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LEDs — Usage and Abusage

LEDs, Usage & Abusage

Gain setting manipulations are greatly eased by the provision of accurate front end peak metering, particularly in view of the difficulty of rapidly locating an overloaded channel when the distortion, whilst plainly audible, is known only to be 'somewhere in the drum mix'.

This obviously excludes VU meters, and it's also unwise to assume that any LED meter or indicator is reading peaks just because its mechanism transcends the inherent sluggishness of mechanical meter movements.

To begin with, many peak indicators register deviations in one direction only; either only the negative or positive peaks. This careless approach is to be condemned, because the percussive sounds that are largely responsible for overloads are frequently highly asymmetrical, i.e. the signal peaks tend to lie predominantly in one direction. As a result, the likelihood of the peak indicator crying out is 50%; or worse, because Sod's law usually ensures that 'one eyed' peak detectors are subjected to asymmetrical peaks in the opposite direction!

Aside from being sensitive to both negative and positive peaks, an LED indicator should also stay on long enough to be (a) physically visible, and (b) mentally visible. That is, it should beg the attention of a possibly harassed engineer. It should also be responsive enough to catch occasional transients; being irregular and quick to disappear, they may not be particularly audible, but awareness of their presence does warn that prolonged overloading isn't far away. Translated into figures, a useful LED indicator will respond to peaks exceeding the threshold level for longer than 5 to 10mS, and regardless of the duration of the peak, will remain lit for a minimum of 300mS. Without this pulse expansion facility, repetitive 50mS overloads will be manifest to the eye only as a faint twinkling of the LED, whilst to the ear, distortion will be plainly in evidence.

For the adventurous, a 555 timer chip in monostable configuration, fed by a full-wave rectifier (conventionally built around 2 or 3 op-amps) will make good the confusing and strictly meaningless LED Christmas decorations on some consoles!

Indicator accuracy

The last requirement is accurate reference. Because there are several points in a channel where gain (or, strictly, signal levels) can be adjusted, well meaning LED indicators can frequently lead the unthinking operator into a sense of false security. Of course, this situation is aggravated in desks without unity gain structures, wherein gain is available after the channel's peak indicator. But then, any console can be overdriven by heavy-handed tweaking of the EQ.

Maybe we should place the indicator after the EQ? It then provides cast iron assurance that EQ adjustments aren't engendering overloading. But perversely, if we cut the EQ settings, then very real overloads at the front end will pass unnoticed by the LED! On balance, the overload indicator is probably best placed prior to the EQ; the controls can then be tested at their squawky maximum settings, and the input control backed off to counteract. Then, with luck, any amount of EQ twiddling will be accounted for. More sophisticated consoles may incorporate LED 'mini' PPMs at the fader, to give an idea of levels at this point, but with a unity gain structure in the channel, this is strictly a luxury.

Aside from positioning, reference also implies a firm knowledge of the level at which the LED shouts 'ouch'. Earlier, we noted +15dBU as a conservative maximum for peak levels, and it's sobering to see specifications where LED overload indicators are optimistically set to turn on at +20dBU, or maybe 1 or 2dB below. In the hands of a conscientious engineer, this hindrance can be overridden, but there are people with a peculiar insensitivity to music who feel that the sound can't possibly be hot enough until every peak—indicating LED is merrily flickering!

Some manufacturers therefore offer psychiatric care by arranging LED indicators to operate at, say, 0 or +5dBU. LED flickering can then be indulged in without detriment to the audience's collective ears. This psychological design ruse has definite commonsense advantages too: the LED set to turn on 1dB below clipping is limited to saying in monotonic Goon Show tones "Oh dear — you've clipped! Oh dear — you've clipped!" And if the signal is dutifully residing below the threshold, there's no other reference to tell you exactly how far you can safely increase the gain. But with the LED turning on at 0dBU, its occasional flickering tells you 'where you are' without luring the music into a duel with distortion.

To end, a pair of clarifying points. Firstly, in discussing the capabilities of microphone inputs, lines signals derived from auxiliary tape machines, FX units and DI'd (Direct Injected) sources have been ignored. The nominal level of tape and FX sources is normally around 0dBU, which implies +12 to +18dBU peaks. For DI'd sources, levels will tend to be greater, up to a maximum of +30dBU peak. Some desks feature an unbalanced line level input (i.e. after the input stage, so no gain is applied to the signal) with suitable attenuation. Others rely on a hefty pad on the input when this is switched from 'mic' to 'line'. In this way, the balancing and isolating facilities of the transformer can be retained for line level inputs. As for DI'd sources, these will normally be derived via DI box on stage, with its own attenuator, which can be set to provide levels well within the capabilities of a well designed microphone input stage.


Secondly, throughout this article, levels have been looked at in terms of peak magnitudes. This doesn't mean the tip of an individual waveform, as in the peak magnitude of a sine wave; all values quoted are equivalent to RMS magnitudes. Rather, peak in this context refers to the short term high level signals that occur in all music: the levels that are read by peak programme meters. Averaged over several seconds, or longer, mean levels for Rock music are typically 10 to 18dB lower than the peak levels quoted here, depending on the character of the music. For instance, Status Quo's peak-to-average ratio is some 8dB, whilst for Reggae, Funk and Rock with strong orchestral or Jazz borrowings, the ratio may be twice as great.

Looking from the opposite direction, allowance for this ratio is desirable in the shape of headroom. In other words, when we consider levels in terms of their nominal or mean values, then we always have to mentally reserve 18dB above the average to accommodate the peaks. Obviously, this gives rise to pointless mental gymnastics when considering the front-end's limits, where mean levels are strictly irrelevant. But elsewhere, levels are best looked at in terms of their average; it is, after all the average level that untrained ears base their perception of loudness upon.

It will therefore come as no surprise that Status Quo are one of the quietest of bands — at least in terms of PPM readings!

Previous Article in this issue

Arthur Brown

Next article in this issue

Ovation 1667 Acoustic Electric

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jun 1983


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