Gain Staging

Analogue equipment is designed around a nominal operating level. Professional equipment runs at +4 dBu, about 1.23 volts, and consumer equipment at -10 dBV, about 0.32 volts. The difference of roughly 12 dB explains why a consumer output connected to a professional input sounds weak and noisy: the input receives far less level than it expects. Digital systems measure levels against 0 dBFS, the absolute clipping point of the converter as defined by AES17. To leave room for peaks, broadcast practice aligns nominal level well below it. The EBU R68 recommendation maps the analogue alignment level of 0 dBu to -18 dBFS, while SMPTE recommended practice RP155 maps +4 dBu to -20 dBFS, leaving 6 dB more headroom. In live sound, a similar target is common practice rather than a standard. The gap between nominal level and zero is the headroom, which absorbs sudden peaks such as a drummer playing harder.

The two failure modes sit at opposite ends. With too little gain, the noise floor is raised along with the signal and everything hisses. With too much gain, the signal clips, and a clipped digital peak cannot be recovered by any later fader. The usual practice is to set the input trim so that the average level of normal programme material sits around -18 dBFS, with peaks well short of zero, then to run every later stage at approximately unity gain so that the level set at the input is preserved.