Research work on PERFORMANCE IMPROVEMENTS IN ADAPTIVE DELTA
MODULATION(ADM)
Study of a Novel ADM Algorithm with Pre-processing for
Performance Improvementby B.K.SujathaM.S. Ramaiah Institute of
Technology, BangaloreGuide: Co-Guide:Prof. Dr. P.S. Satyanarayana
Prof.Dr. K. N. HaribhatThe Head, Dept of Electronics &Comm
EnggThe Head, Dept of Telecomm EnggB.M. Sreenivasaih College of
Engineering Nagarjuna College of Engg and
TechBangaloreBangaloreTopics SPEECH CODINGDISCRIPTION OF EACH
CODING TECHNIQUEADAPTIVE DELTA MODULATIONPRE-PROCESSING STEP SIZE
ALGORITHMEXISTING STEP SIZE ALGORITHMSSONG ALGORITHMMODIFIED ABATE
ALGORITHMPROPOSED ALGORITHM CONCLUSIONBIBLIOGRAPHY
ADVANTAGES AND DIS-ADVANTAGES OF DIGITAL
COMMUNICATIONADVANTAGES:Less distortion in the received
signal.Simple and less expensive digital circuitry.Possibility of
processing digital signals.Better received speech
quality.Possibility of transmission of voice,video,data all in
digital form.Possibility of correction of medium
errors.Encryption/decryption for message
security.DIS-ADVANTAGES:Increased bandwidth.Synchronization
requirement.
SPEECH CODINGConversion of analog speech signals into digital
form Types of speech coding:
Pulse Code ModulationDifferential Pulse Code
Modulation(DPCM)Delta Modulation(DM)Adaptive Delta
Modulation(ADM)
PULSE CODE MODULATIONSteps involved in PCM :Sampling
QuantizingEncoding n = log2L
Bandwidth of PCM depends on bit rate, R = nfs
For no aliasing, fs >= 2 fm BPCM >= R = nfs
5DIFFERENTIAL PULSE CODE MODULATIONTo minimize redundant
transmission
To reduce the bandwidth in comparison with PCMDELTA
MODULATIONONE BIT OR TWO LEVEL VERSION OF DPCM: This one-bit
codeword eliminates the need for' word framing at the transmitter
& receiver & makes DM systems very attractive for many
classes of digital communications.
NOISE IN DM :Smaller step size causes slope overload
distortion.Larger step size causes granular noise.
DM WAVEFORMS
LIMITATIONS OF DM
m(t) stair case approximate m(t)Max slope overload Slope
overload (positive)^
m(t)m(t)Negative slope overloadSlope overload
(negative)^LIMITATIONS OF DM (contd..)
m(t)m(t) Granular noise (slow varying signal)LIMITATIONS OF DM
(contd..)^ADAPTIVE DELTA MODULATIONImproved version of DM by making
the step size of the modulator assume a time varying form.
Here the step size is adapted to the level of the input
signal
ADM WAVEFORMS
Sample speech signalThe sample speech waveform in the
illustration is taken from the speech sound i i i i i which is
shown in Figure. It is one of the waveforms used repeatedly in the
simulation that is about 5s long.
Pre-ProcessingA methodology for further improving the ADM
performance by pre-processing the speech signal prior to the
adaptation is presented. The large variations in the speech are
removed/smoothened by a suitable pre-processing method, one of
which is using an integrator which can smoothen the rapid changes.
At the receiver, the differentiator is followed by a low pass
filter(LPF).
m(t)(2)(1)(1)Slope overload distortin (2)GranuPRE-PROCESSING OF
MESSAGE SIGNAL
m(t)dt smoothes out m(t) , rapid changes may disappear.
tm(t)(2)(1)(1)Slope overload distortion region (2)Granular noise
regiont
16
Frequency response of Pre-Processor (Integrator) at the
transmitter Frequency response of the Differentiator at the
receiver
ENCODERDECODERThe block diagram of Conventional ADM
ENCODERDECODERThe block diagram of proposed ADMSTEP-SIZE
ALGORITHM:
In the step-size algorithm, the processor detects the pattern of
e(t) where e(t) = sgn [m(t)-m(t)]To see if the delta modulator is
operating in the quantization region, in which case e(t) produces
an alternating 1010 pattern, or in the slope overload region in
which case e(t) produces an all 1s or all 0s pattern. These cases
are illustrated as shown.If ADM senses a 1010 pattern, it decreases
the step size, and if it senses 1111 or 0000, it increases the step
size . The manner in which the step size is altered determines the
algorithm.
^Linear delta modulation and the bit pattern produced for each
region
t e(k) 1 01 0 1 0 1 1 1 1 1 1 m(t)m(t)^ EXISTING STEP SIZE
ADAPTATIONS
SONG ALGORITHMHere, we see that as long as e(k) is of the same
sign as e(k-1), the magnitude of the new step size s(k+1) will
exceed the magnitude of the old step size s(k) by so, the min step
size.However, if e(k) and e(k-1) differ in sign , the magnitude of
s(k+1) will be less than the magnitude of s(k) by the amount
so.
The equation describing the song algorithm is given by
s(k)+ so, e(k) = e(k-1)|s(k+1)|= s(k) - so , e(k) e(k-1)
MODIFIED ABATE ALGORITHMThe need to maintain voice
communications as long as possible was a key factor in the
selection of the modified abate algorithm.The equation describing
modified abate algorithm is [|S(k)| + So] e(k); e(k)=e(k-1) and
S(k) < 8SoS(k+1)= |S(k)| e(k); e(k)=e(k-1) and S(k) = 8So So
e(k); otherwiseThe Proposed step-size adaptation The new proposed
technique for the step-size adaptation is described as
[|S(k)|+S0]e(k); e(k)=e(k-1)
S(k+1)= [|S(k)|-S0]e(k); e(k)e(k-1) and | S(k)|> S0 S0e(k) );
e(k)e(k-1) and | S(k)|< S0
is the adaptation parameter, nearly equal to 1 but, greater than
1.
1/()
The Proposed step-size adaptation (cont)This adaptation
parameter gives a better performance to slope overload The
parameter takes care of the granular noise as a result of which a
better performance is obtained as compared to SONG and modified
ABATE algorithms. Where is taken as 1.1 and S0 as equal to 0.1.
SIMULATIONSNR CALCULATION
{Xn } samples of original signal (speech signal)
{Xn } samples of final reconstructed signal
(Xn - Xn ) error signal
(Xn -X n )2 squared error signal where N is the total sample
number of the input. OR
^^^
Performance Comparison of the proposed step-size adaptation
algorithm with the SONG and the modified ABATE algorithms. (b) the
same plot of figure.(a) is shown but the input strength is
displayed for -7db to -1db.(a)(b)
(a)Performance Comparison of the proposed ADM with the SONG,
modified ABATE and the proposed algorithms. (b) the same plot of
figure.(a) is shown but the input strength is displayed for -7db to
-1db.(a)(b)CONCLUSIONSimulations are carried out for all the
schemes. S0 is taken as 0.1 and Simulations have also confirmed
that with the input strength for -7db to -1db on an average a 1.1dB
performance gain in the SNR is got for the new step-size adaptation
algorithm compared to the SONG and a 1.5dB performance gain
compared to the modified ABATE algorithm. Next, with the proposed
methodology(pre-processing) and with the same input strength, on an
average there is 1.4dB performance improvement in the SNR for the
new step-size adaptation algorithm as compared to the SONG and a
1.7dB compared to the modified ABATE algorithm.
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