7
Pipelining, Retiming, Look-ahead Transformation and Polyphase Decomposition
7.1 Introduction
The chapter discusses pipelining, retiming and look-ahead techniques for transforming digital designs to meet the desired objectives. Broadly, signal processing systems can be classified as feed forward or feedback systems. In feed forward systems the data flows from input to output and no value in the system is fed back in a recursive loop. Finite impulse response (FIR) filters are feed forward systems and are fundamental to signal processing. Most of the signal processing algorithms such as fast Fourier transform (FFT) and discrete cosine transform (DCT) are feed forward. The timing can be improved by simply adding multiple stages of pipelining in the hardware design.
Recursive systems such as infinite impulse response (IIR) filters are also widely used in DSP. The feedback recursive algorithms are used for timing, symbol and frequency recovery in digital communication receivers. In speech processing and signal modeling, autoregressive moving average (ARMA) and autoregressive (AR) processes involve IIR systems. These systems are characterized by a difference equation . To compute an output sample, the equation directly or indirectly involves previous values of the output samples along with the current and previous values of input samples. As the previous output samples are required in the computation, adding pipeline registers to improve timing is not directly available to the designer. ...
