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Why are my two DAT machines giving me different analog levels with the same digital signal?

“I’ve been reading your excellent tips about +4 versus -10 level issues, and about digital levels. I understand what 0 dBFS is, but what I don’t understand is why I can run one digital signal into my two different DAT machines and get two different analog levels out of them. Help!” Man, this stuff IS confusing. I’ll be the first to admit it. Here’s the deal. In past issues we’ve already established that 0 dBFS is, for all practical purposes, the full code level on a DAT machine (or any other digital recorder). We’ve also explained that different machines may use different zero reference levels. It’s easy to let this go in one ear and out the other until you finally run into it, as you have. What does it really mean though?Recall what a zero reference level is. In a digital recorder some people think the 0 dBFS level is the zero reference level. This isn’t the case. The zero reference level is a signal level defined in the machine that corresponds to some other standard reference level, which will generally be +4 dBu or -10 dBV. When you input a signal at one of those two zero reference levels (the appropriate one will depend upon the type of I/O your machine has) your digital meters will not go all the way up to zero. They will go to some amount below zero. Where the level falls depends upon how your machine is set up. A typical DAT machine will use anything from -12 dBFS to -20 dBFS as its zero reference. Some can be set by the user. Put a +4 dBu sine wave into a Panasonic SV3700 or 3800, for example, and its meter will show -18 dBFS. On a Fostex D-10 (as configured from the factory) you’ll get -12 dBFS (you can program it for other levels). The difference is mainly related to headroom because in digital metering you are mostly concerned with how far below full code you are. You can still hit the machine with whatever level you want, but someone has to decide on what the reference level is so it can closely approximate the behavior of analog gear. The level your machine uses should be documented in the manual and most likely is indicated by somehow highlighting that level in its meter display.Now, here’s where it gets tricky. Suppose you are doing a digital dub from one DAT machine to another. We know with digital the level recorded to the new tape will be the same as the source tape (errors and error correction aside). But if the machines are set up with different zero reference levels they each will put out a different analog voltage even though they are both reproducing the same signal. Yes, that’s a different analog audio level with the same digital signal. This is because the reference level just tells the machine how the analog levels relate to the digital levels. We only care what goes on the tape so this isn’t all that big of a deal. It affects our monitoring only. One machine is simply “louder” than the other, but they both have the same signal on tape.So what happens if we record an analog signal into a digital device? Take a sine wave at +4 dBu and make a recording of it on your Panasonic DAT (it should read at -18 dBFS on the meters). Play that tape back on other DAT machines and what happens? It shows up at -18 dBFS on all of them. That’s the level on the tape. It isn’t going to change. However if you play it back on a machine with a different reference level you’re going to get a different analog output level for that -18 dBFS signal. On the Fostex D-10 your output will be at -2 dBu. Pretty nutty, huh? That’s what happens when these things don’t get standardized.Most of us rarely come face to face with all of this is minutiae because in digital we were all correctly taught to simply record the level as hot as possible. Most novice users never spend too much time figuring out the zero reference levels of everything in their rooms and have thus been blissfully ignorant of these inconsistencies with digital recorders. Fortunately one can make a perfectly acceptable digital recording without any knowledge of the zero reference level of the machine in question. The problem shows up mostly as an annoyance during dubs or when playing back tapes on machines that are calibrated differently.(For more background on the distinctions between +4 dBu and -10 dBV please consult our Special Edition in the Summits area of inSync.)

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