COMMUNICATION SYSTEM EEEB453Chapter 5 (Part IV)

DIGITAL TRANSMISSION

Delta Modulation Standard PCM systems have no memory i.e each sample value is

separately encoded into a series of binary digits. An alternative, which overcomes some limitations of PCM is to use past

information in the encoding process. One way of doing this is to perform source coding using Delta Modulation. It is a digital modulation technique in which the encoder transmits

information regarding whether the analog info increases or decreases in amplitude.

Only a single bit is transmitted, which simply indicates whether that sample is larger or smaller than the previous sample.

Transmission of the signal can be achieved by sending ‘0’ for –ve transition and ‘1’ for +ve transition.

Note that this means the quantized signal must change at each sampling point.

The key to using delta modulation is to make the right choice of step size and sampling rate.

Slope overload – when the analog signal has a high rate of amplitude change, the encoder can produce a distorted output results. An incorrect selection will mean that the signal

changes too fast for the steps to follow – overloading.

Granular noise – occurs when analog input has relatively constant amplitude.

Adaptive Delta Modulation PCM (ADPCM)

OBJECTIVE to minimize slope overload noise while holding the granular noise at a reasonable level.

One way to improve is to use adaptive DM, where the step size is not required to be constant.

The step size of the DAC is automatically varied, depending on the amplitude characteristics of the analog input signal.

With ADPCM, after predetermined number of consecutives 1s or 0s, the step size is automatically increased - reduce slope overload noise.

When alternative sequence 0s and 1s is occurring, DAC will reduce the step size - reduce granular noise.

ADPCM

Example 1

(a)      An analog signal w(t) is to be converted to a digital signal using PCM. The frequency spectrum of the analog signal is given below. The maximum acceptable error of the sampled amplitudes is 0.4% of the peak-to-peak amplitude.

Determine:

(i)      The minimum sampling rate of the sampler. [2]

(ii)    Quantization level. [2]

(iii)   Number of bits. [1]

(iv)  Coding efficiency. [2]

(v)    Transmission bit rate. [2]

 

1

W(f)

-200 0 200 f (Hz)

Example 2(b)   A digitizing system specifies 72dB of dynamic range.

(i)      Determine the number of samples bits required to

meet the dynamic range requirement. [2]

(ii)    What is the signal-to-noise ratio for the system? [2]

Example 3

(c)      The voltage range being input to a PCM system is 0 to 1 V.

A 3-bit A/D converter is used to convert the analog signal to

digital values.

(i)      How many quantization levels are provided? [2]

(ii)    What is the resolution of each level? [2]

(iii)   What is the value of the quantization error for this system? [2]

Example 4

What is the binary code sent for the original signal in Figure 2, using delta modulation?