Jitterbuggin' 2020 Measurements

In the November 1994 issue of Stereophile, Robert Harley reviewed three products that were intended to reduce word-clock jitter in an S/PDIF or AES/EBU serial datastream: the Audio Alchemy DTI Pro, the Digital Domain VSP, and the Sonic Frontiers UltrajitterBug. (All three handle 16-bit data sampled at rates of 44.1kHz and 48kHz.) This review was published before Paul Miller and the late Julian Dunn created the J-Test signal, which combines a high-level tone at exactly one quarter the sample rate (Fs/4) with a low-level squarewave at 1/192 the sample rate produced by toggling the LSB. (This signal is not dithered, as this would interfere with its diagnostic function—the level of the noise floor between the harmonics of the squarewave is due to the processor's analog noise.)

I still have Stereophile's review samples of the Sonic Frontiers UltraJitterBug and Digital Domain VSP, so I performed some measurements using a 16-bit J-Test signal, generated by my Audio Precision SYS2722 and transmitted to the D/A processors via a 15' optical S/PDIF link. (As the UJB doesn't have optical S/PDIF outputs, the outputs of both processors were taken to the DACs via a short coaxial datalink.)

The first question was what D/A processors to use for the tests? In his review Robert Harley had used a PS Audio UltraLink, which had relatively poor jitter rejection. I dug around in my storage unit and found that UltraLink sample, along with the Parts Connection Assemblage DAC-1 kit, which I had assembled back in the 1990s and was reviewed by the late Wes Phillips in April 1995. For completeness sake, I also examined the effect of the UJB and VSP with a more recent product, an inexpensive E-Mu 0404 A/D and D/A converter box.

2020Jitterfig01.jpg

Fig.1 Assemblage DAC-1, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Looking first at the performance with the Assemblage DAC-1, fig.1 shows a narrowband spectrum of its output when fed 16-bit coaxial J-Test data. The sloping green line shows the correct levels of the harmonics of the low-frequency squarewave-these are all reproduced too high in level, especially those closest to the spike that represents the Fs/4 tone, due to jitter. These were even higher in level via the optical datalink and inserting the Sonic Frontiers UJB had no effect (fig.2).

2020Jitterfig02.jpg

Fig.2 Assemblage DAC-1 via Sonic Frontiers UJB, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

2020Jitterfig03.jpg

Fig.3 Assemblage DAC-1 via Digital Domain VSP, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Replacing the UJB with the Digital Domain VSP with its jitter-reduction button pressed, which processes the incoming data with an asynchronous sample-rate converter chip before sending it to the Assemblage, gave the spectrum shown in fig.3. It looks as if the squarewave harmonics are now at the correct level but are obscured by a noise floor that is around 12dB higher in level than in figs.1 and 2. (This is presumably due to the mathematical limitations of the DSP chip.) In addition, new sideband pairs make an appearance, at ±1375Hz and ±2746Hz. A sideband pair at ±50Hz can also be seen in the left channel (blue trace).

2020Jitterfig04.jpg

Fig.4 PS Audio UltraLink, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

2020Jitterfig05.jpg

Fig.5 PS Audio UltraLink via Digital Domain VSP, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

2020Jitterfig06.jpg

Fig.6 PS Audio UltraLink via Sonic Frontiers UJB, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

The changes in the behavior of the E-Mu DAC with the two jitter-reduction boxes was similar. With the PS Audio UltraLink, however (fig.4), while the effect of the VSP (fig.5) was the same as what it had been with the Assemblage—a reduction in the levels of the squarewave harmonics, a higher analog noise floor, and new sidebands at ±1375Hz and ±2746Hz—the UltraJitterBug (fig.6) increased the levels of the sidebands closest to the Fs/4 tone. While this is not what I had found with the Assemblage DAC-1 (fig.2), this was not what I was expecting from a device that is supposed to reduce jitter.

For my final series of tests, I used the Audio Precision SYS2722's ability to output a serial datastream corrupted with specific amounts and types of jitter. I created S/PDIF data representing a full-scale, 16-bit 10kHz tone and sent it to the PS Audio Ultralink via the 15' optical link, first without jitter, then afflicted with jitter at 1kHz with an amplitude of 1 nanosecond.

2020Jitterfig07.jpg

Fig.7 PS Audio UltraLink, spectrum of analog output signal, 10kHz at 0dBFS, sampled at 44.1kHz: 16-bit data (left channel blue, right red). Center frequency of trace, 10kHz; frequency range, ±2kHz.

2020Jitterfig08.jpg

Fig.8 PS Audio UltraLink, spectrum of analog output signal, 10kHz at 0dBFS, sampled at 44.1kHz and modulated with 1ns of 1kHz jitter: 16-bit data (left channel blue, right red). Center frequency of trace, 10kHz; frequency range, ±2kHz.

Fig.7 shows the spectrum of the PS Audio's output converting the 10kHz data to analog. The noise floor is even and close to the level of the dither in the 16-bit test signal, though some low-level enharmonic spikes are present. By contrast, with the jittered tone (fig.8), strong sidebands have appeared at ±1kHz and the noisefloor shows significant modulation, acquiring a strange scalloped appearance.

2020Jitterfig09.jpg

Fig.9 PS Audio UltraLink, spectrum of analog output signal, 10kHz at 0dBFS, sampled at 44.1kHz and modulated with 1ns of 1kHz jitter: 16-bit data via Sonic Frontiers UJB (left channel blue, right red). Center frequency of trace, 10kHz; frequency range, ±2kHz.

2020Jitterfig10.jpg

Fig.10 PS Audio UltraLink, spectrum of analog output signal, 10kHz at 0dBFS, sampled at 44.1kHz and modulated with 1ns of 1kHz jitter: 16-bit data via Digital Domain VSP (left channel blue, right red). Center frequency of trace, 10kHz; frequency range, ±2kHz.

I then repeated the test, transmitting the data first by the Sonic Frontiers UJB (fig.9) then by the via Digital Domain VSP (fig.10). You can see that the UJB has not eliminated the jitter. The sidebands at ±1kHz are as strong as they were in fig.8, though the low-level enharmonic spikes have been removed. The VSP, however, has completely got rid of the 1kHz sidebands, though this is at the expense of a raised noise floor and the generation of sidebands at ±1375Hz.

Conclusion: With the 20/20 hindsight offered by these 2020 tests, it looks as if the Sonic Frontiers UltraJitterBug doesn't so much eliminate jitter in the S/PDIF datastream as replace it with its own jitter signature. The Digital Domain VSP's asynchronous sample-rate converter certainly eliminates incoming jitter, but by raising the noise floor reduces the resolution of the data. And. Of course, neither the UJB or VSP can reduce the damage done to the decoded audio by jitter that is generated within the D/A processor itself.

The real solution to the problem products like these were intended to address a quarter-century ago was for the designers of D/A processors to incorporate data receivers with much better jitter rejection than those used by the legacy models I used for these tests. See, for example, the exemplary manner in which a modern DAC, the dCS Bartók, handles 16-bit J-Test data (fig.11).—John Atkinson

2020Jitterfig11.jpg

Fig.11 dCS Bartók, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

COMMENTS
hollowman's picture

It's good that now-classic gear is being re-measured with modern tools (and evolved skill sets).
I hope JA can get around to measuring digital gear that is STILL sought after. Such as DACs and CDPs with classic Philips chipsets: TDA1541, SAA7220, etc.

John Atkinson's picture
hollowman wrote:
I hope JA can get around to measuring digital gear that is STILL sought after. Such as DACs and CDPs with classic Philips chipsets: TDA1541, SAA7220, etc.

To that end, I dug out the sample of the Magnavox (Philips) CDB472 that I bought in 1988 to run some modern tests on it. Unfortunately, while the transport still worked, the display just said "ERR" (for error) with every CD I tried. It looks as if the laser pickup had died in the 30 years since I last used it. :-(

John Atkinson
Technical Editor, Stereophile

hollowman's picture

I used to own that model. It was an improvement over the famous CDB650, while still retaining the 650's Hall-effect turntable motor (later transports at that price range used a cheaper "toy" DC motor).
About your issue:
Well, it could be a dead laser. You may be able to know for sure if you remove the cover, hold your Smartphone camera over the laser and press PLAY. (The phone camera will allow you to safely see the laser redness w/o hazard to direct eyesight).
If the laser is okay, some of those electro caps may be completely dried out.
FWIW, I've owned over a dozen (now) vintage CDPs and have yet to encounter a bad laser.

There are plenty of vintage audio refurb/repair houses in big cities. I recall a Stereophile YouTube post featuring one in NY (Leeds Radio???).

In any case, it's worth having device of this vintage in good working cond ... you never know when science my re-require its utility ;)

John Atkinson's picture
hollowman wrote:
Well, it could be a dead laser. You may be able to know for sure if you remove the cover, hold your Smartphone camera over the laser and press PLAY. (The phone camera will allow you to safely see the laser redness w/o hazard to direct eyesight). If the laser is okay, some of those electro caps may be completely dried out.

Will try this. Thanks.

John Atkinson
Technical Editor, Stereophile

JRT's picture
John_Atkinson wrote:

"...the Magnavox (Philips) CDB472 that I bought in 1988..."

My first CDP was a Magnavox (Philips) CDB-473. Not sure now, but I think I bought that in 1987.

hollowman's picture

I do think measuring legacy digital gear is important (preferably equip that's been refurbished-- replace dried caps -- but NOT modified).
I don't think such a project has been undertaken by any other major publication -- specifically one with a standardized lab and routines (test methodology) as Stereophile.

It would be good to gain insight into issues, such as certain audiophiles' preference of classic multi-bit sound, or non-oversampling.

Having a database of revised metrics would be beneficial to both manufactures and audiophiles. It's not only test gear and test methods that have improved. But the list of test parameters has also increased (linearity, jitter, Dirac pulse, etc).
And there is room for growth to this list as well . For example, back in the mid-80s, Bob Carver published some measurements (in Audio mag), regarding his Digital Lens technology. Carver introduced 'scope measurements via Lissajous pattern showing (L-R)/ (L + R) ratio from an LP record vs. CD. They looked different:

Carver 1985
http://gammaelectronics.xyz/audio_013-1985_carver.html

I don't think manufs and journalists paid too much attention. There may very well be many more objective metrics lost in the journal archives.

AudioIdiot63's picture

Hi John,
Very interesting again after a long time. Around 2000 I designed the Assemblage DAC3.1 and "jitterbug" D2D-1. I replaced the AES21 that was getting obsolete and couldn't doe 96k with a discrete dual PLL using a VCXO. According to the Clock Jitter analyzer designed by Dr Remy Fourre it had an intrinsic jitter of 1,5ps and a much lower jitter attenuation frequency. If you ever come across one of these I would be very interested how it performs on this test. Nice article thanks.

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