Audiolab Q-DAC filters

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Audiolab Q-DAC filters
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Audiolab's M-DAC and less expensive Q-DAC  both have no fewer than seven digital filters. Noel Keywood looks at what they do.

Audiolab's Q-DAC. See our review.

From the moment I heard Chord's newly released DAC64, back in 2002, I realised that perhaps CD as we knew it back then could sound better than it did. Designer Rob Watts told me that his unique Watts Transient Aligned (WTA) filter might not measure "as well" as the traditional industry solution, but it sounded better all the same. Since Rob Watts knows more about digital signal processing than most, I listened intently to DAC64 and was impressed, like so many others. His WTA filter rolls off upper treble and does give a slightly warmer sound from CD than usual, but it was also less aggressive and gave a somehow more solid feel to images, as if they were bigger bodied and more stable in position. In the end I just liked DAC64: it was clean, stable and assured and I could listen to CD for hours without suffering the slightly unsettled feeling I often get from it.
    That was then and now, no less than a decade later, Audiolab have also addressed the issue of digital filtering and given us no fewer than seven filter options, first in their upmarket M-DAC and, recently, in their cut down Q-DAC (£400, UK). Three of Audiolab's filters remind me of DAC64, since they give a very similar measured result and sound. The great thing about them is that they can be used to subtly tune – tame I could say – a system. They also improve on digital as we know it: the changes aren't big, but they're worth having in my view.
    Oh, and as an aside, M-DAC comes from another Brit, John Westlake who, like Rob Watts, is a renowned digital guru; both have had the vision and courage to move outside the design box and present us with something innovative and different. Here's a close look at M-DAC's filter set and what to expect from it. All were measured with a Rohde&Schwarz UPL audio analyser, the same instrument John Westlake uses.
The seven filters are -

1) Sharp roll off
2) Optimal Spectrum
3) Minimum phase

4) Slow roll off

5) Optimal transient
6) Optimal transient XD
7) Optimal transient DD

I've put them into groups so you can more easily understand their role. At top we have the fast filters (fast in sense of short rise time) that give strongest treble and the brightest sound. These are the usual industry choice that will give best measured result in terms of frequency extension: they reach highest in other words, if only by a small margin.
    In the middle sits a single medium speed filter, termed Slow roll-off, that under measurement was the best filter of all – something that surprised me.
    At bottom sit three slow filters that Audiolab interestingly place most emphasis on. They quite radically roll down treble from CD, giving a slightly warm balance. Under measurement they are identical. And all three are different from the others in behaviour, measuring quite badly in terms of alias suppression, although an alternative view (used by Pioneer for example, who use similar filtering) is that they have the most analogue-like behaviour.
    In practice, as John Westlake admits, they throw up uncorrelated digital audio rubbish, generally considered a bad thing, but the alternative view is that their energy spectrum is very analogue like and their sound possibly likewise. Is what we hear from these filters coloured more by the way they roll off higher frequencies than any other property, I have to wonder?
    At the end of the day, if you don't like 'em then you can switch 'em out, but I gravitated to Optimal Transient XD and it seems most others prefer the slow filters too, so as "bad" as they are, they may be good, if you see what I mean. But I have more to say on this.
     At a technical level digital filters have both frequency and time domain behaviour, but traditionally it is only the frequency domain that gets attention, meaning frequency response and rejection of uncorrelated digital data. However, looking at an impulse in the time domain shows that digital filters also impose ringing on a signal, both pre and post ringing, meaning before and after the signal. Audiolab's three slow filters "exhibit no ringing" they say. "The transient nature of the music is preserved". So these filters concentrate on the preservation of transients, at the expense of all else. 
    Is all this true, and which filter is best? I was surprised when closely measuring these filters that they they don't fully match Audiolab's claims. Also – and importantly –  you can expect different sonic results from high resolution digital than low-res CD. There are some separate and distinct issues here – and illuminating ones. Let's take a closer look.


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