Chapter 7 Delta Modulator

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Chapter 7
Delta Modulator
Chapter 7 – DELTA MODULATOR
7-1: Curriculum Objectives
1.
To understand the theory of delta modulation.
2.
To understand the signal waveforms of delta modulation.
3.
Design and implementation of delta modulator.
4.
Measurement and adjustment of delta modulator.
7-2: Curriculum Theory
1. The Operation Theory of Delta Modulation
Delta modulation is a kind of source coding which can convert the analog signal to digital signal.
After that we can deal with the digital signal easily such as encoding, filt ering t he unwanted
signal and so on. Furthermore, the transmission quality of digital signal is better than analog signal,
this is because digital signal can recover the original signal easily by the comparator.
The block diagram of delta modulator is shown in figure 7-1. From figure 7-1, the subtraction
between the low frequency signal x(t) and the signal x s(t) will produce a difference signal
d(t) , where x s(t) is a reference signal, which is the former sampling value. Therefore the
expression of the difference signal d(t) is given as
However the difference signal d(t) will be converted by a limiter, then we can obtain a signal Δ(t)
given as
Figure 7-1 Block diagram of delta modulator.
Hence, we know that if the difference signal d(t) is larger than zero, this means that the reference
signal xs (t) is lower than the low frequency signal x(t), i.e. the estimated value is too small and we
need to increase the next estimated value by Δ, on the other words, we only need to send a
variation value, which is Δ during the transmission. Nevertheless, if the difference signal d(t) is lower
than zero, then we only need to transmit a variation value, which is - Δ.
Thus it can be seen that every sampling value is related to the former sampling value, therefore, we
can estimate the next sampling value base on the former sampling value. Then we only need to
concentrate on the t ransmissio n of t he estimat ed value and need not care about t he
quantification encode, so this situation can greatly save the transmission bandwidth. As for PCM
modulation, every sampling value is independently and need to be quantified for encoding, thus, this
situation will increase transmission bandwidth. For example, if the PCM signal after encoded is 8 bits,
then the transmission bandwidth will be B T ≥ 8fs / 2 = 4fs = 8 W , which is 8 times more than
the original bandwidth. Alt hough PCM modulation increases the quality of transmission, it also
increases the transmission bandwidth. However delta modulation can reduce the
transmission bandwidth and achieve the quality of transmission as PCM modulation.
Assume the sampling signal be
After the signal Δ(t) is sampled, we can obtain the expression of delta modulation signal as
Finally we integrate the delta modulation signal to take as the next reference signal x, (t) of delta
modulation signal. The expression is given as
Although delta modulation has the advantages of simple structure and small transmission bandwidth,
it also has the disadvantage of slope overload. As a result of delta modulation is the capacity of the
variation Δ in the T, region, therefore the maximum slope of the delta modulation is Δ / T, = f Δ. If
the maximum slope of the input signal exceeds Δ / T, = fsΔ, that means the step size is too small,
then slope overload will occur in the delta modulation. Consequently the slope of the input
signal must satisfies the prerequisite condition of delta modulation, which is given as follow
Slope overload will cause the modulation signal changes cannot follow closely enough to the
input signal, and then the recovery of the original signal will become distorted. In order to prevent
slope overload, we notice that from equation (7-6), the slope of the input signal cannot be too high
and we can also increase the values of fs or Δ.
2. Implementation of Delta Modulator
Figure 7-2 is the basic circuit diagram of delta modulation. The audio signal will pass through a lowpass filter to remove the unwanted signals, which can prevent the interference from noise. The
comparator is to compare the audio signal and the output signal of integrator, then the difference
will be sampled by the D-type flip-flop and the output signal is a TTL digital signal. After that the
output signal will feedback to integrator for integration and the output signal of integrator will again
compare to the input signal to obtain the value of Δ or – Δ.
We modified the circuit diagram of delta modulation in figure 7-2 to figure 7-3. From figure 7-3, we
add a multiplexer to control the gain of the integrator. This is because the gain of the integrator will
affect the slope of the output signal of integrator, therefore, this method can prevent the
occurrence from slope overload. U1 is the comparator, which can compare the audio signal and the
output signal of integrator, then the output square wave signal will be sampled by a D-type flip-flop
and finally the output signal is the delta modulation signal. U2 is the conversion of unipolar to
bipolar circuit. Since there is no output signal from integrator by inputting the unipolar square wave
signal, therefore, we need to convert the unipolar signal to bipolar signal. Analog switch is a structure
of multiplexer. The purpose of the analog switch is the selection of the amplified gain of
integrator. When AB=00, the signal will pass through R 1 4 , R 1 3 , R 12 , R11 and send into
integrator; when AB= 11, the signal will pass through R14 to integrator. U3 is an inverse integrator.
The expression without R16 is given as
By adding a shunt resistor R16 between integrator U3 and capacitor C 1, we can improve the low
frequency response of the integrator. Assume that R16 and C1 are equivalent impedance, then we get
where
Figure 7-2 Basic circuit diagram of delta modulator.
Figure 7-3 Circuit diagram of delta modulator.
7-3: Experiment Items
Experiment 1: Delta modulator
1. To implement a delta modulator circuit as shown in figure 7-3 or refer to figure DCT7-1 on
GOTT DCT-6000-04 module.
2. Let J2 and J3 be short circuit, i.e. the connection between X 0 and X is on. At the signal input
port (I/P1), input a 2 V amplitude and 500 Hz sine wave frequency. Next at the CLK input port
(I/P2), input a 5 V amplitude and 32 kHz TTL signal. Then observe the input signal (T1), the
output port of comparator (T2), the output port of the conversion from unipolar to bipolar (T3),
the output port of tunable gain (T4), the output port of integrator (T5) and the output port of
delta modulation signal (O/P) by using oscilloscope. Finally record the measured results in table
7-1.
3. According to the input signals in table 7-1, repeat step 2 and record the measured results in table
7-1.
4. Let J2 and J4 be short circuit, i.e. the connection between X 1 and X is on. At the signal input
port (I/P1), input a 2 V amplitude and 1 kHz sine wave frequency. Next at the CLK input port
(I/P2), input a 5 V amplitude and 64 kHz TTL signal. Then by using oscilloscope, observe
on the output signal waveforms of T1, T2, T3, T4, T5 and O/P signal. Finally record the
measured results in table 7-2.
5. According to the input signals in table 7-2, repeat step 4 and record the measured results in table
7-2.
6. Let J1 and J3 be short circuit, i.e. the connection between X2 and X ison. At the signal input port
(I/P1), input a 2 V amplitude and 1.5 kHz sine wave frequency. Next at the CLK input port (I/P2),
input a 5 V amplitude and 128 kHz TTL signal. Then by using oscilloscope, observe on the
output signal waveforms of T1, T2, T3, T4, T5 and O/P signal. Finally record the measured
results in table 7-3.
7. According to the input signals in table 7-3, repeat step 6 and record the measured results in table
7-3.
8. Let J1 and J4 be short circuit, i.e. the connection between X3 and X ison. At the signal input port
(I/P1), input a 2 V amplitude and 2 kHz sine wave frequency. Next at the CLK input port (I/P2),
input a 5 V amplitude and 256 kHz TTL signal. Then by using oscilloscope, observe on the
output signal waveforms of T1, T2, T3, T4, T5 and O/P signal. Finally record the measured
results in table 7-4.
9. According to the input signals in table 7-4, repeat step 8 and record the measured results in table
7-4.
7-4: Measured Results
Table 7-1 Measured results of delta modulation with 32 kHz CLK signal.
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
500 Hz
2V
Table 7-1 Measured results of delta modulation with 32 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
1 kHz
2V
Table 7-1 Measured results of delta modulation with 32 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
T6
2 kHz
2V
Table 7-1 Measured results of delta modulation with 32 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
3 kHz
2V
Table 7-2 Measured results of delta modulation with 64 kHz CLK signal.
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
1 kHz
2V
Table 7-2 Measured results of delta modulation with 64 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
2 kHz
2V
Table 7-2 Measured results of modulation with 64 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
3 kHz
2V
Table 7-2 Measured results of delta modulation with 64 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
4 kHz
2V
Table 7-3 Measured results of delta modulation with 128 kHz CLK signal.
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
1.5 kHz
2V
Table 7-3 Measured results of delta modulation with 128 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
3 kHz
2V
Table 7-3 Measured results of delta modulation with 128 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
5 kHz
2V
Table 7-3 Measured results of delta modulation with 128 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
7 kHz
2V
Table 7-4 Measured results of delta modulation with 256 kHz CLK signal.
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
2 kHz
2V
Table 7-4 Measured results of delta modulation with 256 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
4 kHz
2V
Table 7-4 Measured result of delta modulation with 256 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
6 kHz
2V
Table 7-4 Measured results of delta modulation with 256 kHz CLK signal (Continue).
Input Signals
Measured Results
T1
T2
T3
T4
T5
DM O/P
8 kHz
2V
7-5: Problems Discussion
1. Try to sketch the block diagram of delta modulation.
2. Explain the purposes of comparator and conversion of unipolar to bipolar in figure 7-2.
Why?
3. Explain the purposes of analog switch in figure 7-3.
4. If the signal of CLK in figure 7-3 is not TTL signal, then describe what will the outputbe.
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