PDSP16256GC(1998) データシートの表示（PDF） - Mitel Networks

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PDSP16256GC
(Rev.:1998)

Programmable FIR Filter

Mitel Networks 

FILTER ACCURACY

Input data and coefficients are both represented by 16-bit

two’s complement numbers. The coefficients are converted to

twelve bits by rounding towards zero. This is achieved as

follows. If the coefficient is positive then the least significant

4 bits are discarded. If the coefficient is negative then the

logical ‘OR’ of the least significant 4 bits are added to the

remainder of the word. Twelve bit coefficients can be used

directly provided the least significant four bits are set to zero.

The FIR filter results are calculated using a multiplier

accumulator structure as shown in Fig. 10. The truncation and

word growth allowed for in the data path are explained in

Fig. 10. The 16-bit data and 12-bit coefficient inputs (each with

one sign bit before the binary point), are presented to the

multiplier. This produces a 28-bit result with two bits before the

binary point. Producing the full 28-bit result ensures that if both

the data and coefficients are set to logic 1 a valid result is

generated. Prior to entering the accumulator the least signifi-

cant 4 bits of the multiplier result are truncated and the resulting

24 bits sign extended to 32 bits. The final accumulator result is

32 bits with 10 bits before the binary point. Thus 9 bits of word

growth are allowed within the accumulator. All accumulator

bits are made available on the output pins.

In cascade mode the middle 16 bits from the network A

accumulator are fed round to the network B data inputs, see

Fig. 11.

PDSP16256

INPUT DATA

COEFFICIENT

ADDER

ACCUMULATOR

RESULT

Fig. 10 Multiplier Accumulator

INPUT DATA S -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15

COEFFICIENT S -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11

Multiplication producing a 28-bit result

MULTIPLIER RESULT S 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

ACCUMULATOR RESULT

Sign extended to 32 bits, least significant 4 bits truncated

S S S S S S S S S 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

ACCUMULATOR RESULT

S 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

These bits are passed to filter network B during cascade mode

-22 -23 -24 -25 -26

-22

-22

Fig. 11 Filter accuracy

9

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