This document discusses different digital transmission techniques for analog signals. It covers the bandwidth requirements of PCM and introduces delta modulation (DM) and its limitations like slope overload and granular noise. Adaptive delta modulation (ADM) is presented as an improvement over DM. Time-division multiplexing (TDM) is also introduced along with the standards DS1/T1/E1. The transmission modes of parallel, asynchronous, synchronous and isochronous serial transmission are defined.

1. REVIEW:DIGITALTRANSMISSIONOFANALOG SIGNALS
• 1. BANDWIDTH REQUIREMENT OF PCM
• 2. Disadvantages of DM
• 3. ADM
• TDM
• BANDWIDTH OF TDM
• TRANSMISSION MODES

2. 1. BANDWIDTH REQUIREMENT
OF PCM

3. Bandwidth Requirement of PCM

4. Bandwidth Requirement of PCM
• Bandwidth needed for PCM transmission is given by half
of the signaling rate.
• BW>= (0.5)(r)
• r= signaling rate
• R=(no. of bits)(fs)

5. Noise consideration inPCM
• The performance of a PCM system is influenced by two
major sources of noise.
• Channel noise, which is introduced anywhere between
the transmitter output and the receiver input, channel
noise is always present, once the equipment is switched
on.
• Quantization noise, which is introduced in the transmitter
and is carried all the way along to the receiver output.

6. Quantization noise

8. DPCM

9. • Delta modulation, a special case of DPCM where the
differences eQ[n] are represented with 1 bit as ±∆

10. 2. REVIEW OF DMAND ITS
DISADVANTES

11. Delta Modulation
• Demodulation

12. Delta Modulation
• Next form of pulse modulation
• Transmits information only to indicate whether the analog
signal that is being encoded goes up or goes down.
• The Encoder Outputs are highs or lows that “instruct”
whether to go up or down, respectively
• DM takes advantage of the fact that voice signals do not
change abruptly

13. The process of delta modulation

14. Resulting Waveforms

15. Simple Implementation of a DM system

16. Limitations of thesystem
• Slope overload
• When the analog signal has a high rate of change, the
DM can “fall behind” and a distorted output occurs

17. Tradeoffs
• Simplicity versus Quality
• In order to obtain the high quality DM requires very high sampling rates,
typically 20× the highest frequency of interest, as opposed to Nyquist rate of
2×fm

18. Tradeoffs

19. Delta PCM (DPCM)
• Instead of using one bit to indicate positive
and negative differences, we can use more
bits -> quantization of the difference.
• Each bit code is used to represent the value of
the difference.
• The more bits the more levels -> the higher the
accuracy.

21. Delta modulation transmitter

22. Delta modulation Receiver

23. DISADVANTAGES OF DM

24. Delta Modulation - Example

25. DM circuit’sproblem

26. Granular Noise
Granular noise occurs when the original analog signal has a relatively constant
amplitude and the reconstructed signal has variations that were not present in
the original information signal. Reducing the quantizing step size can reduce
this noise, but this increases the risk of slope overload.

27. OVERSLOPE DISTORTION

29. Cont’d…

30. Cont’d...
• Analog input is approximated by a staircase function
• Move up or down one level ()at each sample interval (by one
quantization level at each sampling time)  output of DM is
a single bit.
• Binary behavior
• Function moves up or down at each sample interval
• In DM the quantization levels are represented by two symbols:
0 for - and 1 for +. In fact the coding process is performed
on eq.
• The main advantage of DM is its simplicity.

31. •Slope overload distortion is due to the fact that the staircase
approximation mq(t) can't follow closely the actual curve of the
message signal m(t ). In contrast to slope-overload distortion,
granular noise occurs when  is too large relative to the local slope
characteristics of m(t). granular noise is similar to quantization noise
in PCM.
•It seems that a large  is needed for rapid variations of m(t) to
reduce the slope-overload distortion and a small  is needed for
slowly varying m(t) to reduce the granular noise. The optimum  can
only be a compromise between the two cases.
•To satisfy both cases, an adaptive DM is needed, where the step
size  can be adjusted in accordance with the input signal m(t).
Cont’d…

32. Cont’d...
• Insummary
• Slope overload
• Due to the input analog signal amplitude changes faster than the
speed of the modulator
• to minimize : the product of the sampling step size and the
sampling rate must be equal to or larger than the rate of change
of the amplitude of the input analog signal.
• Granular noise
• Due to the difference between step size and sampled voltage.
• To minimize : increase the sampling rate, decrease the step size
of modulator

33. DM Performance
• Good voice reproduction
• PCM - 128 levels (7 bit)
• Voice bandwidth 4khz
• Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
• e.g. Interframe coding techniques for video

34. Cont’d...
• Adaptive Delta Modulation (ADM)
• A Delta Modulation system where the step size of the DAC is
automatically varied depending on the amplitude characteristics of
the analog signal.
• A well designed ADM scheme can transmit voice at about halfthe
bit rate of a PCM system with equivalent quality.

35. ADAPTIVE DELTAMODULATION

36. OUTPUT SNR

38. COMPARISON OF PCM

41. COMPARISON OF PCM

42. TDM

43. TDM

46. Time-Division Multiplexing
Figure Block diagram of TDM system.

47. DS1/T1/E1
• Digital signal 1 (DS1, also known as T1) is a T-carrier signaling scheme
devised by Bell Labs. DS1 is a widely used standard in
telecommunications in North America and Japan to transmit voice and data
between devices. E1 is used in place of T1 outside of North America and
Japan. Technically, DS1 is the transmission protocol used over a physical
T1 line; however, the terms "DS1" and "T1" are often used interchangeably.
• A DS1 circuit is made up of twenty-four DS0
• DS1: (8 bits/channel * 24 channels/frame + 1 framing bit) * 8000 frames/s =
1.544 Mbit/s
• A E1 is made up of 32 DS0
• The line data rate is 2.048 Mbit/s which is split into 32 time slots, each
being allocated 8 bits in turn. Thus each time slot sends and receives an 8-
bit sample 8000 times per second (8 x 8000 x 32 = 2,048,000).
2.048Mbit/s

48. T CarrierSystem
Twisted Wire to Cable System

49. Fiber Communication

50. Bandwidth Requirement ofTDM

51. TRANSMISSION
MODES

52. TRANSMISSION MODES
The transmission of binary data across a link can be
accomplished in either parallel or serial mode. In parallel mode,
multiple bits are sent with each clock tick. In serial mode, 1 bit is
sent with each clock tick. While there is only one way to send
parallel data, there are three subclasses of serial transmission:
asynchronous, synchronous, and isochronous.
Topics discussed in this section:
 Parallel Transmission
 Serial Transmission

53. Data transmission and modes

54. Parallel transmission

55. Serial transmission

56. 4.57
In asynchronous transmission, we send
1 start bit (0) at the beginning and 1 or
more stop bits (1s) at the end of each
byte. There may be a gap between
each byte.
Note

57. 4.58
Asynchronous here means
“asynchronous at the byte level,”
but the bits are still synchronized;
their durations are the same.
Note

58. 4.59
Asynchronous transmission

59. 4.60
In synchronous transmission, we send
bits one after another without start or
stop bits or gaps. It is the responsibility
of the receiver to group the bits. The bits
are usually sent as bytes and many
bytes are grouped in a frame. A frame is
identified with a start and an end byte.
Note

60. Synchronous transmission