Texas Instruments | General to Implement Logarithmic and Exponential Operations on Fixed-Point | Application notes | Texas Instruments General to Implement Logarithmic and Exponential Operations on Fixed-Point Application notes

Texas Instruments General to Implement Logarithmic and Exponential Operations on Fixed-Point Application notes
Application Report
SPRA619 - December 1999
General Guide to Implement Logarithmic and
Exponential Operations on a Fixed-Point DSP
Jason Jiang
TI (Shanghai) CAC
ABSTRACT
Modern audio and video compression algorithms usually take the advantage of logarithmic
characteristics of human ears and eyes. This approach greatly reduces the redundancy in
signals being processed. However, it poses a requirement on fixed-point DSPs to handle
these logarithmic and exponential operations. This application report provides a general
guide to implement these operations on fixed-point DSPs and sample codes based on Texas
Instruments (TI) TMS320C54x DSP.
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Implementation of Logarithmic Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3
Implementation of Exponential Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1
Introduction
Modern audio and video compression algorithms usually take the advantage of logarithmic
characteristics of human ears and eyes. This approach greatly reduces the redundancy in the
original signals. However, when implementing this processing on fixed-point DSPs, there is a
requirement to maintain enough SNR without noticeable degradation at the output. For example,
the popular MP3 algorithms achieves 12:1 compression ratio with the help of logarithmic
processing. The decoding requires the expanding operation:
Xr = is 4/3 * 2 * exp
(1)
where –85 < exp < 12, 0 <= is < 8196. Eq.1 is a typical equation. By studying the
equation, it is impossible to calculate directly on a fixed-point DSP. However, after taking a
logarithm on both sides of the equation, it is simplified to multiplication and addition that a
fixed-point DSP can handle.
Y = ln Xr = 4/3 ln is + exp * ln2
(2)
Now the problem is how to calculate (ln is) and compute the final result Xr from Y. These two
problems will be tackled in detail in the following sections and example codes will be provided.
1
SPRA619
2
Implementation of Logarithmic Operation
Now, let’s first examine the logarithmic part (ln is). According to Taylor’s Equation
ln(1–x) = – x – x2/2 – x3/3 ….– xn/n
(3)
where –1 <= x < 1, it is easy to expand the logarithmic operation to a sequence of multiply and
add operations. Since (is) is an integer, it should be scaled to an appropriate range before
using the Taylor’s equation.
is = C (1 – x)
(4)
Where C = 2N. Then it leads to
ln is = ln C + ln (1–x) = N ln2 + ln (1–x)
Experimental results show that for N=11, a good audio decompression can be achieved.
Following is the example code based on C54x DSP. The detail explanation is embedded.
.mmregs
.def log
;
;
;
;
;
;
;
This routine is used to calculate the logarithm of an integer
which is between 0 and 32768. It is programmer’s responsibility
to validate the input value and make sure DP points to the data page
of N and X.
Input:
AL = integer, AH = 0
Output: AL = fraction part in Q11 format and MSB 5bits are integer
;
part
;
AH = 0
; Register used: B, T, AR0, AR3
; Memory: 30 words
;
.data
;
;logtbl is in Q11 format. Logtbl is generated by equation 2048*n*ln2.
;This table is starting from n=15 down to n=0.
;
logtbl .int 21293, 19874, 18454, 17035, 15615, 14196, 12776
.int 11357, 9937, 8517, 7098, 5678, 4259, 2839, 1420, 0
;
;a9 is in Q15 format. It is generated by equation –32768/n. In order
;to take the advantage of instruction POLY, it is organized in reverse
;order. Which means n start from 10 down to 1. Extra 2 word memory
;space is reserved to reduce the possibilities of data over–written.
;
a9
.int –3277, –3641, –4096, –4681, –5461, –6554, –8192
.int –10923, –16384, –32768, 0, 0
.bss N, 1
;N stores scaling number
.bss X, 1
;X is temporary storage. Must
;be at the same data page of N.
.text
2
General Guide to Implement Logarithmic and Exponential Operations on a Fixed-Point DSP
(5)
SPRA619
log:
ADD
EXP
LD
#0, A, B
B
#0x4000, 16, A
ST
ANDM
T, N
#0Fh, N
MVDM
NORM
AND
BC
N, AR0
B
#0x3FFF, 16, B
taylor, BNEQ
STM
MAR
LD
RET
#logtbl+1, AR3
*AR3+0
*AR3, A
SUB
B, 0, A
STH
A, X
STM
a9, AR3
LD
X, T
LD
*AR3+, 16, A
LD
RPT
*AR3+, 16, B
#10
POLY
*AR3+
SFTA
SFTA
STM
MAR
ADD
RET
.end
A, –16
A, –4
#logtbl, AR3
*AR3+0
*AR3, A
;B=A=is
;T=leading 0s
;AH=16384, the largest
;supported scale
;store scaling number
;compensate extra 16 leading
;bits
;AR0 index to segment table
;normalize to Q15 format
;BH=BH–0x4000
;if (B==0) which means is takes
;2^N form
;then index to the pre–stored
;table
;and return the value
taylor:
3
;A=A–B, A is the x in Taylor’s
;equation
;X is the fractional part in
;Q15 format
;AR3 points to coefficient in
;Taylor’s equ.
;set up running environment
;using
;powerful poly instruction on
;c54x DSP
;
;loop 11 times enough for audio
;app.
;AH=fractional part in Q15
;format
;AL=AH
;AL is now in Q11 format
;sum up scaling part
Implementation of Exponential Operation
After getting Y from the previous calculation, it’s time to calculate Xr from Y by performing a
exponential operation. The Taylor Equation states:
ex = 1 + x + x2/2! + x3/3! + …+ xn/n!
(6)
where –∞ < x < +∞. The Y given by previous calculation is in Q11 format. Therefore, the MSB5
bits are integer part and LSB11 bits are fractional part. It is convenient to rewrite the equation as
following,
General Guide to Implement Logarithmic and Exponential Operations on a Fixed-Point DSP
3
SPRA619
Xr = eY = eIk * ex
(7)
Since Ik has only 12 possible occurrences, it is convenient to pre-store the result in a table.
Now look into the fractional part. If x is smaller than 0.5, the Taylor equation converges quickly.
However, if it is larger than 0.5, it converges very slowly. Therefore, if x is larger than 0.5, the
Eq.8 should be rewritten as
Xr = eY = eIk * ex = eIk+1 * ex–1
Following gives the example code based on C54x DSP. The detail explanation is embedded.
.mmregs
.def exp
;
;
;
;
;
;
This routine is used to calculate the exponential of an input
which is given by log in Q11 format. The integer part should be
less than 10 according to MP3 algorithm. Programmer should be
responsible for make sure DP is pointed to the data page of N and X.
Input:
AL = fraction part in Q11 and MSB 5bits are integer part, AH ;
= 0
; Output: AL = integer, AH = 0
; Register used: B, T, AR0, AR3
; Memory: 30 words
;
.data
;
;exptbl is generated by equation en. n starts from 0 to 10.
;
exptbl .int 1, 3, 7, 20, 55, 148, 403, 1097, 2981, 8103, 22026, 0
;
;a9 is in Q15 format. It is generated by equation 32768/n!. n starts ;from 8
down to 1 in order to take advantage of instruction POLY.
;
a9
.int 1, 7, 46, 273, 1365, 5461, 16384, 32767, 0, 0
.bss N, 1
;N stores scaling number
.bss X, 1
;X is temporary storage. Must
;be at the same data page of N.
.text
exp:
ADD
#0, A, B
;B=A=Y
AND
#400h, B
;Check if it is larger than 0.5
BCD
adj, BNEQ
;Yes, then need adjustment
ADD
#400h, A, B
STL
B, –11, N
;store scaling index
AND
#3FFh, B
;truncate fractional part
STL
B, 4, X
;store fractional part in Q15
;format
B
taylor
adj:
STL
B, –11, N
;store scaling index
AND
#7FFh, B
;truncate fractional part
SUB
#400h, B
STL
B, 4, X
;store negative fraction in Q15
;format
4
General Guide to Implement Logarithmic and Exponential Operations on a Fixed-Point DSP
(8)
SPRA619
taylor:
4
STM
a9, AR3
LD
X, T
LD
*AR3+, 16, A
LD
RPT
*AR3+, 16, B
#7
POLY
*AR3+
ADD
MVDM
STM
MAR
MPYA
SFTA
RET
.end
#7FFFh, 16, A
N, AR0
exptbl, AR3
*AR3+0
*AR3
B, –16, A
;AR3 points to coefficient in
;Taylor’s equ.
;set up running environment
;using
;powerful poly instruction on
;c54x DSP
;
;loop 8 times enough for audio
;app.
;AH=fractional part in Q15
;format
;taylor equ. has 1 constant
;index into exptbl
;multiply the scaling part
;AL=BH
Summary
Although a fixed-point DSP does not have the advantage of calculating logarithmic and exponential operations, it does offer ways to approximate the result with limited precision. The
tradeoffs between precision and DSP chip cost are made according to different applications. In
modern audio/video applications such as MP3, a fixed-point DSP presents sufficient precision of
these logarithmic and exponential operations.
5
References
1. TMS320C54x DSP CUP and Peripherals (SPRU131)
2. International Standard ISO/IEC 11172–3
General Guide to Implement Logarithmic and Exponential Operations on a Fixed-Point DSP
5
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