The future will always be tomorrow; perhaps that is why it is hard or impossible to define exactly how far we will go—or, as the popular saying goes, where we are headed. What we do know, to a certain extent, is what has already happened (where we come from). With what I have just said, I am referring only to the field of humankind's inventions and/or scientific advances, not to mysteries or, shall I say, strange things. Since we undoubtedly owe a great deal to our predecessors, especially the most distinguished ones (each in their own field or area of research), it is often almost obligatory to mention, however briefly, some of their feats: first as a preface, second as a heartfelt tribute to their memory, and third as thanks for their praiseworthy work on our behalf—without a doubt, on behalf of all of us.
Whether they are values (physical variables) of tension or voltage, intensity or amperage, power or wattage, etc., whose magnitude we need to know, they must be objectively measured. To achieve that, we use the corresponding measuring instruments—in these cases, electrical ones.
In their early days, measuring instruments such as the voltmeter and the ammeter were mainly analog (the measurement indicated by a needle moving over a graduated scale), whether they were, at first, moving-magnet or later moving-coil types (around 1882, Jacques Arsène d'Arsonval). All of them were based—and still are, since they are still widely used today (especially in control panels)—on electromagnetic principles. Their clear forerunner was the instrument known as the galvanometer.
Following the research and theories of distinguished physicists such as Hans Christian Oersted, André-Marie Ampère, etc., it was the Italian physicist Luigi Galvani who pointed the scientific community in the right direction toward today's measuring instruments. In his studies and experiments, Galvani managed to build an instrument capable of measuring both the intensity of a flowing electric current and its direction (a center-zero instrument). Precisely in his honor, that instrument has been called the galvanometer ever since (around 1836), a compound of Galvani and the Greek term metron, or measure.
To clearly grasp the tremendous importance of that discovery, just consider that later, thanks to it, another physicist, the German Georg Simon Ohm, established the famous Ohm's law during the course of his research in galvanometry—the basis and foundation of everything to do with electricity, electronics and, of course, by extension, electroacoustics.
Voltage values in a sine wave
Here I will present a more or less rigorous (rather less than more) definition of the voltage values intrinsic to a sine wave. Do not lose sight of this, since in audio this knowledge is essential.
The horizontal axis (abscissa) shows the progression values of a sine wave over time, but in reference to its phase rotation over the course of one cycle or period, that is, from 0 to 360 degrees. The vertical axis (ordinate), meanwhile, shows the value of the voltage (U) at each specific time interval.
With this in mind, we have:
U effective = 0.707 x U peak
U average = 0.9 x U effective or 0.637 x U peak
U peak = 1.414 x U effective
These three formulas can be developed even further from the data we have seen, effective voltage and peak voltage; however, it is vital to know that one of the main factors in these calculations is the square root of 2, that is, approximately 1.414, as well as its inverse, 0.707. That is why effective voltage is also called RMS voltage, from root mean square. I won't go any deeper into this since it is not the main subject of this article, but anyone who wishes to is already on the right track to continue on their own.
VU meter and peak meter
Because of the need for a measuring device capable of displaying the signal levels generated by speech, music and audio signals in general (a voltage or dBm meter is only useful for a sine signal—since we are dealing with alternating current—whose frequency does not vary), more than a century after the appearance of the galvanometer (in 1939), an instrument especially designed for audio work saw the light of day, calibrated in volume units. This meter is called the VI meter, from Volume Indicator (jointly developed by Bell Labs, CBS and NBC). This is something that causes confusion (many people say VU meter), and the clarification is that the VI meter is used to measure volume units (VU).
There is apparently no difference between a VU meter and a peak meter, and both perform the same function, although in different ways; their operation (understood as resulting from their own drive circuitry) is noticeably different. While a VU meter shows the overall program level (audio signal), a peak meter (PPM) shows peak levels.
Taking as a reference the difference in level, in dB, between the peak voltage and the effective voltage of a sine wave, we find it is around 3 dB [20 log (1 / 0.707)], but in the case of a complex wave, as with all non-sinusoidal waves, the difference between those values can be on the order of 12 to 20 dB. This difference between VU (RMS) and PPM (peak) readings is called the crest factor.
Another big difference between these two audio level displays lies in their different ballistics; that is, their rise and fall speeds and/or ratios (how long they take to reach and then leave a given level indication) are different. Broadly speaking, it can be estimated that, compared directly with a PPM, the VI meter is considerably slower to rise. For these two reasons, crest factor and differing ballistics, even though both instruments end up showing the same thing, they do it differently. The VU display corresponds more closely to what we hear (to the way our ears work), so it is a common indication for comparing subjective loudness between different programs. Its drawback is that, because of its ballistics, it is not very accurate at measuring peaks: we can easily be dealing with an instantaneous signal peak (a strong transient) while the VU meter is still showing a level quite far from 0 dB. As you would expect, in a recording studio, for example, and especially on recording machines, it is much more advantageous to have peak meters.
We will always find one or more of these instruments, whether VU meters or peak meters (Peak Program Meters), on any mixing desk or console, even the most modest ones, as well as on a great many audio devices. With their help we mainly set—visually as well as by ear—the input and output levels of a given device. That is why their presence and proper use on sound desks is of the utmost importance; on studio consoles they are vital, and in broadcast studios they are crucial, because there, a signal level that exceeds maximum modulation (overmodulation) will cause the transmitter to generate harmonics (spurious emissions) that interfere with other channels or services, which can lead to heavy fines from the relevant regulatory body. It is true that to prevent this, stations also have dynamics limiters that sharply cut the modulating audio signal if it exceeds 100%—program levels that their meters display at all times. At a radio station, it is common to see the level meters for the fully produced program somewhere in the rack that holds the link transmitter (obviously ahead of it).
The graduated scale of an analog VI meter represents the modulation level up to 0 dB in two different but equivalent ways: on one side we see a scale calibrated in decibels, and alongside it another showing the modulation percentage. As you can see, 0 dB coincides exactly with 100% modulation, a level best not exceeded. And in the case of meters that display a digital signal (the input level to the A/D converter, synonymous with bit depth)—although that is not exactly the matter at hand, it is related—whether in hardware or software, exceeding it is forbidden, since doing so overloads the converter with a disastrous and easily recognizable effect. In this case the maximum level reference is 0 dBFS (decibels full scale). Never exceed this level on those devices. Keep in mind that a digital signal level expressed in dBFS should not be used to express an analog value. Please don't get them mixed up.
On an analog recording medium, overmodulation can even be beneficial, but only for creative reasons; that is, when you are after some tape-style saturation on one of the tracks or even on the master—much as when we want a certain vintage feel and use tube gear or some kind of plug-in that emulates that effect. Likewise, as many of you already know, there are also plug-ins for that tape saturation. So much so that one of the devices found in a mastering studio is an open-reel magnetic recorder, naturally of high quality, used to give a production a certain analog flavor; the ones made by Revox usually take the prize for that job.
LEDs and more
The main advantage of analog meters is that their reading, besides being simple, is continuous, meaning there are no jumps in the scale—though it could probably be said that TFT level meters, and in general those that are part of audio software, are also continuous. LED (light-emitting diode) VU meters and PPMs, as well as those made with plasma tubes, read in discrete steps, though these are perfectly defined and/or calibrated; their resolution undoubtedly depends directly on the number of LEDs or scale divisions they display.
A peak LED on desk channels
A special case of peak meter (although it is not really one, but rather a warning of peak risk) is found on many sound desks. That LED called Peak on each channel of a mixing desk or console warns us when it lights up that we are about to saturate or overload (a potential source of harmonic distortion) that channel. In other words, it doesn't indicate that we are working at peak level, but that we are getting dangerously close to it. That is why a slight flickering of that LED is tolerable as long as it doesn't light up brightly, and much less stays on permanently. Make sure you know (manual in hand) the level difference between when the peak LED triggers and the true peak level, usually about 6 dB.
Personally, I light up right along with any of those indicators. I certainly want them to be there and to warn me at a given moment, but I normally keep them nice and dark. To put it bluntly, if you see that red light, or another one also labeled Clip (very common on power amps), or whatever else the indicator for that function is called, putting on a light show: bad news.
Signing off
I remember a concert review I read back in the day that mentioned a technician devoted exclusively to watching the power amp racks, whose only job during the performance was to work the attenuator of an amp whenever the clip indicator came on in one of its channels. That shows how serious the matter was, as well as the value of that indicator.
If you want to dig deeper into this whole subject, you can take a look here, for example: www.rane.com/note169.html. While you're at it, it wouldn't hurt to look at the rest of the documents on that website; highly recommended. Best regards, and see you next time, friends.
