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Chapter 5

PCM Modulator

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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 modulation is commonly used in audio and telephone 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 quantizationcan 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 2

4=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.

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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.

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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

A741 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 +41

The cutoff frequency is

fo =1

2

R2 R3 C1 C2

Figure 5-3 Circuit diagram of PCM modulator.

I f R2 = R3 = R a n d C 1 = C 2 = C , t h e n

fo =1

2

(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

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modulation in this chapter. The analog signal wil l pass through R5 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 structure and the expression of the gain is

AV = 65

(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 -Law format, 8-bit A-

Law 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.

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Table 5-1 Output data format of PCM modulation.

Figure 5-4 Circuit diagram of 2048 kHz square wave generator.

FS0 FS1 Data Format

0 0 8 bits -Law

0 1 8 bits A-Law

1 0 16 bits Liner

1 1 8 bits CVSD

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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 500Hz sine wave frequency. Then by using oscilloscope, observe on the output terminal of low-

pass 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 kHzsquare 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 table5-2.

4. Let J2 short circuit and from the signal input terminal (I/P), input 250 mV amplitude and 500 Hzsine 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 table5-3.

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5-4: Measured Results

Table 5-2 Measured results of PCM modulator when J1 short circuit.

Input Signals Output Signal Waveforms

500 Hz

250 mV

T1 T2

T3 OP

T4 and T6

T5 and T6

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Table 5-2 Measured results of PCM modulator when J1 short circuit. (Continue)

Input Signals Output Signal Waveforms

1 kHz

250 mV

T1 T2

T3 OP

T4 and T6

T5 and T6

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Table 5-3 Measured results of PCM modulator when J2 short circuit.

Input Signals Output Signal Waveforms

500 Hz

250 mV

T1 T2

T3 OP

T4 and T6

T5 and T6

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Table 5-3 Measured results of PCM modulator when J2 short circuit. (Continue)

Input Signals Output Signal Waveforms

1 kHz

250 mV

T1 T2

T3 OP

T4 and T6

T5 and T6

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5-5 Problems Discussion

1. From figure 5-3, what are the objectives of the first A741?

2. From figure 5-3, what are the objectives of the second A741?

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?