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.