Nearly everyone knows that you can use an Exciter to synthesise a new top end to enhance your recordings but did you know your dbx can do the same for the bass end?

The item on review comes from the dbx range of processors targeted at the hi-fi market and in that context, the 120X-DS can be used as part of a conventional stereo setup to create a sense of bass depth never present in the original recording. Because this newly added bass may be as low as around 25 Hz, many speaker systems can't do justice to it and so a separate sub-bass output is provided for those who wish to use an additional mono subwoofer. Subwoofers are generally mono because little or no directional information is imparted by such low frequencies.

Though this is foremost a hi-fi product, a few studio trials proved that it can be of great use in beefing up the sound of drum machines and bass instruments. And, unlike harmonisers, the dbx system only works on frequencies below around 100Hz so there are no undesirable effects in the upper bass, mid-range or treble regions. As is my custom. I'll try to explain how the machine goes about its job before describing the hardware in detail.

How it Works

The secret of this device is that it only operates over a fairly narrow part of the input frequency spectrum - from around 50 to 100Hz. This band is then further divided into four frequency bands by the action of four selective bandpass filters. Each of these filters feeds its own subharmonic generator which is really a refinement of the octave divider system used in some effects pedals. Traditionally, such devices suffer mistracking when more than one note is played at once, but because each divider circuit is fed from such a tightly controlled frequency band, this doesn't seem to be a problem, even when fed from complex mixes. Furthermore, the divided waveform, which is really just a square wave, isn't used directly but is used to switch a phase inverter working on the filtered audio input. I know this sounds a bit heavy, but all it means is that the new sub-octave is derived from the original signal and isn't just a simple square wave.

This new signal is then further filtered to remove any unwanted upper harmonics and mixed back with the original sound. Each of the four bands has its own level control so the user can tailor the sub bass to suit the sound being treated. The best way to describe the result is to say the dbx does for the bottom end what an Exciter does for the top.

Layout

Though it conforms to the 19" rack-mount format, the 1U module comes with bolt-on rack ears so these can be omitted if you're using the unit as part of a stacking hi-fi system. Constructionally, the case is all metal finished in black with gold screening and there are extensive block diagrams and wiring options printed on the top panel. As explained, the heart of the machine is in the sub-bass synthesis circuitry but there is more to the 120X-DS than that. There is also an LF boost equaliser which is said to be specifically designed to produce a musical result at the bass end of the spectrum without sounding boomy. No further details are provided but it does seem to work smoothly.

Controls are provided to regulate the level of the four sub-harmonic frequencies centered at 28Hz, 34Hz, 40Hz and 50Hz. Above each control is a simple three-LED meter which shows the activity taking place in that band which can help when setting up. A bypass button is fitted to disable the sub-octave generation part of the system when not needed and there is a separate sub-woofer level control for use in hi-fi systems where a separate sub-woofer and amplifier are being used.

Turning to the rear panel shows the unit to have phono connectors for both inputs and outputs, colour-coded red and white for right and left as you might expect (nominal —10dBv operating level). Also built into the processor is an active crossover which provides a single output for a sub-woofer system and a rotary control allows the crossover frequency to be set anywhere from 50Hz to 200Hz. A pushbutton then determines whether the main stereo output will carry the full audio range or just the high end from the crossover. For studio use, it is possible to use the unit on an effect send system and just use the sub-woofer output to provide the return signal. Being mono, this should be panned centre. Alternatively, the unit may be used in the insert points of a channel or stereo subgroup in which case the main signal path would be switched to Full Range.

"The great thing is that it doesn't mess with the mid and top end so the music doesn't sound unnatural."

Effectiveness

Having disposed of the theory, the big question is 'does it work and is it of any use in the studio?' It must be made clear that the unit will only produce sub-bass from bass sounds that already exist around the 50 to 100Hz region so higher pitched sounds will be unaffected no matter how much you crank up the levels. This can be demonstrated by feeding a drum machine into the inputs and applying processing while watching the four meters. On the bass drum beats there is a lot of meter activity and the sound takes on a most satisfying kick that certainly wasn't there before. Low toms also benefit from the treatment but the higher toms, snare drum and cymbals were totally unaffected. You could feed a lower snare drum sound into it in which case you would get an effect -I'm sure you get the idea. The great thing is that it doesn't mess with the mid and top end so the music doesn't sound unnatural.

Used on a sampled bass guitar, the low notes again took on an extra dimension and you don't really need to add that much effect to do the job. If you do go over the top and add lots of effect, then the result starts to sound a little like an octave divider rather than like a single note with low harmonics. This can be a useful creative effect so it's nice to be able to get it when you want it, but most of the time I think that restraint is the answer, as it is with most effects.

There is a danger in all this. For a start, you may be generating frequencies well below the range your monitors can handle so apart from the more obvious risk of damaging your speakers, you may be adding frequencies to the mix that you can't hear so you won't know exactly how the mix is going to sound played on full range speakers. If your monitors cut off around 50Hz or so as do most small studio monitors, then err on the side of caution.

Summary

Whether the dbx 120X-DS appeals to you or not will depend largely on the type of music you are trying to create and also on the instruments and mics at your disposal - but it certainly works. In theory, if you get everything right at source, then you'll never need to add any effects at all, but with pop music, that's never the way is it? I'd say that if you're doing disco, house or hip-hop music, then it would be a very useful device to have, especially when you consider the bass capability of the speakers used to reproduce such music in clubs and suchlike. And, if you need to fatten a thin bass guitar or bass drum for whatever reason, then you'd be hard pushed to find a better way of doing it. In fact, I can't think of another machine that does quite the same job as this - pitch shifters affect the whole of the sound, not just the bass end, and ordinary octave dividers are only monophonic. The effect is really solid and you need to hear it to appreciate it. Furthermore, it seems just as happy on complete mixes as on individual instruments, though it may not be appropriate for classical music.

And so we come to the inevitable bottom line which might read something like 'unique, effective and affordable' - so if you need the effect, this is what you have to buy because there's no other way I know of doing the job. Which leads to the cost - which, all things considered, is about right for an effect of this type and you never know, you might want to use it on your hi-fi as well!

The dbx 120X-DS costs £279.90 inc VAT

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