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Voice Digitization

 

Digital Voice Quality Categories

Throughout this section, comparisons can be made in terms of voice quality. Therefore, it is necessary to define the quality categories:

• FM BROADCAST and HI-FI QUALITY
   FM Broadcast has 50 Hz – 15 KHz bandwidth with no perceptable noise.
  High Fidelity (HI-FI) has wide bandwidth (0 Hz – 20 KHz), with no noise.
• AM BROADCAST QUALITY
  Wide bandwidth (50 Hz – 6 KHz), with no perceptable noise.
  Used within the broadcasting market for commentary, remote correspondent audio, etc.
  Landlines may be equalized up to 8 KHz.
• TOLL QUALITY
  The “Standard” telephone speech signal quality.
  Bandwidth of 200 Hz – 3200 Hz with a Signal/Noise Ratio (SNR) of > 30 dB
  Total Harmonic Distortion (THD) is below 2-3%.
• TRANSPARENT QUALITY
  Quality similar to that of Toll Quality.
  The difference in quality can only be distinguished by direct A/B comparison
  
• CONVERSATIONAL QUALITY
  Highly intelligible, but with noticeable distortion
  While worse than Toll Quality, the speaker at the other end can be identified and understood
  
• SYNTHETIC QUALITY
  More than 80% intelligibility, but with degraded quality
  The synthetic sound is machine-like, and it may not be possible to identify the far-end speaker

As a general guideline, the diagram below indicates voice quality as a function of the digital channel’s bit rate and algorithm used.