Data are either digital or analog

Signals are either digital analog

Therefore, encoding of data into signals falls into four categories:

A/A, A/D, D/A, D/D.

Encoding is unipolar, polar, or bipolar (polarity means positive or negative)

Unipolar:

• uses polarized voltage to encode 1, idle line to encode 0 (could be opposite).
• Major problem is synchronization, when data has long string of 1s or 0s.
• Not used much.

Polar:

• Uses two voltage levels, one positive and one negative
• Note how they deal with synchronization problem
• NRZ-L
• Non-Return to Zero
• 1 is represented by positive (or neg) voltage, 0 is opposite
• RS-232 is example
• NRZ-I
• Non-Return to Zero - Inverted
• 1 is represented by voltage transition, 0 by no transition
• good synch on strings of 1's, poor synch on strings of 0's (less frequent)
• RZ
• Return to Zero
• Uses 3 levels: positive (1), negative (0), and 0 volts.
• signal goes to 0 volts midway through bit interval (synch)
• Note: requires double the bandwidth!
• Manchester
• 1 is represented by negative-to-positive transition in mid-bit
• 0 is represented by positive-to-negative in mid-bit
• also have transition at beginning of bit (to get back).
• Used in Ethernet
• Differential Manchester
• 1 is represented by no transition at beginning of bit
• 0 is represented by transition at beginning of bit
• There is a bit transition in mid-bit in either case (for synch)
• Used in Token Ring

• Analog wave encoded by digital pulses
• Multistep process:

1. Quantize (sample signal, assign integer value to sample)
2. encode to binary (binary string that represents integer value)
3. use D/D encoding to transmit (above)

• Used to transmit analog telephone data on digital T-lines (e.g. T-1, T-2).
• PCM (Pulse Code Modulation)
• Sample analog signal at fixed intervals
  (using PAM - pulse amplitude modulation)
• Quantize signal value into integer in range -127 to +127
• Integer encoded into 8-bit sign-and-magnitude number
  (bit 7 is sign, bits 0-6 are absolute value)
• 8-bit value encoded for digital transmission
• Would like to small number of bits per sample, small number of samples
• Small bits per sample limits precision of decoded signal
• Nyquist: sampling rate must be at least double the highest frequency.
• What is minimum sampling rate for voice-grade telephone comm?
• PCM standard is 8000 samples/sec, 8 bits per sample (64000 bps -- not 64k!)

• Encode by modulating a fixed carrier signal 
• Carrier has known amplitude, frequency, phase 
• Receiver compares received signal to carrier -- differences reveal the code 
• Amplitude Shift Keying (ASK)
• Two amplitude values: one for 1 and one for 0
• Can generate by: adding unipolar digital signal to carrier
• Susceptible to interference (voltage spike affects amplitude)
• One bit per baud

• Frequency Shift Keying (FSK)
• Encode by shifting between two carrier frequencies
• Not susceptible to interference
• Can generate by: applying unipolar digital signal to one carrier, inverse digital to other, then summing the result
• High bandwidth requirements: baud rate plus difference between two carriers
• Example: for 600 bps, half-duplex (signal gets full bandwidth).
  Carriers must be > 600 Hz apart -- assume 1000 Hz apart.
  Lower half of lower carrier bandwidth is 300 Hz
  Upper half of upper carrier bandwidth is 300 Hz.
  Total is 300+1000+300 = 1600 Hz.

• Phase Shift Keying (PSK)
• Encode by modulating phase of carrier
• Not susceptible to interference, low bandwidth requirements
• For 2-PSK, define two phases, e.g. 0 and 180 degrees: one bit / baud
• For 4-PSK, define four phases, e.g. 0,90,180,270 degrees: two bits / baud
• For n-PSK, define n phases: log₂n bits / baud
• Often shown as dots on a constellation diagram
  (cartesean coordinate system, phase = angle of line from origin to dot, amplitude = length of that line)

• Quadrature Amplitude Modulation (QAM)
• Combination of ASK and PSK
• 8-QAM: 2 amplitudes, 4 phases for each amplitude (3 bits/baud)
• Possible 16-QAM: 2 amplitudes, 8 phases for each amplitude (4 bits/baud)
• Possible 16-QAM: 4 amplitudes, 8 phases, some combinations not used
• Given 2400 baud limit on phone line, what is required for 14.4Kbps, 28.8?

• Example is radio transmission.
• start with fixed carrier, modulate its amplitude or frequency or phase

• Amplitude Modulation (AM)
• Encode signal by modulating carrier amplitude
• Susceptible to noise (static on AM radio)
• space between carriers based on frequency range
• AM radio has 5KHz freq range, 10KHz between carriers

• Frequency Modulation (FM)
• carrier frequency changes with amplitude of information signal
• Not susceptible to noise
• Space between carriers based on frequency range (plus guard band)
• FM radio has 15 KHz freq range, 200KHz between carriers

• Phase Modulation (PM)
• Carrier phase changed with amplitude of information signal
• None of my resources describe application of this method

Last reviewed: 18 February 1998