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April 27, 2011 - 4:42pm
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Impulse test for Stereophile digital Measurements
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I just noticed that JA did perform an impulse test for the Ayre DX-5 review from Dec 2010. Interesting that it looks way better than the Meridian 808.2 from April 2009. Alas ... these units may, in their latest versions, have been upgraded in *so many ways not related to MP/apodizing filters*, that it's hard to truly know why their SQ *actually* improved.
Because in 99.99% of cases, the impulse response is that of a conventional FIR filter, ie, symmetrical in the time domain with as much "ringing" before the peak as after: see the Benchmark DAC1's impulse reponse in fig.1 at http://www.stereophile.com/content/meridian-8082808i2-signature-reference-cd-playerpreamplifier-measurements. I only publish the impulse response when it is _not_ like that. You can infer the impulse response, BTW, from the undithered waveform at -90.31dBFS, which I always publish in my digital reviews. A conventional FIR filter gives symmetrical overshoots to the edges of the squarewave.
That is correct.
John Atkinson
Editor, Stereophile
JA, thx for your reply. That Meridian post-ring is still awfully ugly. What kind of analog filter are they using with unit (eg, no. of poles, etc.)?
Something else I realized is that newer MP/apodizing filters -- used in Meridian, Ayre, etc. -- may have (in addition to better traditional oversampling) also been written to correct for shortcomings in the A/D part of chain. So NOS's "clean" impulse plot may have v. little, if anything, to do with "NOS sound".
Back to how efficacious MP/apodizing filters are ... well, the Keith Howard-engineered tests from Jan 2006 didn't help the case for these filters! But, maybe, those were old-school MP filters; Meridian/Ayre have been busy since then, so perhaps their math is better??
Wadia and (I think) Theta putatively addressed time-domain issues quite early on in the reconstruction game. Anyone know how their impulse responses looked? (I went thru most of my 28-year Stereophile magazine stack (uh .. stacks!!) but found nothing.
Wadia, yes, and Sony and Pioneer, but not Theta.
I just posted the impulse response of the Wadia 850 to the Stereophile magazine Facebook page. It is symmetrical in the time domain with minimal ringing.
John Atkinson
Editor, Stereophile