Analog and Digital Data Transmission

A bunch of definitions.
- Data is the information that contains meaning
- Signals are the electromagnetic representations of the data.
- Signaling is the propagation of the signal along the media.
- Transmission is the communication of the data by the process of signaling.
Three examples:
- audio communications
  - Range in frequencies from 100 Hz to 7 kHz.
  - Most energy is in the lower frequencies.
  - Most frequencies below 600-700 Hz add very little to understanding.
  - The range is about 25db from whisper to yell.
  - db is a log scale,
  - This is very much an analog signal.
- Analog video
  - Electron guns and pixels
  - Scan lines and fly-back
  - Interlaced
  - 30 frames per second = flicker
  - 60 frames per second is fine.
  - A time dependent signal
  - This is also an analog signal.
- Text
  - Encoded as (Morse code, ASCII, Unicode)
  - ASCII - 7 bit code, 1 bit error detection
  - Even parity, odd parity
Analog signals are continuous
Digital signals are a sequence of pulses
- Digital signals are usually cheaper to send.
- They a binary thing however, once they are lost, they are lost.
- Attenuation is the loss of signal strength over distance.
- The higher the frequency, the greater the attenuation.
Back to the examples:
- Sound:
  - Sound waves, amplitude measured in db are converted to electromagnetic signals, amplitude measured in volts.
  - Range is reduced to 300 to 3400Hz.
  - Analog, analog, analog
- Video
  - There are 525 lines (in the US)
  - There are 30 frames/second
  - If we interlace, we half the first and double the second, so it is a wash.
  - ```
  1 second    1 frame       1 second
  --------- x---------    = --------     = 1/15,750 sec/line
  30 frames   525 lines     30*525 lines
  
  or 63.5 μsec/line
  	  
```
- Stallings tells us that 11μsec are lost in traceback.
- So the final rate is 52μsec / line
- To estimate bandwidth
  - Black would be a constant signal low
  - White would be a constant signal high
  - The maximum frequency would be switching between the two each pixel.
  - While there are 525 lines, 42 are lost to traceback, and we only see about 70% of what is left. (525-42)*.7 = 338
  - The ratio of the screen is 4:3, so 4/3*338 = 450 dots per line.
  - So we would need to cycle from low to high 450 times per line.
  - But remember, we get two bits per cycle.
  - So we need to achieve 225 cycles/ 52.5 μsec
  - This is 4.29 MHz
  - Stallings tells us that this is a fairly accurate figure.
  - Since the constant screen is a single value, the frequency is 0.
  - So the bandwidth is about 4MHz.