Last month, we talked about dynamic range, headroom and signal-to-noise ratios, and how these concepts relate to the metering used on analog and digital gear. Since there was a bit of brain damage involved with last month’s T&P, we’ll quickly review a few important highlights. First, Zero on a VU meter is known as “operating level.” Second, In the pro audio world, when we see 0 on a VU meter, the signal level is approximately 1.23 volts. This signal level “standard” is known as +4 dBu. Third, Digital gear — such as mixing consoles, drive processors — often use meters calibrated in dBFS (deciBels below full scale), whereby 0 is at the top of their scale. When you hit 0 on a dBFS meter you are out of headroom. If we send +4 dBu (1.23 volts) into a dBFS meter it will typically show up at -18 dBFS.
It’s Complicated
Now for a monkey wrench: Not all audio gear operates at +4 dBu. A lot of semi-pro or consumer gear operates at a “standard” level of -10 dBV (with a capital “V”). Zero VU for such gear indicates a voltage level of 0.316 — very different from that of pro gear. Most audio gear is commonly described as having operating levels of either “+4” or “-10.” Got that?
Fig. 1 shows the display on a multimeter measuring the voltage of a 1 kHz test tone from a signal generator. Note the value: approximately 1.229 volts. We have also connected that test tone to the input of a dbx 162SL compressor (which BTW, makes a very nice L/R bus compressor for your P.A. system). Notice that the dbx 162SL’s meter reads 0 VU. If we look on the rear panel of the dbx 162SL. we’ll see that its operating level switch is set to +4 dBu — meaning that the 162SL’s input is “looking” for a nominal signal level of 1.23 volts. All is well.
If we send that same 1.23 volts into a dBFS meter (see screen shot from Metric Halo’s SpectraFoo, Fig. 2) we’ll see that it indicates -18 dBFS. So, when we see -18 dBFS on our digital mixing console, our bus compressor shows 0 VU and everyone is happy because they are operating at the same level. The signal level “space” between 0VU and +6 on the VU meter represents headroom. The “space” between -18 dBFS and 0 dBFS on our console also indicates headroom, and as they are running at the same operating level, you can be fairly confident that when you run audio through the mixing console below 0 dBFS, you’ll still have headroom on the bus compressor — even though the raw numbers don’t agree.
Take a look at the rear panel of the dbx 162SL (Fig. 3). It has a switch that toggles the operating level between +4 and -10. Why? Because dbx is smart enough to recognize that some people may use their compressor with semi-pro gear, or with pro gear that has -10 inserts. Let’s switch it to -10 and look at Fig. 4: the meter is pinned, and its audio circuitry is overloaded. Why? When we set the operating level to -10, the 162SL is expecting a nominal signal level of 0.316 volt, but we fed it much more than that. By overloading the 162SL’s input, we have consumed all of its headroom and sent it into distortion. That’s why you need to match the operating levels of the devices in your audio system.
If we look at Fig. 5, we’ll see the VU meter returns to 0 with a voltage of 0.315. Sorry, I couldn’t get the tone generator to precisely produce 0.316 volts, but we are dealing with the real-world here.
The Rear View
The reverse of the situation is no better. See Fig. 6, where the 162SL is set to +4 and we are feeding it 0.316 volts. The meter (and more importantly, the output level) of the 162SL is very low — which means it will not drive the next stage of the system properly. You have lost gain between the mixing console and the compressor.
Issues and Answers
Similar problems arising from operating level mismatch were discussed in T&P a few years back (April 2011) but bear revisiting. Some noise gates can be switched between —10 and +4. This setting should match the operating level of the console’s channel insert. If the gate is operating at -10 and the insert is at +4, you’ll have to raise the threshold very high to close the gate. You lose the ability to use a large portion of the threshold control for fine adjustment and may overload the gate’s audio path. How about interfacing a console with outboard reverb or delay? Let’s say your console’s aux send operates at —10 and the reverb is at +4. Even when you crank the aux way up, the processor receives too little signal, and you can’t hear enough of the effect. To compensate, you turn up the aux return, introducing noise into your mix. Booooo.
A more serious issue comes up when you patch a +4 limiter on a -10 L/R output bus or master insert. You won’t get protection for the power amps, because the limiter’s threshold is too high for the console’s output to reach — so limiting never happens.
One way to avoid some of these problems is to route the digital output of a mixer to the digital input of a drive processor — but make sure they are both looking for the same digital standard! We’ll save that discussion for another time.
