Basic components of a modem

The figure given below is a block diagram of a modem suitable for the interfacing a home or business computer with telephone line. The modulator, or D/A converter, changes digital computer data into audio tones. The demodulator, or A/D converter, changes the incoming audio tones into the digital signals for computer. The audio tones fall within frequency range, or band, of around 300 Hz to 3 kHz. This is the band required to clearly transmit a human voice.

It is amazing how much computer data can race over the single telephone or radio circuit having a narrow bandwidth. Even the pictures can be sent and received in the brilliant color and in quite good detail. As you may imagine, color images take longer than the gray scale images to send and receive; also, more detail an image contains, the longer it takes to be transferred at the given data speed.