• There are 4 basic blocks here too

9
General AR filters

• There are many ways to implement the general AR
• Note the FIFO on outputs
• and iteration on basic blocks

10
ARMA filters

• The straightforward implementation
• Note LM memory points
• Now we can demonstrate
• how to use graph theory
• to save memory

11
ARMA filters (cont.)

• We can commute
• the MA and AR filters
• (any 2 filters commute)
• Now that there are points representing
• the same signal !
• Assume that LM (w.o.l.g.)

12
ARMA filters (cont.)

• So we can use only one point
• And eliminate redundant branches

13
Allowed transformations

• Geometrical transformations that do no change
  topology
• Commutation of any two filters
• Unification of identical points (signals)
  and elimination of
  redundant branches
• Transposition theorem
• exchange input and output
• reverse all arrows
• replace adders with splitters
• replace splitters with adders