Special Edition, Part 6 – Final Installment
Welcome to the final installment of our look at the dB and the -10 dBV, +4 dBu problem. Now that we know the difference between the two we will continue discussing some of the practical considerations.
What does this discrepancy really mean to my levels?
For these conclusions you can follow along proving each statement using the math unveiled in the discussion so far, or you can just take our word for it. A -10 dBV device outputting a signal at its “zero” metering level is outputting -10 dBV, which we have shown to be .316 volts (a high impedance is assumed, but not specified). This is its zero reference and that level is equal to -7.8 dBu, but shows up on the meters of a +4 dBu device as -11.8 dB. Of course this assumes the input device is of sufficiently high input impedance not to “load down” the output device. In order to get the +4 dBu device up to its “zero” level on its meter you must drive the -10 dBV device until its meters register +11.8 dB, at which point it is outputting 1.78 dBV, or 1.23 volts (which equals +4 dBu). Most -10 dBV devices can deliver this much voltage, but it puts them awfully close to clipping. At this point you are about out of headroom on the output device (at nearly its maximum level) while you are just beginning to drive the +4 dBu device. When the -10 dBV device peaks or clips there is still tons of headroom (probably at least 11.8 dB) available in the +4 dBu device. There is no way to drive the +4 dBu device to maximum level unless it has some sort of amplifier on the front end you can turn up to raise the voltage of the signal. Of course the tables get turned the opposite way if you are driving a +4 dBu device into a -10 dBV device. In most situations a reasonably high signal in the +4 dBu device will overload -10 dBV equipment. Maybe you can turn the level down at one end or the other, but you may be compromising your signal to noise ratio. For these and other reasons it is generally not advisable to interface the two different standards together without some sort of gain stage in between that properly changes from one level to the other. There are many such devices made in various price ranges.
Here is a handy chart that summarizes much of the above paragraph (some values have been rounded off for clarity).
| -10 dBV |
Voltage |
+4 dBu |
| Meter Reading |
dBV |
dBu |
Meter Reading |
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Yeah, but does it “really” matter that much?
It depends on what you are doing. In critical situations where you want the lowest noise floor and maximum dynamic range, yes, it really is important to have the levels properly matched. In other applications you can often get by “fudging” it, especially on equipment with input and output level controls. As long as you understand what you are doing and what you are asking of the equipment you can make it work okay in many situations. Your ears will be the final judge.
Why is -10 dBV equipment sometimes considered consumer or semi-pro?The -10 dBV standard arose because manufacturers were looking for less expensive ways to build equipment. The componentry required to drive .775 volts (with headroom to spare) into a 600 ohm load is relatively expensive. When input impedances got up in the 10,000 ohm range designers realized they could use much lower voltages and save money. The -10 dBV standard took off in the hi-fi industry while the pro audio and broadcast industries were still building and using gear referenced to dBu and dBm. Once -10 dBV gear began to crossover into professional use the trouble started and the rest is history, as they say.
It’s important to understand that there really isn’t much of an inherent disadvantage to a .316 volt (-10 dBV) nominal level compared to a 1.23 volt (+4 dBu) signal in the modern equipment we use today. However, since (as a matter of convention) the +4 dBu standard is usually applied to high quality balanced connections on high quality “pro” gear, while the -10 dBV standard is often used with unbalanced connections – sometimes of unknown quality on gear of sometimes questionable quality – the stigma that -10 dBV is “not as good” as +4 dBu has lingered in our industry. It really isn’t always true, and in fact it is sometimes preferable to use the -10 dBV unbalanced connections on gear rather than routing your signal through another gain stage to balance it and raise the level up to +4 dBu. It all just depends on the specific equipment and your specific circumstances.
Is all equipment rated at -10 dBV or +4 dBu?
Of course not. Why have two standards when you can confuse the public with more? 0 dBu, +8 dBu, 0 dBm, 0 dBV, and 0 dB (whatever that means) are a few of the operating levels you may find on equipment. And don’t forget about the old dBv (little v). The cool thing is that you now have the background to figure out what they actually mean in terms of voltage and real world interfacing with your existing gear. Sometimes impedances will be specified, but usually not. If no impedance is specified you should assume a very high input impedance and a very low output impedance, which means you only need to worry about voltage levels, not power transfer.
That’s all for now. We’ve laid a great foundation we can build on. There are many other related issues we can now delve more deeply into. We will create a direct link to this segment of inSync out in our Summits link area so you can always refer back to this discussion when you get confused. And you will get confused from time to time about some of this stuff.
Now go connect your equipment and make music.
By the way, did anyone notice that +11.8 + 2.2 = 14?
