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Triple 'X' Rating

Dbx 'X' Processors

Apart from noise reduction systems, dbx make a studio quality range of 'budget' signal processors. Engineer Gareth Stuart put three of them - the 163X compressor/limiter, 263X de-esser, and 463X noise gate/expander - through their paces.

Engineer/musician Gareth Stuart takes a 'peep' at the three signal processors that form the new 'X' series from dbx: the 263X de-esser, 163X over-easy compressor/limiter, and 463X over-easy noise gate expander.


Priced at £151.80 each, dbx describe these units as relatively low-cost, easy to use, high quality signal processors. Well, yes, they're all of that, but if you'd like to know how they do what they do, and what they can do it to... read on.


Once out of their protective cartons, and in bits on the floor, you'll realise that dbx have marketed this 'X' series along with a self-assembly kit which enables each unit to be either 19-inch rack-mounted solo, or side-by-side, or to stand independently on a flat surface. All the necessary tools for assembly are included in the package.

Having assembled the units for 19-inch rack-mounting, don't be put off by what seems at first touch to be a rather flimsy structure. As soon as they're fastened into the rack, they gain sufficient solidity and stiffness. However, I feel I must say that when you come to set up two units to be mounted side-by-side, don't over-tighten their locking mechanism (as I did)... cos it'll break.

For the record, it took me about half an hour to put the three units plus blanking panels together. Pat on the back to any of you who get it done more quickly. Now on to the units themselves...

"I woke up thith morning and the thun wath gone", but the sun would have still been there if I hadn't used such a disproportionate amount of de-essing! The quotation, in case you're not familiar with it as it stands, is from Boston's 'More Than A Feeling'.

A de-esser, and this dbx 263X is no exception to the rule, reduces the amount of sibilance in a signal. It's most commonly used for processing the voice. Words like 'shape', 'synthesizer' and 'scream' are examples of ssss... sibilance. Very often in a mix, an over-sibilant sounding voice causes a balance problem. It's not a problem that can be satisfactorily solved using equalisation either, no matter how subtle, or by turning down the volume. Well, I mean, you wouldn't be able to hear it then would you?

The 263X tackles this problem for you in a jiffy. I say that because it doesn't take long to learn how to use this unit and understand exactly what it's meant to be doing, and also because it takes no time at all to get it set up.

The three controls which govern the amount of de-essing are the 'High Frequency/Broadband' mode selector switch, the slider controlling the amount of gain reduction, and the thumb knob which pinpoints the frequency band in which the de-essing takes place.

In the 'High Frequency' mode (a green LED lights up) the unit acts as a low pass filter, letting low frequencies pass unprocessed, and effecting high frequencies only.

In 'Broadband',mode (an orange LED lights up) the entire frequency band is processed. But, you may say to yourselves, it's only high frequency sounds which cause this sibilant effect, so why do you need to process any low frequencies?

Well, first of all you'd be right in thinking what you think, but if an excessively sibilant signal is recorded it can cause tape saturation, which in turn causes low frequency distortion. So, in this mode, the 263X serves a dual purpose - to reduce low frequency distortion and to remove high frequency sibilance.

The horizontal slider governs the amount of processing, and this is well displayed on the LEDs in terms of dB of gain reduction (-1dB to-20dB), ie. the amount by which the sibilance is being reduced relative to the unprocessed input signal.

The 'Frequency' knob sets the crossover point between the higher and lower frequency bands. So, just to make its function absolutely clear, in the 'High Frequency' mode the only frequencies getting de-essed are those above the setting on this thumb knob. For example, having set the thumb knob to 4kHz, only those sibilant sounds above 4khz will be attenuated (the amount by which they're attenuated being governed by the slider).

So, if it's so quick to set up, how do you set it up?

Right then, it's best that you start off from a setting using too little, rather than too much effect. Having set the 'High Frequency/Broadband' switch to the 'High Frequency' mode, push the slider to a mid-way position, and set the 'Frequency' thumb knob at 4kHz.

Why 4kHz?

Because, as the manual states, sibilants contain predominantly high frequency components with a sharp rise above 1kHz and most of the energy in the 4-10kHz frequency band, centered on 6-8kHz.

However, on starting from this point, I found that the effect was too strong and even though it didn't cause the 'ess' to turn into a 'th' sound (as in the earlier Boston quotation) it was still too noticeable. Rather than reduce the effect, I raised the 'Frequency' control and this solved the problem - reducing the gain only on the extra sibilance (which wasn't required) which was merrily hissing away just above the 5kHz mark.

It's worth bearing in mind that the 263X is meant to do its job as unobtrusively as possible - which it does. It's not supposed to eradicate all sibilance, but just tone it down. I mean, you wouldn't saw off the top of your door just because there was a notch in the middle not allowing it to fit snugly, would you? No, you'd smooth down the notch with a file or sandpaper. De-essing a signal is just like removing that offending notch. Equalising it would be like sawing off its top end.

When the dbx unit is set up correctly, subtly removing all unwanted sibilance, it seems at first as though it hasn't actually changed the signal in anyway. The LEDs flash telling you that so many dBs of gain reduction have just smoothed out the last sibilant word but the effect is not that noticeable. This is very much a point in the 263X's favour.

I said earlier that the de-esser is most commonly used to remove excessive vocal sibilance, but it needn't be confined to the voice. I tried using it on several instruments, in a mixdown context, and in a simple plug-in-and-play context.

My Roland TR707 drum machine responded well. With the 263X's slider fully right, for maximum effect, the switch in 'Broadband' mode, and the thumb knob set at 8kHz, the drums sounded that much brighter, with an added crispness to the bass drum and rimshot.

The same setting tended to mellow out an alto saxophone, giving a much rounder, soothing sound. The instruction manual suggests that it may be particularly useful in losing the spitting, sizzling sound created by some brass instruments.

You wouldn't expect acoustic guitar, bass, or congas to be sibilant would you? Well, you'd be right, for testing the 263X on those instruments caused no noticeable effect.

Removing the 263X from its rack-mount, I plugged my guitar into the Hi-Z (high impedance 391 kOhms) input and set the guitar amp to give a particularly dirty sound. Very often in playing with a distorted, sustaining guitar sound, you have to put up with quite a bit of harshness - the nasty edge which leaves your ears ringing after a fairly short practice session. I was able to tune into those jangling nasties... and waste them!

To do this, the 263X worked best in 'Broadband' mode, the slider still set maximum right, and with the thumb knob at 800Hz. Even though I'd managed to lose all the harshness in the sound, I still had the same volume, and the lower notes remained lovely and bright. What's more, I was left without the ringing in my ears - and that can't be bad!


The aim of a compressor is to reduce the gain of the input signal relative to the output signal when it has exceeded a certain threshold. Limiting is an extreme form of compression.

There are normally several parameters governing the type of compression needed for a particular signal. In a standard compressor/limiter of this quality (but costing much more than the 163X) those parameters would be independently adjustable, enabling the user to set the attack time, recovery time, compression ratio, and threshold. Dbx have designed the 163X to allow the user to set the type of compression required using one, rather than four, controls. Now they'd be the first, I am sure, to point out that the 163X is less flexible than its more expensive counterparts, but it is much easier to use, faster to set up, and yields an equally high quality result.

So, what's the catch?

Well, the catch, if you like, is that the unit won't allow you to do silly things, like set a low threshold with a high compression ratio, or set too slow a recovery time, or too fast an attack time, for unsuitable input signals. In other words, you can't play with lots of controls, but that's good for you in the long run you know.

Dbx have set the attack and recovery times for you - that is, the rate at which the 163X responds to a sudden increase in input level, and how long it takes for its gain to return to normal. On a more flexible unit the attack time could probably be set from around 0.25ms to 25ms, and you'd set it to suit a particular input signal. For instance, you'd use a different (hopefully much slower) attack time when compressing a piano than a bass drum. Why?

Well, a piano produces a quality of transient (ie. the initial attack of the struck key) which is susceptible to square wave distortion if too fast an attack time is used. A fast attack will literally clip the tops off those transients in compressing the signal by a desired amount. A bass drum on the other hand tends to improve when it's compressed in this way - gaining in punch, and cutting through the mix. With experience you come to realise what degree of attack is likely to sound good with whatever input signal you're compressing.

Dbx know all that, and have produced this unit to cover the commonest accepted uses of compressor/limiters. Their over-easy concept, in a nutshell, is to keep the music sounding natural even under extreme amounts of compression.

Now for a guided tour of the front of the 163X...

There's a slider that increases the amount of compression (the compression ratio) the further right it travels. This slider also automatically raises and lowers the threshold level. A 'Level Set' thumb knob enables the unit to work properly in any set up, and the Hi-Z input allows a microphone or guitar to be plugged directly into it.

To clarify the term 'compression ratio': most compressors of this quality have the ratios 1:1, 2:1, 3:1, 10:1, and increase to something like 20:1. The 163X states its highest ratio as infinity:1, but more of that later. I'd like a volunteer from the audience to pick a ratio... yes, you madam... pick a ratio, any ratio....5:1?.. lucky for some.

With a compression ratio of 5:1 there will be a rise of 1 dB of the output level for every 5dB rise in input level above the threshold. Now, how about a round of applause?

I think it's late enough to talk about the infinity: 1 ratio. What it means according to dbx, is that "as the signal levels increase, the output level increases toward a certain maximum but will never go above it no matter how much signal is put in". At this stage, the 163X has become a 'limiter', there's no longer any change in output level. For every infinitely large signal that enters the 163X, only 1dB escapes.

Right, that's come to terms with the slider's purpose to increase the compression ratio, but to understand its other function in setting the threshold level, it is best to imagine the 163X's use in a real situation...

You want to compress something, a voice for instance, but the only place where the level needs to be brought down is during the chorus when the singer is straining on the high notes and forcing the level to rise by an unacceptable margin. All you want to do is compress those high strained notes, the rest is fine.

The threshold point therefore must be set relatively high, enabling most of the track to pass below it and consequently not be effected by any compression. On the other hand, the threshold must be set below the level of the strained notes. So you push the slider across 3/4 right, which sets the threshold. Remember, that the slider also sets the amount of compression, and 3/4 right is a lot. Don't worry though, the theory is that because the main part of the vocal track is fine, only a small signal will be sent to the 163X... Consequently, the compressor will look down from its high threshold and mutter 'what's happening down there man', but it won't actually effect the signal (since it's passing below the threshold point).

But as soon as the straining starts, it is taken by surprise... 'Stay cool!' it exclaims, and begins to reduce the output level to one that is acceptable to the mastering machine and your ears. I say, 'begins to reduce', because this is a laid-back (over-easy) compressor - very cool, and doesn't like reducing the level in a hurry. So what, if it missed a tiny bit of the straining transient? Do you prefer your signals clipped or squashed sir? Oh, I dunno. I'll take them over-easy.

Because dbx have had to compromise on the attack time, it is possible for the initial peaks of a transient to momentarily slip through the compression net. But this is really nit-picking, and not meant to be destructive criticism in any way.

There's another way in which the 163X can be used, and that's in conjunction with a second 163X. No, this isn't salesman's talk, but an explanatory note on how to use two units for true stereo compression.

To begin with you must use a stereo lead, one that is 2-conductor shielded (3 wires) with standard quarter-inch (tip/ring/sleeve) jack plugs at the ends.

The 163X you'd like to set the amount of compression on is called the 'master', and in accordance with this, a switch on the back of the unit has to be set to 'master'. The second 163X then takes all the information from the master, so that equal amounts of compression will effect both channels, and it's called the 'slave'.

The reason for this 'stereo strapping' facility is to prevent image shift. Two mono compressors set up to compress the left and right channels of a stereo mix would not work in unison, causing the image to move erratically from side to side.


Okay, let's kick off on this device by first tackling the principle of 'expansion'. The reason for explaining this first is because 'gating' is best understood as an extreme form of expansion.

This time I'll put the 463X into a working situation immediately, so rather than seeing what it's supposed to do, you'll see what it does. For the sake of argument then, assume all tracks have been recorded - bass, guitar, vocals, drums etc. Let's look at the vocal track.

Very often, as a result of wearing headphones when recording the vocal track (to stop sounds from the other tracks being picked up by the vocal mic) the vocalist may breathe more heavily, or even hum along to the backing track, without realising that they're doing it. Also, you've got the rest of the band muttering in the background about last night's TV, or all the notes they reckon you've just sung flat! All of that extra sound is unnecessary, and unwanted.

They may not be loud on the track compared with the level of the singing, but they do add to the overall noise, and it would help to silence them.

This is where the 463X comes into play. Before using any jargon, let me explain how it works. Imagine you're listening to that vocal track, you've decided how loud it should be relative to the other tracks, so you make a note of the volume (fader) setting. When you come to the end of the verse or chorus, you think there's no point in leaving the volume turned up, the singing's ended, and won't start again until the next verse. Having made a note of the setting, you manually turn down the vocal track. The 463X could have saved you the bother, but that's only the simplest case for using it.

That's fine then, simple; but what about the space between each line or each word? There's nothing on the track at those points, so why not turn the volume down, making sure you turn it back to exactly where it was set for the next sound to be heard? The reason why not, is because you'd have to change the volume constantly and like grease lightning (and very accurately to maintain a natural flow). Anyway, while you're doing that, assuming you've got quicker reactions than most of us, who's looking after the overall balance of the mix?

Now, with that in mind, if I told you that an expander can be thought of as an automatic variable gain amplifier capable of controlling the level of an audio signal much faster than you could control it manually, you'd understand wouldn't you?

No? ...don't worry. Think of setting the volume, turning it down, and then back up again to the original setting - that's the job of an automatic variable gain amplifier. That's also the job, or rather part of the job, of the 463X.

So, why call it an expander? Well, because by turning down the volume in those empty spaces, what you're effectively doing is expanding the dynamic range (downwards). The difference between the level of the quiet sections (the gaps between the words, lines, and verses) and the actual singing is much greater now.

Sitting comfortably? Good, here comes the next molehill for you to climb.

Taking a look at the front of the 463X, there are two controls: the slider determines the amount of expansion (you get more the further right you push the slider), and the thumb knob controls the threshold.

In setting the threshold, remember that you want to keep the singing and exclude the breathing. The 463X makes this job easy for you by having a little green LED come on whenever the signal rises above the threshold. In this case then, the breathing would occur beneath the threshold and the light would go off, only to come back on when the next word was sung.

You may be wondering now, why it's called a 'noise gate expander'. It's because as you push the slider right and increase the downward expansion (ie. the rate at which the unwanted noise is attenuated), there comes a point at which the attenuation is so quick, it's as if the track has been switched off. So, a signal above the threshold triggers it to open, and a signal below the threshold for it to close.

The thing you can't do on this 463X unit, even on maximum expansion, is to cause it to shut off in the way that a classic noise gate can, so you can't abbreviate a sung word by shutting off the sound mid-flow. This is a point in the favour of the 463X, and makes mistreating a signal very difficult.

The 463X, like the 163X, also has a stereo strapping facility which enables two noise gates to be coupled and operated in sync.

One facility peculiar to the noise gate is its 'Key In' input. This allows one signal to control the gating of another. Its typical use is in fattening up drum sounds, by having a drum trigger an external source such as an oscillator.

The way you set up this effect is by plugging the oscillator into the 463X's normal input, and the drum into the 'Key In' jack socket. Whenever the drum sounds, it causes the gate to open allowing the oscillator to be heard as well. In beefing up a snare drum sound for instance, you might consider triggering white noise.

The other way you can use the 'Key In' facility, is by having an external source opening and shutting the gate on the main signal. I tried this using an electric guitar as the main signal, and a sequencer as the external trigger. The effect produced was a type of tremolo, the speed of the sequencer deciding the tremolo rate.


I think the thing that's made the greatest impression on me is just how quickly it is possible to come to terms with these signal processors. They're most definitely 'user friendly'. All three units are easy to operate, and to start using them efficiently takes as long as you take to plug them in and put the right leads in the right sockets. After that, you can't go wrong.

It could be argued that the skill of setting up a certain effect is taken away from you, as the units work along such automatic lines, but then if you're that fussy about your effects patch, perhaps you should be looking at a different price range. For what they offer, all three units represent excellent value for money at £151.80 inc VAT each.

Again, the self-sufficiency of these dbx products may make it more difficult for the user to fully understand which process is happening and when and by how much. However, that argument hasn't prevented drivers from buying automatic cars! If the product does its job well and with the minimum of fuss, then it's a product worth buying.

At the beginning of the review, I mentioned the rather flimsy structures which you have to assemble yourself to rack-mount the units. It would be nicer to have a more robust system, rather than have to rely on other rack-mounted equipment to give them support.

But really, having heard what the dbx units have done and what they've done it to. I'm impressed.

Perhaps the next time you're suffering from tense, nervous, headaches you should consider buying the new 'X' series from dbx.

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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - Oct 1986

Donated by: Gavin Livingstone

Review by Gareth Stuart

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