Dr. AIX's POSTS — 04 July 2015


Happy Fourth of July! I got up early and headed to the recreation area of my little city for the annual Will Rogers 5 and 10K race…with Charlie, the family dog (border collie). I haven’t been running much over the past 6 months due to the pneumonia thing and recent achy knee, but I figured what the heck. Charlie and I stayed at the back of the pack with the strollers and kids and just coasted through the 3.1 miles. It was a beautiful morning and Charlie does manage to make a bunch of friends.

This is Part III of a series of posts that corrects some inaccuracies found in the blog post at BitPerfect regarding click here“>DSD vs. PCM. Richard, the author of the lengthy post, was somewhat critical of my position that 96 kHz/24-bit PCM audio provides better fidelity than DSD 64 (we don’t even talk about the difficulties doing any work with 1-bit DSD). His technical argument seemed flawed at first read. He makes some statements that either aren’t relevant or simply wrong. It’s troubling that he spends 8000 words reinforcing his position with information and “facts” about digital conversion (both DSD and PCM) that when examined in great detail by an acknowledged expert prove problematic. Here’s the next installment:

BitPerfect: “If we confine ourselves purely to digital anti-aliasing filters used in a SDM-ADC, a slope of 300 dB/8ve inevitably requires an ‘elliptic’ filter. Whole books have been devoted to elliptic filters, so I shall confine myself to saying that these filters have rather ugly phase responses. In principle they also have a degree of pass-band ripple, but I am willing to stipulate to an argument that such ripple is practically inaudible. The phase argument is another matter, though. Although conventional wisdom has it that phase distortion is inaudible, there is an increasing body of anecdotal evidence that suggests the opposite is the case. One of the core pillars of Meridian’s recent MQA initiative is based on the assumed superiority of “minimum phase” filter architectures, for example.”

John Siau Wrong again! The slope is not 300dB per octave (for the reasons stated in yesterday’s post). Also the filter is almost always a linear-phase filter and not an elliptical filter. This claim is completely erroneous.

Dr. AIX There are filters involved in the conversions process in both DSD and PCM. The use of anti-aliasing filters in 96 kHz/24-bit PCM systems doesn’t require “brickwall” filters as stated in the blog or “elliptic” filter either. How or why could Richard include elements in his argument that are incorrect. Perhaps the knowledge base of those reading the post would simply take it on faith and thus his position could be taken as true. As for the recent MQA Meridian invention by Robert Stuart, he does advocate for higher sample rates to close the gap on timing differences but I’m not yet convinced that higher than 96 or 192 kHz really gets us anything…except much larger files.

BitPerfect: “By increasing the sample rate of PCM we can actually reduce the aggression required of our anti-aliasing filters. I have written a previous post on this subject, but the bottom line is that only at sample rates above the 8Fs family (352.8/384kHz) can anti-aliasing filters be implemented with sufficiently low phase distortion. And Dr. AIX poo-poohs even 24/352.8 (aka ‘DXD’) as a credible format for high-end audio. Here at BitPerfect we are persuaded by the notion that the sound of digital audio is actually the sound of the anti-aliasing filters that are necessary for its existence, and that the characteristic that predominantly governs this is their phase response.”

John Siau This phase-response argument is only applicable to analog filters used in front on non-oversampling A/D converters. It does not apply to oversampled (SDM) A/D converters. Ironically, it is DSD that has a phase-response issue.

Dr. AIX I can’t say I recall “pooh poohing” “DXD” otherwise known as 352.8 KHz/24-bit PCM without the anti-aliasing filters. I have questioned the misleading name because the DSD community didn’t want their “dirty little secret” – that 1-bit DSD requires the use of ultra high sample rate PCM – to pollute the purity of DSD encoding. With very few exceptions, the DSD files and SACD discs that are sold are has tasted PCM.

Just this morning, I received the new Blue Coast Music Group Newsletter. They announced a relationship with Mack Avenue Records and will be selling a variety of formats of some great new music. But they state, “Recorded at Avatar Studios in New York City by renown engineer, Joe Ferla, the project was initially recorded to 96 kHz/24-bit PCM WAV. It is our opinion these will offer the audiophile the best sonic experience. We have converted the files to DSD and FLAC for your convenience and performance on your listening system.”

They don’t mention that by converting and offering the source 96 kHz/24-bit PCM files to DSD (64 and 128), they’ve substantially increased the price. A FLAC version of the original 96/24 master is $15…converted to DSD 128, it will set you back $30. Stick with the PCM…get the “best sonic experience” and save some cash. Is having the DSD converted copy worth twice as much?

BitPerfect: “PCM requires an anti-aliasing filter, whereas DSD does not (actually, strictly speaking it does, but it is such a gentle filter that you could not with any kind of a straight face describe it as a ‘brick-wall’ filter). DSD has no inherent phase distortion resulting from a required filter. Instead, it has ultrasonic noise, and this is where Dr. AIX’s argument encounters difficulties. The simple solution is to filter it out. However, if we read his post, he seems to think that no such filtering is used…I quote: ‘It’s supposed to be out of the audio band but there is no ‘audio band’ for your playback equipment’. Seriously? All it calls for is a filter similar to PCM’s ‘anti-aliasing’ filter, except not nearly as rigorous in its requirements.”

John Siau The requirements of this filter are nearly identical to the requirements for the anti-aliasing filter in a 96 kHz PCM system. The difference is that the DSD post filter must be implemented in the analog domain and this means that the DSD post filter will cause phase distortion.

Dr. AIX If, as Richard claims, the ultrasonic noise (with no inherent phase distortion) is imply filtered out, then why is still present whenever I do a spectral analysis on DSD files. Theoretically, the noise shouldn’t be there…but it is. John’s response seems to provide the answer. I’m analyzing the digital files prior to the application of the analog filter…the one that causes phase distortion. I suppose there are circumstances where the ultrasonic frequencies are filtered out but in my experience and in talking to other engineers, there have been problems with the ultrasonic noise.

Another post that is way too long. I apologize…but I feel it’s important to establish who’s doing the spinning here.

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About Author


Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

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