Regardless of its size, a good hi-fi sound system must have certain characteristics. Here are two of the most important technical considerations.
Linearity is the extent to which the output waveform of an amplifier is a faithful reproduction of the input waveform. In hi-fi equipment, all the amplifiers must be as linear as the state of the art allows. If you connect a dual-trace oscilloscope (one that lets you observe two waveforms at the same time) to the input and output terminals of a hi-fi audio amplifier with good linearity, the output waveform is a vertically magnified duplicate of the input waveform. When the input signal is applied to the horizontal scope input and the output signal is applied to the vertical scope input, the display is a straight line. In an amplifier with poor linearity, the instantaneous output-versus-input function is not a straight line.
The output waveform is not a faithful reproduction of the input, and distortion occurs. In some RF amplifiers this is all right. In hi-fi audio systems, it is not. Hi-fi amplifiers are designed to work with input signals up to a certain peak (maximum instantaneous) amplitude. If the peak input exceeds this level, the amplifier becomes nonlinear, and distortion is inevitable. In a hi-fi system equipped with VU or distortion meters, excessive input causes the needles to kick up into the red range of the scale during peaks.
Dynamic range is a prime consideration in hi-fi recording and reproduction. As the dynamic range increases, the sound quality improves for music or programming having a wide range of volume levels. Dynamic range is expressed in decibels (dB).
At low volume levels, the limiting factor in dynamic range is the background noise in the system. In an analog system, most of this noise comes from the audio amplification stages. In tape recording, there is also some tape hiss. A scheme called Dolby (a trademark of Dolby Laboratories) is used in professional recording studios, and also in high-end consumer tape equipment, to minimize this hiss. Digital recording systems produce less internal noise than analog systems.
At high volume levels, the power-handling capability of an audio amplifier limits the dynamic range. If all other factors are equal, a 100-W audio system can be expected to have greater dynamic range than a 50-W system. The speaker size is also important. As speakers get physically larger, their ability to handle high power improves, resulting in increased dynamic range. This is why serious audio enthusiasts sometimes purchase sound systems with amplifiers and speakers that seem unnecessarily large.