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dB or not dB

— Dat is de Question

Paul White does his level best to unscrew the inscrutable.

A bit of basic theory to remind you of the difference between a dB, a dBm and a dBv along with a few reasons why you should want to know in the first place.

The decibel is the unit commonly used to measure sound levels but in this case, 0dB does not stand for silence but for the level below which an average listener can no longer hear a sound. In terms of actual power, this corresponds to one millionth of one millionth a watt per square metre which just goes to show what sensitive instruments our ears are.

Back in the studio however, our meters are not monitoring sound levels but electrical ones so what are we really measuring?

The Decibel

This unit was developed by Alexander Graham Bell in the pioneering days of telephony and though it might be a little out of date, it has remained as the accepted standard of voltage and power comparison.

The dB itself is simply the ratio between two voltages or powers; it is not an absolute measurement of value. Expressed mathematically, NdB = 10log(P1/P2) where P1 and P2 are the two power levels being compared. If we were to compare two voltages rather than power, the formula would be NdB = 20log(P1/P2). In the latter case, every 20dB we add means an increase of ten times. However, we are concerned with both ratios and absolute levels when it comes to the recording studio and that's where the dBm comes in.

dBm and dBv

Again, courtesy of the Bell telephone corporation, the dBv came onto the scene.

For some reason best known to himself, Mr Bell decided that he should measure the voltage across a 600R resistor in which was being dissipated one watt of power. This voltage, being 0.775 volts, developed across 600R was then to be taken as a standard reference and the mystic title of 0dBm was bestowed upon it. Whilst this was all well and good for the telephone people who no doubt spent all day connecting resistors to 0.775 volt power supplies, it still doesn't relate to real life in the studio where 600 ohm lines are seldom used. What then transpired is that a new unit was invented which ignored the load resistance and changed the voltage to a more convenient one volt standard and this was of course the dBv.

Because nothing is ever as standard as we would like it to be, dBv measurements are also made to the 0.775 volt level which all adds to the general confusion. This is because some multi-meters with a dB scale are incapable of telling what the line impedance is and so measure everything to the dBm standard but ignore the line impedance, assuming it to be 600R regardless of its actual value. As they only measure a voltage across a line of indeterminate impedance, the measurement is expressed in dBv. Some manufacturers even hedge their bets by printing both scales onto the meter but fortunately, this unwelcome source of confusion should not find its way into the studio to disturb your piece of mind.

More Chaos

Back in the early days of broadcasting, it was found that the problems involved in building a meter to accurately measure dBm could not be readily solved due to the loading effect of the meter coil which was then around 4000R. Not to be outdone, someone suggested connecting a series resistor of 3.6K to reduce the loading. This was all well and good but now the meter read 4dBm too low because of the power dissipated in this extra resistor — so guess what they did next. Yes you've guessed it, instead of re-defining the zero reference or waiting until someone invented the high impedance meter, they decided to leave +4dBm showing 0VU on the meter and that is where our professional operating level comes from.

So, from all this historic chaos and confusion, we have a universally agreed standard that says 0VU should be +4dBm — or do we?

Along came the Japanese who regarded all this historic telephone exchange stuff with some suspicion and set up their own standard of -10dBm = 0VU which is now commonly used on semi-pro gear by Tascam, Fostex and so on, but then a few bright sparks such as those otherwise laudable Vesta Fire people decided to knock off another 10dBm for good measure and come up with a set of effects units that would only output -20dBm flat out. That might not seem so bad, after all it will just about run with semi-pro gear working at the -10dBm level but there's more.

Some of the very nice semi-pro mixing desks currently available lull you into a false sense of security by giving you nice standard -10dBm inputs and outputs, but when you look at the small print, you find that the insert points work at +4dBm. Infuriating isn't it?

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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Jan 1986

Donated & scanned by: Mike Gorman

Feature by Paul White

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