Digital equalization: two words guaranteed to set teeth on edge among audiophiles. Unless they happen to be incurable technoids who believe all the crap about digital = perfect. So Celestion's DLP 600 processor, like the Marantz audio computer and just about everything from Meridian, has a built-in disadvantage before the listening even starts. But such processors are on the increase, with B&W's due any day now, so try to open your mind for a thousand words, okay?

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Unlike universal, free-standing, stick-it-in-a-tape loop processors, the DLP 600 is (literally) a black box dedicated to Celestion speakers. At present, it's only available for use with the SL600/600si, limiting it's appeal even further. But the programming involves nothing more than blowing an EPROM (talkin' dirty, computer-style), so Celecstion can issue this device for use with all of its own speakers and any others for which it has the driver and crossover details. Which leads me to a brief aside.

Equalization in the digital domain when using digital programmme material has undeniable appeal, but it requires R&D financing which may not be available to smaller companies. I would suggest some serious pride swallowing and advise smaller companies to co-operate with companies like Celestion, B&W, Meridian and any others offering similar devices, to enable the processors to be used with their products. Badge engineering and OEM purchasing is neither a sin nor an admittance of inferiority. And access to such black boxes is worth hoving. As you'll see.

The DLP 600 is a box measuring a mere 239x45x180 (WHD). On the front, one button for in/out; this bypass allows for A/B comparisons and will probably find more action in dealer show rooms than any button since the days of the speaker 'comparator'. To its left are three red LEDs. The first indicates 'in' or 'out', the second shows that the device has locked on to the digital signal, while the third indicates power on.

At the back is an IEC three pin mains receptacle. the on/off switch and inputs for digital signals via TOSlink fibre optics or coaxial. (edge among audiophiles. The former sucks like a Hoover, so I stuck with coaxial.) That's it. You insert it between a digital source, such as a CD transport, and the D/A converter. Which tells you instantly that this has no effect when you're listening to analogue sources unless you own a pre-amp which digitizes them.

What the DLP 600 does, in a nutshell, is this:What the DLP 600 does, in a nutshell, is this: the DLP uses digital sound processing to rectify time domain errors. It doesn't deal with bass or room characteristics, the former because the company believes its 600-series speakers to be linear and smooth down below, while the latter would give rise to unnatural low-frequency psychoacoustic phenomena. And, hey, no two rooms are alike, so you'd be looking at something a lot more complex, like the Marantz computer, with its microphone input and on-board signal generator.

Even the best speakers suffer from time-domain problems, resulting in non-linear phase characteristics. Until the arrival of devices such as the DLP, correction involved mechanical solutions, such as stepped baffles, sloped baffles and other solutions which could lead to cabinet diffratcion problems. The DLP provides the SL600/600si (and, eventually, other Celestion speakers) with a more perfectly linear phase response by using finite impulse response (FIR) digital transversal filtering, to improve phase characteristics from above 400Hz to beyond 25kHz.

Sited between the source and the D/A converter, the DLP 600 uses an Analog Devices ADSP-2100-family DSP to process the signal via algortihms written by Celestionrt and stored on the aforementioned EPROM, loaded into the DLP on switch-on. The Analog Devices chip calculates the coefficients to correct both the ampliftude and phase response of the SL600. The equalization is achieved during real-time, with Celestion having performed the measurements in an anechoic chamber at two metres from the speaker rather than one, for this was felt to be more of a real-world listening situation. This, as you'd expect, allowed the designers to measure with greater relevance for integration of the two drivers at a practical listening position.th the SL600/600si, limiting itdirty, computer-style), so Cele and a sample to stick in an anechoic chamber ---- aAt the back is an IEC three-or DAT device neitheran on other signals nor even mere 'circuit intrusion' ... ith could lead to cabinet diffractversal filtering, to improve incoming ients to correct both the ampli achieved during real-time,; t listening situation. Ait the withthe signal arriving (A simple illustration, allowing for level changes, would be to listen first at one metre from the speakers, then at two metres. Believe me, you'll hear all sorts of differences. Which is why they call it a hot spot.)