Why Digital Communications?
In the past, analog modulation techniques were used for analog signals and digital modulation techniques were used for digital signals. This model is now going through fundamental changes. It makes more sense to use digital modulation for analog signals as well as digital signals. For example, in digital broadcasting we can transmit CD quality sound with less bandwidth than FM radio. There are numerous other examples where analog information is transmitted via digital modulation schemes in applications such as digital voice in cellular and digital video transmission in HDTV.
We will witness a tremendous growth in digital communications applications well into the 21st century. The greatest growth area is in digital transmission of analog information. In these applications, deep down in the physical layer, there are two major functions - Codec and Modem.
The first part is the codec (coder-decoder). Codec is responsible for analog-digital conversion and compression-decompression of the analog information. Most of the complexity is in the compression side of the codec.
The second half is the modem (modulator-demodulator). The transmit modem takes the compressed information bits and generates waveforms suitable for the transmission channel. The receive modem, which usually is much more complex, takes the received waveform and maps that information to bits. And if everything works, the receivers bit stream will match the transmitters.
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