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Digital Transmission

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Digital Transmission

- Line coding
- Encoding considerations
- DC components in signals
- Synchronization
- Various line coding methods

- Process of converting binary data to digital signal

- Number of signal levels
- Number of different voltage levels allowed in a signal

- Number of data levels
- Number of voltage levels that actually represent data values

00

11

01

10

01

10

11

00

+3

+1

t

-1

-3

One pulse(one signal element)

b – number of bits per pulse

L – number of different signal elements

BitRate = PulseRate× b = PulseRate × log2L

Bit rate Bits per second

Pulse rate Baud (pulses or signals per second)

- Example: In Manchester Encoding, if the bit rate is 10 Mbps, what is the pulse rate?

0

1

0

0

1

1

0

1

t

One pulse(one signal element)

One bit

- Signal spectrum
- Lack of DC components
- Lack of high frequency components

- Clocking/synchronization
- Error detection
- Noise immunity
- Cost and complexity

0

1

0

0

1

1

0

1

t

0

1

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1

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1

t

- DC components in signals are not desirable
- Cannot pass thru certain devices
- Leave extra (useless) energy on the line

Signal with DC component

Signal without DC component

0

1

0

0

1

1

0

1

Sender sends:

01001101

t

0

1

0

0

0

1

1

0

1

1

t

- To correctly decode a signal, receiver and sender must agree on bit interval

Receiver sees:

0100011011

data

Sender

Receiver

clock

0

1

0

0

1

1

0

1

t

- Separate clock wire
- Self-synchronization

- Unipolar
- Uses only one voltage level (one side of time axis)

- Polar
- Uses two voltage levels (negative and positive)
- E.g., NRZ, RZ, Manchester, Differential Manchester

- Bipolar
- Uses three voltage levels (+, 0, and –) for data bits

- Multilevel

0

1

0

0

1

1

0

0

t

- Simplest form of digital encoding
Rarely used

- Only one polarity of voltage is used
- E.g., polarity assigned to 1

- Two voltage levels (+,-) represent data bits
- Most popular four
- Nonreturn-to-Zero (NRZ)
- Return-to-Zero (RZ)
- Manchester
- Differential Manchester

0

1

0

0

1

1

1

0

t

0

1

0

0

1

1

1

0

t

- Nonreturn to Zero
- NRZ-L (NRZ-Level):Signal level depends on bit value
- NRZ-I (NRZ-Invert): Signal is inverted if 1 is encountered

N = Bit rate

Save = Average signal rate

0

1

0

0

1

1

0

0

t

- Return to Zero
- Uses three voltage levels: +, - and 0, but only + and - represent data bits
- Half way thru each bit, signal returns to zero

?

= 0

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1

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1

t

= 1

- Uses an inversion at the middle of each bit
- For bit representation
- For synchronization

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1

0

1

t

- The inversion on the middle of each bit is only for synchronization
- Transition at the beginning of each bit tells the value

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0

1

t

- Bipolar encoding uses three voltage levels: +, - and 0
- Each of all three levels represents a bit
- E.g., Bipolar AMI (Alternate Mark Inversion)
- 0V always represents binary 0
- Binary 1s are represented by alternating + and -

1

1

0

0

0

0

0

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Bipolar AMI

t

0

0

0

V

B

0

V

B

B8ZS

- BnZS – Bipolar n-zero substitution
- Based on Bipolar AMI
- n consecutive zeros are substituted with some +/- levels
- provides synchronization during long sequence of 0s

- E.g., B8ZS

V – Bipolar violation

B – Valid bipolar signal

00

11

01

10

01

10

11

00

+3

+1

t

-1

-3

Bit sequenceVoltage level

00-3

01-1

10+3

11+1

- mBnL
- m data elements are substituted with n signal elements
- E.g., 2B1Q (two binary, 1 quaternary)

t

- Improves the performance of line coding
- Provides
- Synchronization
- Error detection

Division

Substitution

LineCoding

…01011010001…

:

001011010001

:

:

101100101101010

:

PAM

PAM signal(Sampled analog data)

Analog signal

- Pulse Amplitude Modulation (PAM)
- Converts an analog signal into a series of pulses by sampling

- Converts an analog signal into a digital signal
- PAM
- Quantization
- Binary encoding
- Line coding

6

4

Output

2

0

1

2

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4

5

6

7

Input

- Converts continuous values of data to a finite number of discrete values

Quantization

- Assume sine-wave input and uniform quantization
- Known as the 6 dB/bit approximation

See also: http://en.wikipedia.org/wiki/Quantization_error#Quantization_noise_model

- A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample?

Solution

We can calculate the number of bits as

Telephone companies usually assign 7 or 8 bits per sample.

- Maps discrete values to binary digits

t

sampling interval

- Nyquist Theorem:

Sampling rate must be greater than twice the highest frequency

Ex.Find the maximum samplinginterval for recording human voice(freq. range 300Hz – 3000Hz)

See also: Wagon-wheel effect

Demo: sampling.py

- Ex.Calculate the minimum bit rate for recoding human voice, if each sample requires 60 levels of precision