Narrowband speech is important between 300 and 3400 Hz. The bandwidth is 3100 Hz. It is also referred to as 3k (or 3K) flat band, which means 3000-Hz bandwidth. Wideband is gaining popularity through VoIP and multimedia terminals. Wideband voice frequencies are from 100 to 7000 Hz. Sometimes wideband is also referred to in the range 50 to 7000 Hz.
The ratio of two quantities in logarithmic scale is represented in deciBel (dB) units. In power representation, when the reference power is 1 milliWatt, the dB units are denoted as dBm, where “m” represents 1 milliWatt of power. Speech power levels are expressed in dBm units. Noise power is low and expressed using picoWatt units represented as dBrn; 0 dBm = 90 dBrn (rn is referenced for noise power of 1 picoWatt).
The undistorted sine wave amplitude in μ-law is 3.17 dBm, which gives quantized sample values in the range of ±8159. The full-scale square wave gives 6.17 dBm. In A-law, sine wave power is 3.14 dBm, and samples are in the range of ±4096. Typical speech levels are at −16 dBm, and ideal channel noise is lower than −68 dBm (22 dBrn). When speech levels are approaching −40 dBm, speech is not perceivable clearly.
Signals are measured with milli-Watt (mW) reference and noise is measured with pico-Watt reference (pW). The noise measurement without ...