External Signal Levels
There are several standards for line signal levels. The −10 dBv standard is used for a lot of semi-professional recording equipment as it gives more headroom with unbalanced connections – the professional levels of +4 and +6 dBu assume balanced outputs that inherently give twice the output level for the same supply rails as it is measured between two pins with signals of opposite phase on them. See Table 14.1
Signal levels in dBu are expressed with reference to 0 dBu = 775 mVrms; the origin of this odd value is that it gives a power of 1 mW in a purely historical ...