Audiolab Q-DAC filters - page 2 Performance with CD

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Audiolab Q-DAC filters
page 2 Performance with CD
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1) Sharp roll off


If you want flat frequency response to a little beyond 20kHz with CD, then use this filter. It will give you the sound you are used to, meaning quite bright in balance, although Audiolab's filter doesn't peak like some. Impulse response in the time domain suffers substantial pre-ringing and post-ringing, which is bad. Rejection of unwanted data is good.
    Subjectively, this is most like conventional CD, offering the same balance, but it has a 'clamped' into place sound stage that lacks air around it.

2) Optimal spectrum

This filter is very similar to Sharp roll off, if with fractionally and inconsequentially less bandwidth. Time domain ringing is again substantial, considered poor, but rubbish rejection is good.
    Subjectively, this is a better sounding version of Sharp roll off I found, offering a clear, open sound that seems less constrained than the other filters. It's a nice filter to use, I felt.

3) Minimum phase

Technically interesting, this filter eliminates pre-ringing our measurements show, without affecting frequency response in any significant way. If ringing is subjectively important then this filter should sound obviously different and better, but in my experience it does not. Is time domain ringing audibly important then?
    Subjectively, I could not detect any improvement in transient quality, or any other quality, due to this filter's lack of pre-ringing.

4) Slow roll off.

This is a very interesting filter. Audiolab say it has "significantly less time domain ringing" than filters 1) and 2), but not "no ringing" of the slow, optimum time domain filters, 5), 6), 7). It would appear to be a compromise then.
    However, under measurement it exhibited less time domain ringing than all the other filters, including the Optimal transient set. It also rejected unwanted rubbish very well and retained frequency response quite well too. Under measurement at least this was clearly the best filter of all seven.
    Subjectively, this filter was a disappointment. It seemed to have no special qualities at all, sounding clean but constrained. The sound stage was much like that of the fast filters.

5) Optimal transient

This filter well suppresses pre and post ringing, but it did not match the 'Slow roll-off' filter. It significantly rolls down upper treble and poorly rejects unwanted digital rubbish. Technically – not good.
    Subjectively, the topography of the sound stage changed with this filter. Instead of instruments being displayed in a line between the earpieces, the feeling of a sound stage with instruments on it appeared. This injected a better sense of realism, making the presentation less contrived and mechanical – always a criticism of CD.

6) Optimal transient XD

A variant of Optimal transient, it is identical in behaviour under measurement. It has different "mathematics" to Optimal transient though, designer John Westlake says, and offers an improvement in bass quality.
    Subjectively, Optimal transient XD was a slightly fuller sounding and better sorted version of Optimal transient. Focus was improved and there was a better sense of rhythmic grip and push. This is one nice way to hear CD, loosening a lot of the subjective constraints it suffers.

7) Optimal transient DD

A further variant of Optimal transient, it is again identical in behaviour under measurement but offers more balanced signal current draw on the silicon die and apparently has a different bass quality to XD.
    Subjectivley, Optimal transient DD was difficult to pin down I felt. It didn't quite have the impact of XD, but was only marginally less attractive. Sound staging was as good as that of the other Optimal transient filters.

It is only CD's low sample rate of 44.1kHz that cause the slow Optimum transient filters to intrude into the audio band, audibly suppressing treble. When sample rate is more than doubled to 96kHz even the Optimum transient filter set ceases to affect upper treble, our analysis below (at right) shows. The theoretical upper limit is now 48kHz (half the sampling frequency). Optimal spectrum reaches this limit but no further, whilst Optimal transient extends much higher.


Optimal spectrum and Optimal transient filters with 96kHz sample rate data


The question now is: do these filters still affect the sound? If not, then the audible impact of the Optimum transient filters upon CD is likely more down to their frequency domain response than any other property. Bear in mind here that 96kHz and 192kHz sample rates (multiples of 48kHz used in professional circumstances) were proposed so that anti-alias filter design could be eased, using better damped filters with slower roll-off rates that would give better all-round results than those of CD. So results from the M-DAC here show whether this is a promise fulfilled or not, and what we can expect from high resolution digital in future.
    Listening to these filters with a variety of 24/96 material, Rock and live Classical, showed that as measurement suggested, differences were not easily obvious. The differences heard with CD were repeated, but they were smaller. The Optimum transient filter set was again the most arresting to listen to and again the main difference between them and the others lay in sound stage presentation: it broadened beyond the earpieces of my headphones (Philips Fidelio X1s) as if unconstrained by them. There was more air around images and a better sense of depth, especially with orchestra. Of the three Optimal transient filters XD again seemed the most engaging, putting a bit more bulk and character into instruments, again with better stated rhythm and timing.



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