Sometimes we’re just curious and we don’t even know why. This was the case with an inSync reader recently.
“The other day I was doing a dub from a cassette into a DAT machine and I noticed how far apart the meters are from each other. Zero on the cassette’s meters corresponded to about -16 on the DAT machine. I’m aware of how digital machines meter some amount down from zero, but I’m not sure of the difference between analog and digital and why everything is referenced to zero in the first place.”
We’ll start with the second half of your question first. Why do all of our levels reference zero? The zero reference, as it is sometimes known, exists because back in the old days of audio the electronics in a lot of equipment worked in a fundamentally different way than they do today. The phone company in particular had a unique set of concerns – they had to transmit audio signals over very long distances. Anytime you move electrical signals over long lengths of wire you will have losses, and the phone company had to pay very close attention to this. Their concern was that when the signal reached its destination they hadn’t lost too much signal. This primary concern over the loss of signal had them adopt the convention that any signal loss would be measured as some amount of dB “down” from where it started. Zero was considered no loss of signal so if a signal lost 3dB of level along the line it would show up at the destination reading -3dB on a meter. No loss would read as 0dB on the meter, indicating no loss.
This convention has worked out pretty well over the years. Companies can design gear to work with a standard reference level for the type of gear it is and the user knows he has some amount of headroom above that level, depending upon the equipment. When a level of +4dBu is applied to the input of a tape deck rated at +4dBu nominal input level its VU meters will read 0VU (assuming the input control is set to unity gain). If your mixer is calibrated so that its meters read zero when it is outputting +4dBu then all is well. You can use either set of meters to know what your levels are. You can go above or below zero by some amount as dictated by the dynamic range of the equipment and your tastes.
Digital threw this all off because, unlike analog, digital equipment actually produces more distortion at lower levels. It’s desirable to record at the highest level possible without going over full code in digital (0dBFS). Unlike analog there is a brick wall there beyond which the signal will not get any louder, but will become very distorted very quickly. So digital equipment was set up so that zero on the metering is full code, or the highest level that can be encoded, and all usable levels are some amount below this zero maximum level. For the sake of interfacing with the outside analog world, digital equipment uses a reference level below this new zero by some number of dB (usually 12 – 18 dB below zero on the meters). Apply a +4dBu signal to a DAT machine with +4dBu inputs and you’ll get meter readings down around -12 to -18 depending upon how the machine is calibrated. The idea is to give you a similar amount of headroom as the upstream gear when recording. If your mixer can put out +26dBu before crapping out you sure wouldn’t want your DAT machine going into full distortion at +5dBu. So digital meters are necessarily calibrated differently to allow the equipment to be interfaced with analog gear, yet also remaining true to the idea that once you hit zero in digital there is no where to go from there but into full distortion.