Before converting the signal from analog to digital, the unwanted frequency components of the signal are filtered out. The human voice uses frequencies between 100Hz and 10,000Hz, but it has been found that most of the energy in speech is between 300 Hertz and 3400 Hertz - a bandwidth of approximately 3100 Hertz. The combined encoder/decoder is known as a codec. The main parts of a conversion system are the encoder (the analogue-to-digital converter) and the decoder (the digital-to-analogue converter). Pulse code modulation is the most frequently used analogue-to-digital conversion technique, and is defined in the ITU-T G.711 specification. Each analogue sample value is quantised into a discrete value for representation as a digital code word. Pulse code modulation is an extension of Pulse Amplitude Modulation (PAM), in which a sampled signal consists of a train of pulses where each pulse corresponds to the amplitude of the signal at the corresponding sampling time (the signal is modulated in amplitude). The accuracy with which the analogue signal can be reproduced depends in part on the number of bits used to encode the original signal. The digital signal is subsequently used to reconstruct the analogue signal. Waveform coding is used to encode analogue signals (for example speech) into a digital signal. Pulse Code Modulation (PCM) is the simplest form of waveform coding. The conversion of analogue signals to digital signals therefore eliminates the problems caused by attenuation. Digital signals are more easily separated from noise and can be regenerated in their original state. When the signal-to-noise ratio of an analog signal deteriorates due to attenuation, amplifying the signal also amplifies noise.
Analog transmission is not particularly efficient.
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