Audio meters are a poor, misunderstood lot. In many pieces of equipment they are included seemingly as an afterthought, as a signal presence indicator or just “something to go by” rather than as a true measurement device.
For offline setting of levels (with sine waves), almost any kind of meter will suffice, especially if the meter is built into the device you are setting. “Turn it up until the third LED lights” will still result in a repeatable, if only an arbitrary, level. If you are setting up the gain structure of a device, you are often using (relatively) pure sine waves as opposed to a complex signal, such as program material. In that case, the meter indication is relatively easy to read and is reasonably repeatable. It can be tempting to use these metering devices that have reasonable accuracy but unknown frequency response, depending on the application. The use of sine waves, or pure tones for set up allows the use of lower-quality meters or digital multimeters without true RMS algorithms to be used. Inexpensive digital meters often read the average value of a signal, but if they assume that the signal is a sine wave (sometimes not a correct assumption), the display can be scaled to “read” RMS values.
Metering, both for calibration and operation (gain riding), is an area where multiple standards coexist. The traditional method of metering/monitoring audio signals was the VU meter. Learning how to “ride” an audio level is very much an acquired skill. It is sometimes difficult to interpret the movement of a VU meter, and instructing operators how to set program levels based on its “readings” can give greatly varying results. If there is a VU meter somewhere in the room, many operators will, almost subconsciously, glance at it occasionally as a sort of “reality check” during a session or show. More often than not, this is due to the lack of faith in the meter that the operator is looking at.
We have come a long way from the days of the VU meter with an LED to indicate “peak” level. There are metering implementations that give us much more information about the signal than the old VU meter ever could. The trick is to figure out a way to show the most information without the display being cluttered or hard to read and interpret.
As broadcasters work with an expanding number of audio channels (first mono, then stereo, and now even more) it becomes challenging to try to come up with a method to meter and display all of this information in a format that is meaningful, yet still easy to comprehend.
Most audio mixers and engineers are familiar with using a Lissajous, or X-Y display for two-channel systems. But as the number of channels grows to four, five, six, eight or even more, how can that many channels be comprehensively or intuitively monitored? A number of manufacturers have developed displays that can broadly be referred to as “best fish finder” displays. This is because of the resemblance of these displays, at least at first glance, to a sonar-type display used by boaters and anglers to find fish and the depth of the water. (more information: http://gigafishing.com/)
The look, as well as the technology, of these displays varies widely and many designers are trying to come up with the best way to accomplish this task. Measurement devices resembling audio meters have been developed for video signals, and any such approach is fair game for development and approval by the marketplace. It will most likely come down to users forming a consensus on the most useful of these displays. The waveform monitor and vectorscope evolved into the de facto standard measurement devices for monitoring video signals.
Many manufacturers are looking for the best way to monitor multichannel audio signals. If you can, the best way to evaluate these meters is to give them a try. Talk it over with your colleagues and see what they think. If you have the opportunity, visit the manufacturer at trade shows or dealer showrooms.
Digital meters mean different things to different people. Because digital audio assigns a value to every audio sample, it is theoretically possible to build a meter that could indicate every single audio sample. Of course, it would take over 65,000 LEDs to build such a meter, and it wouldn’t be all that useful. These meters can also have ballistics programmed into the device, so they can be changed from VU to PPM to “true peak” at the programmer’s option (and be selected by the operator.) Peak hold is also an option.
Digital meters can be made with different resolutions. An eight-segment LED meter has more resolution than a four-segment LED meter, and less resolution than a 64-segment meter. It is important to remember that these statements alone do not describe the accuracy of the meter at all.
Audio meters are really scientific instruments. But in order for them to be useful in a production environment, they must be easy to use and provide the desired information without the need for time-consuming interpretation. The merger of science and art continues to evolve. Equipment manufacturers are now offering solutions, but they need to have feedback to develop these designs into truly useful tools that broadcasters use in their day-to-day operations.