Problems with Analog Connections

PROBLEMS WITH ANALOG CONNECTIONS

  • Distance limitations – analog signals eventually fade.  Repeaters can be introduced to regenerate signal but also regenerate anything else picked up including white noise.
  • Wiring requirements – for each call, a tip and ring wire is required. Not scalable. 

Digital signals convert analog wave into zeroes and ones.

DIGITIZING VOICE

Step 1 – Sample the signal

  • Nyquist formula: If you can sample at twice the highest frequency, you can accurately reconstruct a signal digitally.
  • Common frequencies – Human ear: 20 – 20,000Hz. Human speech: 200 – 9000Hz
  • Nyquist theorum: 300 – 4000Hz.

Step 2 – Perform Quantization on the Sample

  • Lining samples up to a scale that the samples can be represented with. In this case taking the value of the amplitude and line it up to a scale (known as Pulse Amplitude Modulation (PAM).  The Y access on the chart are labelled segments.  More samples taken around the baseline.

Step 3 – Convert the “quantized” signal to binary

  • Known as pulse code modulation (PCM).
  • 2 PCM methods: A-LAW and MU-LAW.
  • A-LAW makes more sense.
  • Mu-Law used in US, Japan and Canada
  • A-LAW used elsewhere. First digit: 1 presents positive numbers . 0 represents negative numbers. Next three digits represent Segment number. Next four digits are interval.
  • Mu-Law –  The exact opposite of A-LAW.

Step 4 – Optionally compress the samples

You can

  • Send all these samples
  • Send just the changes
  • Build a code book

Standard voice sample is 64Kbps.  Common compressed value: 8kbps (G.729 codec)

 

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