Reading The Meter
They bend, they blink, and they tell you when your mixer is about to explode. Martin Sheehan feeds his meter.
WOULD YOU rather it rose up quickly and then slackened off slowly or would you prefer it to go up and down repeatedly at a fair old lick?
Well, nine out of 10 chickens, when asked whether they would go for PPMs or VUs replied, "Whichever is the cheepest". However, the average chicken knows as much about metering as most people know about laying an egg. For this reason a brief discussion will follow for poultry perusal.
Most audio equipment with an in or through-going signal will sport some sort of level indicator. This may be as basic as a green "OK" or a red "Oh no!" indicator. For individual signals this is often adequate but where the interaction of more complicated signals needs to be monitored, as in mixers and multitracks, then a more sophisticated approach is required.
The bar graph meter can offer more information by supplying a column of LEDs filling in extra steps between the OK and the Oh no! Although this type of meter is capable of great accuracy, if its display were allowed to respond freely to all the ups and downs of a punchy piece of music then the visual result would prove somewhat confusing. In order to present a more usable display to the eye, the circuitry of the LED meter may be designed to produce a display which is less responsive but more comprehensible. The signal peaks, being the most important from the point of view of avoiding overload, need to be made obvious. This can be achieved by slowing the decay of the display following those peaks.
The disadvantage here is that while the display is delayed in order to register its last peak, it is not indicating the subsequent lower level excursions. To give a sort of "best of both coops" response (chicken talk) a peak hold mode is sometimes employed whereby the LED corresponding to the highest peak remains illuminated while all those in the display below continue to dance about appropriately. The decay time of this peak hold might be anything from about three to 30 seconds.
A more common sight still is the moving coil meter (needle moving across a scale). The two basic types, as mentioned earlier, are the Volume Unit and the Peak Programme Meter. The PPMs are normally to be found only on professional equipment due to their higher price.
Although both types operate on the same basic moving coil principle, the ballistic requirements of the PPM are far more stringent. They must have a very fast rise time of just a few milliseconds with strict overshoot limits, and a recovery time of around three seconds. VU meters are not subject to such regulation, however, and although the readings by different models should concur given a steady tone at 0 VU, responses may vary with more irregular types of signal. VUs are well known to under-respond to percussive sounds, the needle never quite indicating the true signal peaks. (The cluck of a hen would thus not register as high as its true value on a VU.)
As well as responding at different speeds, VUs and PPMs are also scaled differently. VU meters (fig. 1) are typically scaled logarithmically from -20 to +3, allowing 0 VU to fall at about ⅔ of the full scale deflection.
Looking at the diagram you'll see that 0 VU on the scale above the line corresponds to a reading of 100% below the line — essentially the tape is full, and if you increase the level and sustain it, distortion is on the cards. (These percentage markings are not present on some makes so it is well to remember what the 0 means.) With all but the steadiest of signals, VU readings should be interpreted from experience rather than taken as gospel.
A PPM (fig. 2) is scaled from one to seven with optimum level occuring centre scale at four. The slow recovery time and more accurate peak indication of a PPM means that less guess work is involved in their use.
VUs, PPMs and bar graphs each have their own individual merits. VUs are cheap, PPMs are accurate and bar graphs can be made very small.
One of the best solutions to accuracy with economy is the VU meter with a peak reading LED built in. The rapid response of the VU to both rising and falling signals is coupled with the peak reading accuracy of the LED. Just which type of meter to use, however, is usually a forgone conclusion dictated by a manufacturer, but it is as well to be aware of the type of response of any meter you may come across in order to get the best out of the piece of equipment.
Feature by Martin Sheehan
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