Pro line level
In pro line levels, +4dBu is commonly used as the reference level.
+4dBu corresponds to a signal level based on 1.228V, and it is the standard signal strength commonly used among professional audio equipment. Transmitting at levels higher than this would be inefficient due to excessive power consumption of the preamplifier, and transmitting at lower levels may result in signal degradation due to the typical impedance of XLR or TRS cables.
One common method for transmitting pro line level signals is balanced transmission. Balanced transmission sends the signal in two electrically opposed states, minimizing external interference and noise. For this purpose, connectors with three contact points like TRS (Tip-Ring-Sleeve) or XLR are used, and each signal consists of a normal signal, an inverted signal, and a common ground. This method increases the signal’s resistance to noise and enhances its accuracy.
Such levels have no loss at 100m when the impedance is 600 ohms. A consistent signal level is required to maintain the quality of the signal while minimizing attenuation or distortion even at a cable length of 100m. Pro line levels are generally designed assuming a 600-ohm impedance, and through impedance matching, they can minimize loss depending on cable length to maintain signal quality. Therefore, even at 100m, transmission between 600-ohm equipment at pro line levels can deliver a precise signal without loss.