| 单词 | voice grade |
| 释义 | voice grade Bandwidth sufficient to support voice communications. In analog transmission systems, a standard voice grade narrowband channel has nominal bandwidth of 4,000 Hz (4 kHz), which is the standard for analog voice. Within that channel and as illustrated in Figure V-1, the 0–300 Hz range generally is ignored, suppressed by the equipment’s lack of ability to deal with voice at those low frequencies, which also avoids picking up hum from AC power lines. The active voice band is approximately 3.0 kHz wide, running at 300–3,300 Hz. As most speech activity takes place within this range, the level of fidelity is considered quite acceptable. Signaling and control functions take place in the 3,300–3,700 Hz band. In analog multiplexers, the lower band of 0–300 Hz and the upper band of 3,700–4,000 Hz are used as guard bands, i.e., for maintaining separation between information channels; on local loops these frequencies are filtered out. Multiple voice grade channels can coexist on an analog transmission facility, each running in a distinct carrier frequency range, and multiplexed using Frequency Division Multiplexing (FDM). Band-limiting filters employed in carrier networks constrain the amount of bandwidth provided for a voice application, which conserves bandwidth without overly compromising fidelity. Capping the bandwidth at 3,300 Hz also prevents aliasing, a phenomenon that occurs when different continuous signals overlap and become indistinguishable when encoded into digital format. In digital systems, a narrowband channel is 64 kbps, based on G.711, which is the fundamental standard for digitized voice.There are numerous other standards for voice encoding, most of which involve considerable compression to improve bandwidth efficiency. Multiple voice grade channels can coexist on a digital transmission facility, each running in a distinct time slots, and multiplexed using Time Division Multiplexing (TDM). See also aliasing, bandwidth, carrier, G.711, guard band, and narrowband. 4,000 Hz Guardband 3,700 Hz Signaling & Control 3,300 Hz Voice 300 Hz Guardband 0 Hz |
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