Real-time digital signal processing: implementations, ... changes in the input signal is limited by its internal clock rate, so that it may be slow to
Document source : notes.ump.edu.my
BIT2
.equ 0x0004
; b2
BIT0
.equ 0x0001
; b0
.def _pn_gen
.sect "rand_gen"
_pn_gen
mov *AR0, AC0
; Get register value
bfxtr #(BIT15jBIT2), AC0, T0 ; Get b15 and b2
bfxtr #(BIT11jBIT0), AC0, T1 ; Get b11 and b0
sfts AC0, #1
j j
xor
T0, T1
; XOR all 4 bits
mov
T1, T0
sfts T1, #À1
xor
T0, T1
; Final XOR
and
#1, T1
or
T1, AC0
mov
AC0, *AR0
; Update register
sub
#0x4000, AC0, T0
; Zero-mean random number
j j
ret
.end
The C program pn_sequence.c and the TMS320C55x assembly program
pn_gen.asm for this example are available in the software package.
9.2.3 Comfort Noise in Communication Systems
In voice-communication systems, the complete suppression of a signal using residual
echo suppressor (will be discussed later in Section 9.4) has an adverse subjective effect.
This problem can be solved by adding a low-level comfort noise, when the signal is
suppressed by a center clipper. As illustrated in Figure 9.3, the output of residual echo
suppressor is expressed as
yn avn, jxnj
xn,
jxnj > ,
9:2:6
where v(n) is an internally generated zero-mean pseudo-random noise and x(n) is the
input applied to the center clipper with the clipping threshold b.
x(n)
v(n)
y(n)
Center
clipper
a
+
+
Noise power
estimator
Noise
generator
Figure 9.3 Injection of comfort noise with active center clipper
408
PRACTICAL DSP APPLICATIONS IN COMMUNICATIONS
Summary :
Get b11 and b0 sfts AC0, #1 j j xor T0, T1 ; XOR all 4 bits mov T1, T0 sfts T1, #À1 xor T0, T1 ; Final XOR and #1, T1 or T1, AC0 mov AC0, *AR0 ;
Tags :
ac0,noise,clipper,xor,center,comfort,get,bfxtr,register,systems,signal,sfts,jxnj