Download Chapter 5 PCM Modulator

Chapter 5
PCM Modulator
PCM Modulator
5-1: Curriculum Objectives
1. To understand the operation theory of pulse coded modulation (PCM).
2. To understand the theory of PCM modulation circuit.
3. To design and implement the PCM modulator.
4. Measurement and adjustment of delta modulator.
5-2: Curriculum Theory
The Operation Theory of PCM Modulation
PCM modulation is a kind of source coding. The meaning of source coding is the conversion
from analog signal to digital signal. After converted to digital signal, it is easy for us to
process the signal such as encoding, filtering the unwanted signal and so on. Besides, the quality of
digital signal is better than analog signal. This is because the digital signal can be easily recovered
by using comparator.
PCM modulat ion is commo nly used in audio and t elephone transmission. The main
advantage is the PCM modulation only needs 8 kHz sampling frequency to maintain the original
quality of audio. Figure 5-1 is the block diagram of PCM modulation. First of all is the low-pass
filter, which is used to remove the noise in the audio signal. After that the audio signal will be
sampled to obtain a series of sampling values as shown in figure 5-2. Next, the signal will pass
through a quantizer to quantize the sampling values. Then the signal will pass through an encoder to
encode the quantization values and then convert to digital signal. In fact, the process of quantization
can be achieved at one time by A/D converter. However, we should pay attention on the
quantization levels. For example if the bits for PCM modulation is 3, then the quantization levels is
23=8, which is 8 steps. If the bits are for PCM is 4, then the quantization level is 24=16, which is 16
steps. The increasing of bits of PCM modulation will prevent the signal from distortion, but the
bandwidth will also increase due to the increasing of the capacity of data.
From figure 5-2, the encoder utilizes n output terminals;therefore, we need to convert the parallel
data to serial data, which is the way that satisfies the data format of PCM modulation.
Figure 5-1 Block diagram of PCM modulation.
Figure 5-2 Diagram ofPCM modulation.
2. The Implementation of PCM Modulator
Figure 5-3 is the circuit diagram of PCM modulator. Capacitors C1, C2, resistors R1, R2, R3, R4 and
µA741 comprise a second order low-pass filter. The structure of this low-pass filter is a voltage
controlled voltage source (VCVS) low-pass filter. The gain can be expressed as
AV = 1 +
𝑅4
𝑅1
The cutoff frequency is
fo =
1
2π R 2 R 3 C1 C2
Figure 5-3 Circuit diagram of PCM modulator.
If R2 = R3 = R and C1 = C2= C, t he n
fo =
1
2𝜋 𝑅𝐶
(5-3)
In this experiment, we use IC CW6694 from Conwise to implement the PCM modulator. This IC
includes the circuits of PCM modulation and demodulation, however, we only discuss the
modulation in this chapter. The analog signal will pass through R 5 and input to pin 10, which
is the inverting input terminal. Then the signal will pass through R6 and feedback to pin 9.
Therefore, the structure of these two pins is an OPA struct ure and the expression of the gain is
AV = −
𝑅6
𝑅5
(5-4)
Besides the input gain control of the modulation circuit, the sampler.quantizer and encoder are built
in the IC, therefore, we just need a few components to implement the PCM modulator. Master clock
(MC1k) is the operation frequency of the system, which is 2048 kHz square wave
frequency. Figure 5-4 is the circuit diagram of 2048 kHz square wave generator. From figure 5-5,
we use 2048 kHz crystal oscillator to match with the TTL inverter, which can produce the
required signal. Sample clock (SC1k) is the sample frequency, which supplies the required
operation frequency of the internal sampler. The sample frequency is 8 kHz, i.e. the sampler will
sample the input audio signal in every 0.125 ms. The sampling frequency is obtained by using the
counter to divide the 2048 kHz square wave signal by 256.
From figure 5-3, FSO and FS1 are the data format selection of PCM encoder as shown in table 5-1.
The data format selection of PCM encoder can encode the sample to 8-bit µ-Law format, 8-bit ALaw format or 16-bit digital data format. Besides, the above-mentioned format, the IC CW6694 also
provides with encode and decode of the continuous variable slope delta modulation (CVSD) format.
The CVSD format can be selected by pin FSO and FS1. However, CVSD is not included in this
chapter, therefore, the FS1 will be grounded and FSO will be in "HIGH" level. At this moment, the
output encode data format of PCM is 16-bit. When FSO is in "LOW" level.the output encode of PCM
is 8-bit. Pin RST is the reset pin of this IC.
From figure 5-3, the data output terminal of pin 26 of PCM modulatorwill connect to a buffer
U1: B, which is used for impedance matching. Thereason is the output of PCM modulator is
bipolar junction transistor type not fieId effect transistor type; therefore, in order to prevent the load
effect, we need to connect a buffer at the output terminal for impedance matching.
Table 5-1 Output data format of PCM modulation.
FS0
FS1
Data Format
0
0
8 bits μ-Law
0
1
8 bits A-Law
1
0
16 bits Liner
1
1
8 bits CVSD
Figure 5-4 Circuit diagram of 2048 kHz square wave generator.
5-3: Experiment Items
Experiment 1: PCM modulator
1. Refer to the circuit diagram in figure 5-3 or figure DCT5-1 of GOTT DCT-6000-03 module.
2. Let J1 short circuit and from the signal input terminal (I/P), input 250 mV amplitude and 500
Hz sine wave frequency. Then by using oscilloscope, observe on the output terminal of lowpass filter (T1), input terminal of audio signal (T2), feedback point of output signal (T3) and
output signal terminal of PCM (OP). After that connect the output terminal (T4) with 2048 kHz
square wave to the CH1 of the oscilloscope and output terminal (T6) of modulated signal to
CH2 of the oscilloscope, then record the measured results in table 5-2.
3. Follow the input signals in table 5-2, then repeat step 2 and record the measured results in table
5-2.
4. Let J2 short circuit and from the signal input terminal (I/P), input 250 mV amplitude and 500 Hz
sine wave frequency. Then by using oscilloscope, observe on the signal waveforms of T1, T2,
T3 and OP. After that connect T4 to the CH1 of the oscilloscope and T6 to the CH2 of the
oscilloscope, then record the measured results in table 5-3.
5. Follow the input signals in table 5-3, then repeat step 4 and record the measured results in table
5-3.
5-4: Measured Results
Table 5-2 Measured results of PCM modulator when J1 short circuit.
Input Signals
Output Signal Waveforms
T1
T2
T3
OP
500 Hz
250 mV
T4 and T6
T5 and T6
Table 5-2 Measured results of PCM modulator when J1 short circuit. (Continue)
Input Signals
Output Signal Waveforms
T1
T2
T3
OP
1 kHz
250 mV
T4 and T6
T5 and T6
Table 5-3 Measured results of PCM modulator when J2 short circuit.
Input Signals
Output Signal Waveforms
T1
T2
T3
OP
500 Hz
250 mV
T4 and T6
T5 and T6
Table 5-3 Measured results of PCM modulator when J2 short circuit. (Continue)
Input Signals
Output Signal Waveforms
T1
T2
T3
OP
1 kHz
250 mV
T4 and T6
T5 and T6
5-5 Problems Discussion
1. From figure 5-3, what are the objectives of the first µA741?
2. From figure 5-3, what are the objectives of the second μA741?
3. From the PCM modulator circuit, how to generate the 2048 kHz and 8 MHz square wave signal?
4. In figure 5-3, what are the functions of R5 and R6?