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RE: Just a thought ...

Posted by Charles Hansen (M) on June 11, 2017 at 17:32:40

In Reply to: RE: Just a thought ... posted by Isaak J. Garvey on June 11, 2017 at 16:32:51:

>> Many of today's DACs have virtually unmeasurable jitter. There are numerous non proprietary ways to deal with it, including synchronous upsampling <<

1) There are only two magazines (both print) of which I am aware that publish jitter tests - Stereophile in the US and Hi-Fi News in the UK.

2) Both have upgraded their measurement equipment at least once, making it impossible to compare earlier tests from their more recent tests. Specifically Stereophile was the first to measure jitter, but using a unit from Ed Meitner. The difficulty with this unit was that it required opening up the unit and connecting test leads to specific pins on the DAC chip itself. This wasn't so bad in the early '90s as all of the parts were large through-hole devices and most of them were R-2R "ladder" DACs that were sensitive to jitter on the word-clock pin.

By the end of the '90s there were new DACs, almost all surface-mount devices that are extremely difficult to probe without damaging the DUT. In addition the ladder DAC chips were changed such that jitter was only important on the bit-clock pin, and new delta-sigma DAC chips were available that are sensitive to jitter on the master-clock input pin (that drives the modulators in the output stage). All of this led Stereophile to switch to a new analyzer, developed by Paul Miller (currently editor of Hi-Fi News). This machine obviated the need to open up the DUT, as it only measures the analog output signals. It is made from plug-in A/D cards made by National Istruments that fit into a desktop computer, along with audio specific test routines developed by Miller. The problem with that early machine was the A/D converters used were only 16-bits and they were inside an extremely noisy environment (a desktop computer with dozens of high-speed clocks and switching power supplies). Jitter tests made with this machine typically show artifacts of the test equipment in addition to actual jitter in the DUT.

Many years ago Stereophile switched to an Audio Precision which has a noise floor low enough to accurately measure audio equipment without the artifacts of the test equipment interfering. Hi-Fi News still uses Paul Miller's equipment, but at some point the National Instruments cards were upgraded to use 24-bit A/D converters and improved shielding. It's unclear if that setup performs equally to Stereophile's Audio Precision, but it likely is quite close.

3) In addition to the constantly improving measurement equipment, there has been a parallel trend in many of the D/A converters that are tested. Specifically they often incorporate Asynchronous Sample Rate Converters (ASRC), either separately or built into many modern DAC chips. When this ASRC is employed, almost any D/A converter will exhibit "textbook perfect" results on the J-TEST jitter test used by both Stereophile and Hi-Fi News. However my personal experience is that ASRC can dramatically improve the measured performance of a converter while at the same time significantly degrading its audible performance. YMMV.

4) Unlike ASRC, so-called "synchronous upsampling" does nothing to reduce jitter (measured or audible). It can affect the sound quality, as it is simply a specific type of digital filter. As such they will all affect the sound quality differently depending on the parameters of the filters used.

The bottom line is that I agree that when comparing jitter measurement from now to 10 (or even 20) years ago that it definitely appears that the equipment has improved. I am much less clear that most equipment has actually improved when it comes to audible jitter, especially when ASRC is employed.

As always, strictly my personal opinion and not necessarily that of my employer or pharmacist.

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