DIFFERENTIAL PULSE CODE MODULATION
AIM: To study the DPCM modulation & demodulation technique.
APPARATUS: DPCM Trainer kit,
Power chords,
20 MHz Dual trace CRO,
Power supply.
THEORY: Pulse Code Modulation (PCM) is different from Amplitude Modulation (AM) and
Frequency Modulation (FM) because those two are continuous forms of modulation. PCM is
used to convert analog signals into binary form. Inthe absence of noise and distortion it is
possible to completely recover a continuous analog modulated signals. But in real time they
suffer from transmission distortion and noise to anappreciable extent. In the PCM system,
groups of pulses or codes are transmitted which represent binary numbers corresponding to
modulating signal voltage levels. Recovery of the transmitter information does not depend on the
height, width, or energy content of the individual pulses, but only on their presence or absence.
Since it is relatively easy to recover pulses underthese conditions, even in the presence of large
amounts of noise and distortion, PCM systems tend to be very immune to interference and noise.
APPARATUS: DPCM Trainer kit,
Power chords,
20 MHz Dual trace CRO,
Power supply.
THEORY: Pulse Code Modulation (PCM) is different from Amplitude Modulation (AM) and
Frequency Modulation (FM) because those two are continuous forms of modulation. PCM is
used to convert analog signals into binary form. Inthe absence of noise and distortion it is
possible to completely recover a continuous analog modulated signals. But in real time they
suffer from transmission distortion and noise to anappreciable extent. In the PCM system,
groups of pulses or codes are transmitted which represent binary numbers corresponding to
modulating signal voltage levels. Recovery of the transmitter information does not depend on the
height, width, or energy content of the individual pulses, but only on their presence or absence.
Since it is relatively easy to recover pulses underthese conditions, even in the presence of large
amounts of noise and distortion, PCM systems tend to be very immune to interference and noise.
Regeneration of the pulse reroute is also relatively easy, resulting in system that produces
excellent result for long distance communication.
Differential PCM is quite similar to ordinary PCM. However, each word in this system indicates
the difference in amplitude, positive or negative, between this sample and the previous sample.
Thus the relative value of each sample is indicatedrather than the absolute value as in normal
PCM. In this each amplitude is related to the previous amplitude, so that large variations from
one sample to the next are unlikely. This being thecase, it would take fewer bits to indicate to
indicate the size of the amplitude change than the absolute amplitude, and so a smaller
bandwidth would be required for the transmission. The differential PCM system has not found
wide acceptance because complications in the encoding and decoding process appear to out
weigh advantages gained.
excellent result for long distance communication.
Differential PCM is quite similar to ordinary PCM. However, each word in this system indicates
the difference in amplitude, positive or negative, between this sample and the previous sample.
Thus the relative value of each sample is indicatedrather than the absolute value as in normal
PCM. In this each amplitude is related to the previous amplitude, so that large variations from
one sample to the next are unlikely. This being thecase, it would take fewer bits to indicate to
indicate the size of the amplitude change than the absolute amplitude, and so a smaller
bandwidth would be required for the transmission. The differential PCM system has not found
wide acceptance because complications in the encoding and decoding process appear to out
weigh advantages gained.
DPCM ENCODING: DPCM Encoding is similar to the PCM encoding, except that initial stage
employs Delta modulation after that PCM encoding isfollowing. The encoding process
generates a binary code number corresponding to modulating signal voltage level to be
transmitted for each sampling interval. Any one of the codes like binary, ASCII etc, may be used
as it provides a sufficient number of different symbols to represent all of the levels to be
transmitted. Ordinary binary number will contain a train of’1’ and ‘0’ pulses with a total of
log2N pulses in each number. (N is no of levels in the full range). This system is very economical
to realize because it corresponds exactly to the process of analog – to – digital (A/D) conversion.
The 1
st
step in the PCM system is to quantize the modulating signal. The modulating signal can
assume an infinite no.of different level between the two limit values, which define the range of
the signal. In PCM a coded no is transmitted for each level sampled in the modulating signal. If
the exact no corresponding to the exact voltage were to be transmitted for every sample, an
infinitely large no of different code symbols wouldbe needed. Quantization has the effect of
reducing this infinite no of levels to a relativelysmall number, which can be coded without
difficulty.
In the quantization process, the total range of themodulating signal is divided into a no of small
sub ranges. The number will depend on the nature ofthe modulating signal and will form as few
as 8 to as many as 128 levels. A number that is an integer power of two is usually chosen
because of the ease of generating binary codes. Theresult is stepped waveform which follows
the counter of the original modulating signal with each step synchronized to the sampling period.
The quantized staircase waveform is an approximation to the original waveform. The difference
between the two-wave form amounts to “noise” added to the signal by the quantizing circuit. The
mean square quantization noise voltage has a value of E(square)np= S(square)/12
Where S is the voltage of each step. As a result the number of quantization levels must be kept high in order to keep the quantizationnoise below some acceptable limit given by the power signal-to-noise ratio, which is the ratioof average noise power.
DECODING: The decoding process reshapes the incoming pulses and eliminates most of the
transmission noise. A serial to parallel circuit passes the bits in parallel groups to a digital to
transmission noise. A serial to parallel circuit passes the bits in parallel groups to a digital to
analog converter (D/A) for decoding. Thus decoded signal passes through a sample and hold
amplifier, which maintains the pulse level for the duration of the sampling period, recreating the
staircase waveform approximation of the modulation signal. A low-pass filter may be used to
reduce the quantization noise.
amplifier, which maintains the pulse level for the duration of the sampling period, recreating the
staircase waveform approximation of the modulation signal. A low-pass filter may be used to
reduce the quantization noise.
BLOCK DIAGRAM:
PROCEDURE:
1) Connect the AC power supply to the trainer kit and switch it ON.
2) Connect the DC output signal to the input of DPCM Modulator.
3) Observe the sampling signal output on the CH-1 of CRO.
4) Observe the DPCM output put on the CH-2 of CRO.
5) By adjusting the DC voltage we can get the DPCM output from 0000 0000 to 1111 1111.
6) Now disconnect the DC voltage and apply AF output to the input of DPCM modulator.
7) Observe the conditioning amplifier output & DPCM output with respect to sampling signal.
8) Connect the DPCM output to the input of demodulatorand observe the output with respect to AF output signal.
9) Calculate the Phase shift of the demodulated signal.
10) Plot the observed waveforms on the graph sheet.
MODEL GRAPH:
