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 and Total Harmonic Distortion (THD) 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.