NOTE: THIS DOCUMENT IS OBSOLETE, PLEASE CHECK THE NEW VERSION: "Mathematics of the Discrete Fourier Transform (DFT), with Audio Applications --- Second Edition", by Julius O. Smith III, W3K Publishing, 2007, ISBN 978-0-9745607-4-8. - Copyright © 2017-09-28 by Julius O. Smith III - Center for Computer Research in Music and Acoustics (CCRMA), Stanford University
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Floating-Point Numbers
Floating-point numbers consist of an ''exponent,'' ''significand'', and ''sign bit''. For a negative number, we may set the sign bit of the floating-point word and negate the number to be encoded, leaving only nonnegative numbers to be considered. Zero is represented by all zeros, so now we need only consider positive numbers.
The basic idea of floating point encoding of a binary number is tonormalize the number by shifting the bits either left or right until the shifted result lies between 1/2 and 1. (A left-shift by one place in a binary word corresponds to multiplying by 2, while a right-shift one place corresponds to dividing by 2.) The number of bit-positions shifted to normalize the number can be recorded as a signed integer. The negative of this integer (i.e., the shift required to recover the original number) is defined as the exponent of the floating-point encoding. The normalized number between 1/2 and 1 is called the significand, so called because it holds all the ''significant bits'' of the number.
Floating point notation is exactly analogous to ''scientific notation'' for decimal numbers, e.g.,
; the number of significant digits, 5 in this example, is determined by counting digits in the ''significand''
, while the ''order of magnitude'' is determined by the power of 10 (-9 in this case). In floating-point numbers, the significand is stored in fractional two's-complement binary format, and the exponent is stored as a binary integer.
Since the significand lies in the interval
,4.11its most significant bit is always a 1, so it is not actually stored in the computer word, giving one more significant bit of precision.
Let's now restate the above a little more precisely. Let
denote a number to be encoded in floating-point, and let
denote the normalized value obtained by shifting
either
bits to the right (if
), or
bits to the left (if
). Then we have
, and
. The significand
of the floating-point representation for
.4.12 It is often the case that
bits for the significand, the encoding of
(left-shifting it
As a final practical note, exponents in floating-point formats may have abias. That is, instead of storing
where
is the bias.4.13
These days, floating-point formats generally follow the IEEE standards set out for them. A single-precision floating point word is
bits (four bytes) long, consisting of
sign bit,
exponent bits, and
significand bits, normally laid out as
where S denotes the sign bit, E an exponent bit, and M a significand bit. Note that in this layout, ordinary integer comparison can be used in the hardware.A double-precision floating point word is
bits (eight bytes) long, consisting of
exponent bits, and
significand bits. In the Intel Pentium processor, there is also an extended precisionformat, used for intermediate results, which is
bits (ten bytes) containing
exponent bits, and
for single-precision floating-point,
for double-precision, and
for extended-precision. The single and double precision formats have a ''hidden'' significand bit, while the extended precision format does not. Thus, the most significant significand bit is always set in extended precision.
The MPEG-4 audio compression standard (which supports compression using music synthesis algorithms) specifies that the numerical calculations in any MPEG-4 audio decoder should be at least as accurate as 32-bit single-precision floating point.
