Instructor: Eng. Nada Khatib
Website: http://www.philadelphia.edu.jo/academics/nkhatib/
Digital Communication (650533) CH 3
Pulse Modulation
Philadelphia University/Faculty of Engineering
Communication and Electronics Engineering
Eng. Nada Khatib Ch 3- lecture 2/5 1
Outline • Introduction
• PCM – Sampling Process
– Quantization Process
– Encoding • Line codes
– Regeneration
– Noise in PCM
• TDM
• DPCM
• Delta Modulation
Eng. Nada Khatib Ch 3- PCM 2
Introduction • Analog signal versus digital signal
• Almost every communication system in use today is a digital.
– Cell phone system (GSM, UMTS,LTE)
– GPS, etc
Why are digital communications becoming popular? • Noise immunity of digital signals
• Information extraction is often easier from analog signal.
• Viability of regenerative repeater in Digital comm.
Why digital communication? • The ability to detect and correct errors by
using error detection and correction techniques.
• The ability to compress data
• The ability to securely send data by using encryption for example.
PCM
• PCM = Pulse code Modulation
• It is a method of converting an analog signal into a digital binary signal (A/D conversion).
• An analog signal can be converted into binary digital signal by means of
– Sampling
– Quantization
– Encoding
Sampling Process
• Ts= sampling interval
• fs ≥ 2fm Nyquist criterion
• Fs=2fm Nyquist rate
• Fs> 2fm oversampling
• Fs<2fm undersampling → aliasing
• Aliasing = spectral folding = tail inversion
Sampling Process
• Use low pass anti-aliasing filter prior to sampling process to combat the effects of aliasing in practice
Sampling Process
• the standard CCITT voice channel, 300–3400 Hz (a bandwidth of 3100 Hz)
• So for telephony the sampling rate is 8000 samples per second, or 8 kHz.
• And the sampling period is therefore 1/8 kHz (i.e., 125 μs).
Quantization
• The process of transformation the sample amplitudes of a signal into discrete amplitude taken from a finite set of possible amplitude (Lo, L1, L2,..).
• Number of representation levels is L
Quantization Noise • Also known as Quantization error and round-off error.
• It is the difference between the input sampled signal and the output quantized signal
• The degradation of the signal due to quantization is limited to (± s/2)
• The maximum quantization error is ± s/2
)()( sqsq nTmnTme
Quantization Noise
• A useful figure of merit for the uniform quantizer is the mean-square quantization error.
• The output signal-to-quantization noise ratio of a uniform quantizer is
12
2Sq
q
q
tmRS
)(2
Non uniform Quantization • In telephonic communication, it is preferable to use a
variable separation between the representation levels.
• Nonuniform quantization uses a variable step size.
Non uniform Quantization
• The use of a nouniform quantizer is equivalent to
– passing the baseband signal through a compressor and then
– applying the compressed signal to a unifrom quantizer.
Non uniform Quantization
• A particular form of compression law is used in practice is
– μ-law: which is defined by
– A-law: which is defined by
Non uniform Quantization
• The compressed samples must be restored to their
original values at receiver by using an expander with a
characteristic complements to that of the compressor.
• The combination of a compressor and an expander is
called a compander.
Encoding • Represent each quantized level by a group of n binary digit
and transmit these n bits.
• The output SQNR of a uniform quantizer for in dB is
• The output SNqR of a uniform quantizer for a full-load
sinusoidal modulating signal in dB is
,2nL
nRSNq 68.1
)12log(1062
pp
m
m
PnSQNR
Bit Rate and bandwidth
• Bit rate (Rb) is the number of bits sent per second.
• The minimum required channel bandwidth is
sb nfR
bRBW2
1min
b
bT
R1
Split-phase or Manchester code.
Unipolar NRZ signaling
Polar NRZ signaling.
Unipolar RZ signaling.
Bipolar RZ signaling or AMI
Line codes:
Waveform of differentially encoded data using unipolar NRZ signaling.
Assuming reference bit 1
Differential encoding:
Noise in PCM system • Quantization Noise:
– Signal dependent
– Under the designer’s control
• Channel Noise:
– The main effect of channel noise is o introduce bit error into
the received signal.
– Bit error rate (BER) = PeRb
Differential Pulse code modulation
(DPCM)
• Estimate (predict) the value of the kth sample m[k] from
the knowledge of the previous sample values.
• Transmit the prediction error
][ˆ][][ nTsmnTsmnTse
Block diagram of a linear prediction filter of order N.
prediction filter a1, a2, .. aN = prediction coefficients
= filter coefficients
DPCM-Processing gain
• The processing gain (Gp) produced by the differential
quantization scheme is defined by:
d
mp
P
PG
Power of m(t)
Power of difference
between m[nTs] and its
predicated value m
SNR improvement due
to prediction
Delta Modulation (DM)
• The incoming signal is oversampled to increase the
correlation between adjacent samples of the signal.
• Quantized into only two levels (encoded by 1-bit).
• DM provides a staircase approximation to the
oversampled version of the message signal.