PULSE CODE MODULATION
AIM: To study the pulse code modulation & demodulation technique.
APPARATUS: PCM 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.
APPARATUS: PCM 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.
PCM ENCODING:
The encoding process generates a binary code numbercorresponding 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 log 2N 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.
QUANTIZATION:
The 1st 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.
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 bitsin 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.
Block Diagram:
PROCEDURE:
1) Connect the AC power supply to the PCM trainer kit and switch it ON.
2) Observe the ANALOG output from AF signal generator by connecting it to CH-1 of
CRO and adjust its frequency to 1KHz and amplitude to 0.5Vp-p with the help of
potentiometers P-1 and P-2.
3) Connect the ANALOG OUTPUT to ANALOG INPUT of sample and hold circuit.
4) Observe the CONVERTION CLOCK OUTPUT by connecting it to CH-2 of CRO and
adjust its frequency with the help of potentiometerP-4.
5) Observe the SAMPLING CLOCK by connecting it to CH-2of CRO and adjust the
frequency to 8KHz and connect it to the sample and hold circuit.
6) Observe the ANALOG OUTPUT and SAH OUTPUT by connecting them CH-1 and CH-2 of CRO.
7) Connect the SAH OUTPUT to ADC SAH OUTPUT and CONB CLOCK OUTPUT to
2) Observe the ANALOG output from AF signal generator by connecting it to CH-1 of
CRO and adjust its frequency to 1KHz and amplitude to 0.5Vp-p with the help of
potentiometers P-1 and P-2.
3) Connect the ANALOG OUTPUT to ANALOG INPUT of sample and hold circuit.
4) Observe the CONVERTION CLOCK OUTPUT by connecting it to CH-2 of CRO and
adjust its frequency with the help of potentiometerP-4.
5) Observe the SAMPLING CLOCK by connecting it to CH-2of CRO and adjust the
frequency to 8KHz and connect it to the sample and hold circuit.
6) Observe the ANALOG OUTPUT and SAH OUTPUT by connecting them CH-1 and CH-2 of CRO.
7) Connect the SAH OUTPUT to ADC SAH OUTPUT and CONB CLOCK OUTPUT to
ADC CONB CLOCK OUTPUT.
8) Observe the PCM OUTPUT and SAMPLE CLOCK simultaneously on CRO.
9) Observe the output of DAC amplifier by connecting them to CH-1 and CH-2 of CRO.
10) Observe the output of LPF with reference to inputanalog signal.
11) Calculate the frequency and time delay of output signal with reference to input analog
signal.
8) Observe the PCM OUTPUT and SAMPLE CLOCK simultaneously on CRO.
9) Observe the output of DAC amplifier by connecting them to CH-1 and CH-2 of CRO.
10) Observe the output of LPF with reference to inputanalog signal.
11) Calculate the frequency and time delay of output signal with reference to input analog
signal.
MODEL GRAPH:
