sigma-delta modulator issues

1) Many sigma-delta modulator's simulink models include some gain blocks, whose gain value may be 0.5/1.0/2.0. How to decide whether the gain block is needed and how to decide these gain values?

P.S., The pic (SDM.jpg) is an example. The a1(a2,b1,b2,b3,c1,c2) block is what I mentioned gain block. This pic is from Richard Schreier?s manual.


2) I downloaded the free sigma-delta modulator toolbox from
h**p://www.mathworks.com/matlabcentral/fileexchange
I do as the example says(write a plot.m file which contains the lines on the right of spectrum) and find the frequency-domain plot cannot work(the time-domain plot can work if I write a plot.m file which contains the lines on the right of time-domain figure).

The matlab prompt hints:
? Attempt to execute SCRIPT plot as a function.
Error in ==> plot at 21
plot(linspace(0,1,N/2),dbv(spec(1:N/2)));

I am sure that the dbv.m exists in the same path with plot.m
P.S., The pic (Toolbox.jpg) is also from Richard Schreier?s manual.

What are your opinions?


Thank you very much.