Number of integer bits needed for fixed-point inner product
innerprodintbits(a,b)
innerprodintbits(a,b) computes the minimum
number of integer bits necessary in the inner product of a'*b to
guarantee that no overflows occur and to preserve best precision.
a and b are fi vectors.
The values of a are known.
Only the numeric type of b is relevant.
The values of b are ignored.
The primary use of this function is to determine the number
of integer bits necessary in the output Y of an
FIR filter that computes the inner product between constant coefficient
row vector B and state column vector Z.
For example,
for k=1:length(X);
Z = [X(k);Z(1:end-1)];
Y(k) = B * Z;
end
In general, an inner product grows log2(n) bits
for vectors of length n. However, in the case of
this function the vector a is known and its values
do not change. This knowledge is used to compute the smallest number
of integer bits that are necessary in the output to guarantee that
no overflow will occur.
The largest gain occurs when the vector b has
the same sign as the constant vector a. Therefore,
the largest gain due to the vector a is a*sign(a'),
which is equal to sum(abs(a)).
The overall number of integer bits necessary to guarantee that no overflow occurs in the inner product is computed by:
n = ceil(log2(sum(abs(a)))) + number of integer bits in b + 1 sign bit
The extra sign bit is only added if both a and b are
signed and b attains its minimum. This prevents
overflow in the event of (-1)*(-1).