What is Voiceband?
In electronics, voiceband
means the typical human hearing frequency range that is from 20 Hz to 20 kHz. In telephony, it means the frequency range normally transmitted by a telephone line, generally about 200-3600 Hz. Frequency-division multiplexing in telephony normally uses 4 kHz carrier spacing. The roll-off rate, or rate at which the amplitude of a signal drops off near the upper and lower limits can vary with the design of the band-pass filters.
The question of frequency spacing was addressed at an international meeting in the 1930s. Germany and Britain favored 2 kHz spacing, while the Netherlands and some other countries preferred 6 kHz. The question was compromised at the American position, which was the 4 kHz spacing that remained standard and also fixed the standard PCM sample rate at 8 kHz, which in turn defines what "voiceband" means for this purpose. The abandonment of in-band signaling in the late 20th century made available a larger voiceband for modems and voices.