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General Studio Tips


Word Clock Tests

DAVID RIDEAU discovers how different clock sources can alter converter sounds and how striking the results are.

I am an analogue man. I know this is a strange way to start an article about digital gear, but I feel it’s an important point. I’ve been in this business long enough to remember when the digital audio medium did not exist and the professional recording process was a simple one. These times fostered sensibilities that made the audio’s path top priority and an engineer’s job (design or recording) was to enable the music to speak without interference. Now with the digital revolution there have been quantum leaps in the storage and manipulation of audio, but not always with the highest regard for audio quality. With this philosophy I am constantly looking for ways to improve the digital signal with analogue serving as my holy grail. This quest fostered this digital clock comparison.

We have all heard from the manufacturers of digital audio equipment on how important an accurate clock is in the performance of their products. Unfortunately, we have also all experienced the dreaded ‘ticks’ that occur when a device finds it’s unable to function due to the quality of its digital clock source. Besides this point where jitter is so high that a unit can no longer function properly, there are also more subtle abnormalities that can harm a signal’s sound quality. I have heard many levels of performance in the multitude of digital rigs that reside here in the LA area, but how much of that can be attributed to clocking issues?


Our idea (I enlisted a couple of colleagues) was to take a sampling of the most popular clock sources and put them through a series of bench tests by an independent lab, followed by a series of listening tests to see what we could discover. The test consisted of four units: the Aardvark Aardsync II, Digidesign Universal Slave Driver (USD), an Apogee PSX100, and the new Rosendahl Nanosyncs.

We purposely ran our listening test before reading any bench test results to avoid any bias. The test was held at Westlake Audio and it began by creating a live mix from an analogue two-inch Studer 24-track machine through an SSL G console. The mix included real drums, vocals with dense reverb tails and lots of complex stereo information — all the material that suffers most during digital conversion. At this point we fed the stereo bus to one of the three A/D and D/A converter sets we selected (Digidesign 888, Apogee PSX100 and the Sony 3348HR) that were alternately clocked by our various test units. Using an external patch point to return the twice-converted audio to the console made it easy to A/B this signal with the live mix.

In all our listening tests we were careful to make sure that all clock runs were as short as possible with the same high-quality 75 Ohm cable. Every other variable, such as power source, was all uniform to the best of our abilities. Keep in mind our goal was to simulate a real-world professional working environment, so we approached the set-up as we would for any other session, except to ensure that all clocks were on a level playing field.



The results were surprising in many ways. First, it was amazing how different the clock source could make the converters sound and how easy it was to hear these differences! These were so dramatic that we had other engineers who were in the building listen and they too were amazed.

I rated my findings with the standard being how well the signal compared to the live stereo bus (a perfect ten rating). For example, with the 888 being fed 256kHz directly, the USD scored a 6.8; the Nanosyncs fared much better with a 7.8; and the Aardsync was best with an 8.5 (the PSX 100 has no 256kHz output).

The conclusions were interesting. In general, the Aardsync consistently performed best with the Apogee and Nanosyncs next and the USD last. I realise that in some instances we are comparing apples and oranges (ie. the USD does much more than generate word clock), but our goal was to create a real-world environment that investigates how much a better clock source means in the performance of hardware that our readers may already own. As an added bonus, we had the opportunity to compare converters under the same conditions producing a test within the test.

The next step was to compare the labs’ bench tests to see if there was any correlation between what we heard and the data received. At first glance the test results were quite confusing. First, there seemed to be no significant difference in jitter between all the units! They all came in at around 200-240ps range. I called several manufacturers and found out that jitter can be measured in countless ways: RMS with many time interval options or peak readings, both of which can be done at any number of frequencies. In general, the ear is more critical to clocking errors at higher frequencies where jitter can cloud the sound stage, so some sort of weighted curve most likely would produce results that better relate to how we perceive clocking errors. Whatever approach our lab used (unknown), the jitter results we’re non conclusive. No wonder audio professionals are so confused; until there are standards, jitter specifications are virtually useless.

Thankfully, our tech tested further to see if there were other areas where she could find hard data of any variation between the units. Where there seemed to be a significant difference in results could be seen on a digital scope across the power spectrum. With the clock source fed directly into a digital scope, it was possible to see exactly how clean the signal was.

In the tests, we found that the output of all the clocks exhibited a power spectrum, which contained, in addition to the primary component at the base frequency, n = 48kHz or 12.288MHz, odd harmonic components occurring at regular intervals of 3n, 5n, 7n, etc., which are directly related to the square wave of the word clock (see Figure 1). Additional smaller amplitude peaks, 60 to 70dB below the primary signal were also observed. We found that the main difference between the different clocks was in the relationship between these smaller higher-frequency components and the main signal. In the case of the Aardvark, we found that the smaller components appeared at regular intervals, at n/2, or half way between the harmonic peaks (see Figure 2). Therefore, we believe these to be harmonically related to the base frequency and therefore inaudible. In the case of the other clocks, the USD in particular, the smaller components occurred randomly between the main harmonic peaks (see Figure 1, small peaks), which could cause audible artefacts. These artefacts seem to have a direct relation to our listening test results.


The manufacturers of all these products have created fine equipment. The Digidesign Universal Slave Driver (USD) is a high-quality, multi-purpose synchronisation device designed for use with TDM-based Pro Tools systems. It supports all major industry standard clocks and formats and can also act as a standalone timecode reader/generator. It can provide solutions for many audio professionals’ problems that are encountered every day. At the price list this unit is still a good buy and fills a specific need in our industry. Obviously many hits have been recorded on Pro Tools with this unit as the clock source, but I personally would invest the extra cash to incorporate an additional high-quality clock into the same rig to achieve the next level of performance.

The Rosendahl Nanosyncs is a real contender in the field of digital clock sources. With six BNC clock outs, Word Clock in, SPDIF and AES outs, Video in and out, Blackburst generator, and the ability to resolve to LTC all as standard features, the price makes this unit a perfect solution for someone in need of a lot of quality WC outs at a great price.

The Apogee PSX 100 also qualifies in the most value for money department. It provides quality 24-bit 96kHz A/D and D/A converters, ADAT and TDIF interfaces, AES SPDIF I/O, bit splitting and all with a great individual clock source.

The Aardvark Aardsync II is an amazing unit! Time after time it turned a good sounding signal into a great one. The common failures of digital conversion were always minimised with the Aardsync leaving an accurate, focussed, open, uncompressed, wide interpretation of the analogue stereo picture. Not the most economical choice compared to our other entries (four WC BNC outs but no LTC resolve, Video Lock, 96kHz or Blackburst generator without considerably more of an investment), but at that price the Aardvark provided excellent results with all of the converters.

The purpose of these tests was to give readers an idea of how important clocking issues are in our age of modern recording. I also hope it can help encourage manufacturers to provide buyers with a more standard jitter test that would only help clear up the cloud of confusion that surrounds this specification. The listening part of the test is subjective, and I encourage buyers to audition clocks in their own environment. I guarantee that the time and money you spend is well worth the investment!


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