Last week’s TTOTD on impedance matching predictably sparked questions about levels. A typical one comes from inSync reader and Sweetwater friend, Wil P.
“In reference to your mention of level matching, I’d love to see inSync go there. I have such level matching concerns with specific pieces of gear. For one, my Mackie 8 Buss and XT20’s. The XT20 manual suggests setting levels around -15dB. That may be good for the faint of heart, but I have to push them harder than that (without overload of course)! Maybe it’s just a meter issue, but my Mackie output meters are riding at +10 to get a -3ish reading on the XT’s. My +4/-10 switches are right on the 8 buss and I’m using Elcos. What’s the deal? Also, the balanced 2-track XLR output of the Mackie to the balanced XLR input of my Sony DAT is terrible. Similar issue, levels pegging on the Mackie to get what I need at the DAT machine. Am I missing the total obvious here?”
This is mostly a metering issue, and represents a fundamental difference between the way metering works (in practice) on digital equipment versus analog equipment. The reason the ADAT manual suggests a -15 dB setting is probably because that is its zero reference. That means that the ADAT will meter at -15 dB on its meters when a +4 dBu signal is applied to its balanced +4 input (the Elco connector). Conversely, when the Mackie is outputting a +4 dBu signal its meters will show zero (just before going into the red). This is its zero reference. Pushing either of their levels beyond this point is a decision you can make, but it will eat up available headroom. When the digital machine’s meters are up at its zero, or near full code level, its input or output will be more in the neighborhood of +26 or +28 dBu. Similarly, the Mackie can put out as much as +28 dBu with minimal distortion, but in so doing its meters will be slammed all the way to the top. Those respective levels represent the highest signal level the devices can input or output without distortion. There is no headroom left at this level.
The thing to remember is that music is dynamic. Your levels need to be set so you can capture the dynamic range of what you are working with (sometimes much easier said than done) regardless of what the meters tell you. So if your setup dictates that the ADAT is at -15 when the Mackie is at zero just keep in mind that there is plenty of headroom there in both devices. If you don’t need that much headroom (often rock or pop music does not) then go ahead and crank it up a bit.
Here’s where it can get tricky with a DAT machine. DAT machines have variable level controls as part of their input stage before the signal hits the A/D converter. In some machines these go above unity gain. If you turn your controls up on the DAT so its level is at maximum when your Mackie is at zero you have actually lowered the signal to noise ratio of the system. Why? This means that when your DAT is at maximum level, your Mackie isn’t. It is at a much lower relative level. You are really better off if both pieces reach their maximum level at the same time. This maximizes your signal to noise level and dynamic range for the system. The only issue is to make sure you set your recording levels so you have sufficient headroom to make the recording without going over this maximum level.
Clear as mud?