Digital Signal Processing Reference
In-Depth Information
Delta-modulator
Delta-Demodulator
+
Digital
transmission
+
TP
1 Bit ADC
Int
-
analog
Integrator
analog
Delta-modulated signal
Int
Integrator
Sigma-Delta-Modulator
Integrator
Delta-modulator
Demodulator
+
-
Digital
transmission
+
TP
Int
1 Bit ADC
analog
analog
Delta-modulated signal
Int
Integrator
Integrator
Digital integrator
+
Int
T
A
Illustration 229:
From the delta modulator to the sigma-delta modulator
At the top the block diagram of the transmission channel of a delta-modulator and demodulator is shown.
Below this only the integrator of the demodulator was placed at the beginning of the transmission channel.
This can hardly have any effect on the output signal as the sequence of linear processes - such as integra-
tion - can be changed around (see Chapter 7 from Illustration 134).
As interference may also occur on the transmission route this results in advantages which will be explained
in more detail in the text.
The two integrators can be replaced by a single integrator after the differentiator (see Exercise 10 in
Chapter 7). Thus the
modulator/encoder is complete. The demodulator now consists only of the ana-
log lowpass. This functions like a "floating averager" and recovers the analog input signal from the digital
sigma-delta modulated signal.
Σ−Δ−
At the bottom the integrator is realised by means of a feedback adder i.e. the instantaneous output value is
added (with a delay) to the input value following. This circuit variant is also used in the DASYLab simula-
tion in Illustration 229.
Sigma-delta modulation or encoding (
Σ−Δ−
M)
A disadvantage of the
-modulator is that the bit errors which occur in transmission lead
to an "offset" i.e. to a offset in the signal received. So-called sigma-delta modulation
(
Δ
modulation) in which significant improvements can be achieved by a skilful
changing round of components is a solution here.
Σ−Δ−
