Re: working with fixed point

[Per Persson]

On Monday, August 25, 2003, at 11:56 AM, Zdenek Hurak wrote:

> Well, I think this will not be as easy as that. Converting the data is 
> one
> step only, but Ravid obviously wants to calculate in fixed point 
> arithmetics
> too.

Hmm,
multiplying two integers will give the same result whether or not that 
multiplication was performed using integer or floting point arithmetics.
With a limited number of bits available for representation, the result 
may overflow. This will have to be handled in some way, such as 
truncating or saturating the result, in the fpga (or VHDL/Verilog code) 
as well as in an octave script.

It is quite possible to have a per-sample correspondance between the 
output of a VHDL simulation and a simulation run in octave.

The following is an example of a first order link of a wave digital 
filter. It was used to simulate the effect of varying wordlength in 
octave before moving to VHDL code and simulation.

function [y state] = qaplink1(x, alpha, B, W, state);
u = alpha*(-x+state);
##
## Quantize u
##
u = truncate(u, B, W-1);
y = u + state;
y = saturate(y, B);
state = u + x;
state = saturate(state, B);
## endfunction

> Considering that the "fixed-point skills" are by far the most expensive
> component in commercial Matlab suite, it can be expected that solution 
> is not
> trivial.

AFAIK commercial software is not normally priced by its complexity or 
the amount of time spent developing it, but rather from an estimate of 
how much that particular functionality is worth to the customer.

/Per

------------
Per Persson
Blekinge Institute of Technology
Dept. of Signal Processing and Telecommunications

www:   http://www.its.bth.se/staff/pee
e-mail: address@hidden



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