Analog to digital

• This is digitization
• This is done with a codec (coder decoder)
• Two techniques are discussed
  • Pulse Code Modulation (PCM)
    • Based on the sampling theorem
    • If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest signal frequency, the the samples contain all the information of the original signal
    • Since voice data is below 4kHz, 8k samples per second are sufficient.
    • So for a bandwidth of B, 2B samples are taken every 1/2B seconds.
    • Each sample can be quantized into 2ⁿ different levels.
    • Each quantized sample represents the amplitude.
    • Then n bits are needed to send the value.
    • Stallings tells us that quantizing violates the sampling theorem and introduces an error.
    • This is called the quantizing error
    • This makes sense, we are taking a sample, then moving it to a different value.
    • 
    • For good voice encoding, we need 128 levels (7 bits)
    • This implies a data rate of 56Kbps
    • Color TV uses 10 bit codes, (1024 levels)
    • This requires 92 Mbps at 4.6 MHz
  • Delta Modulation(DM)
    • The value of the pulse is approximated with a discrete function.
    • Only a change of +/-1 is sent.
    • 
    • This is much simpler than PCM.
  • Why analog to digital?
    • As mentioned before, less susceptible to noise
    • The equipment introduces less noise.
    • Time Domain Multiplexing can be used, and this adds less noise
    • Digital allows different switching techniques.
    • Compression and algorithms can be used to reduce data rate
      • Voice could drop to 4kbps
      • Video to 15Mbps
      • Teleconferencing to 64kbps