The analog audio interfaces ofsound cards, laptops, CD and DVD players, MP3 players or other components with which audio amplifiers and sound cards are equipped include the mic-in input, the line-in input and the line-out output. Line- In is a high-impedance input into which analog audio signals are fed.
A wide variety of consumer electronics devices such as CD players, turntables, tape recorders, synthesizers, camcorders, tuners, or mixers can be connected to the Line-In jack. The input impedance of the line-in input is high impedance and typically ranges from 20 kilo ohms to 70 kilo ohms. The standardized input level depends on the sound card and can range from +/- 0.7 V to a maximum peak voltage of 2 V. The advantage of the high impedance is that low impedance signal sources such as the line-out output are not loaded by the input impedance.
If a line-out output or an audio output with a low-impedance impedance of 50 ohms to 1 kilo-ohm is connected to the line-in input, an optimal voltage transformation takes place because the low-impedance output impedance is in series with the high-impedance input impedance, and most of the voltage drops across the input impedance. The signal fed into the line-in input is first preamplified and then power amplified so that it can be fed to the speakers.
For consumer electronics devices such as MP3 players or DVD players, the nominal level is -10 dBV. This corresponds to a voltage of 0.316 V. For professional audio equipment, the allowable level is +4 dBu.
Line-In inputs are color-coded like the other analog audio interfaces. Line-In is light blue and equipped with jack plugs, Mic-In is pink and Line-Out is light green. There are also audio devices and sound cards with RCA plugs.