Click here to purchase the entire book in PDF format. Operating level You're may already be familiar with electrical operating levels in analog recording and playback equipment. If so, then you know that, on a professional piece of equipment, when your output level VU meter says ``0 dB,'' then you should be getting +4 dBu at the output. (If this is completely unfamiliar to you, don't worry. It'll be thoroughly discussed in Section 10.1.) In the world of analog tape, we have a similar issue. The question is, ``when the record meter on my deck reads 0 dB VU, how much am I magnetizing the tape?'' The answer to this question is your operating level (also known as the reference level or the reference fluxivity). It's a measure of the fluxivity on the tape, measured in Wb per meter. How do you decide on what operating level to use? This depends on the type of tape that you're using, and what kind of music you record. Analog recording tape has some maximum amount of fluxivity that it can ``store'' - an amount called the maximum output level or MOL. Different types of tape have different MOL's. This is defined as the point where the tape starts to saturate, and the difference in the signal you tried to put on the tape and the signal that comes back off the tape causes a distortion of 3% THD. (A definition of THD can be found in Section 7.2.16.) For example, Ampex 456 will produce 3% distortion when you try to put a sine wave with a level of approximately 762 nWb/m (nanoWebers per meter). By comparison, 3M 206 tape will produce the same 3% distortion is you try to put a sine wave with a level of 465 nWb/m. So, if you've set your record head to put a given amount of magnetism on the tape and you record an increasingly high level of signal, you'll distort the 3M tape before the Ampex tape. Therefore, you should set the standard operating level lower if you regularly use the 3M tape than if you use the Ampex tape. Let's also say that you record death metal with absolutely no dynamic range and I record classical music and I've never seen a compressor in my life, but we both use the same tape. You know exactly what your maximum level will be, but I never will, because my orchestra might hit a really loud point and surprise me. Therefore, I need more ``headroom'' when I record - more space to put extra level in case I need it. You don't need this headroom because death metal never surprises. Consequently, you can set your operating level to a higher level than I can. This is because you always know how far away you are from saturating the tape and therefore distorting the signal - I almost never do... Once upon a time, a typical operating level was 185 nWb/m. Back about 15 years ago, 250 nWb/m was a more standard operating level - 3 dB higher than the older level. Once analog tapes with extremely high operating levels were introduced on the marker (such as Ampex 499 tape) people could push their operating level even higher. These numbers are not standard for all recording studios, however, so you will have to know the operating level before you start recording. For example, if you are used to working with a deck that is aligned to 185 nWb/m and you do a gig in a studio that's aligned at 250 nWb/m, you'll distort your tape 3 dB earlier than you're used to because the deck is putting more magnetism on the tape for the same reading on the meters. The moral of this story is to know the behaviour of your tape and your operating level.