Hardware Reviews

Antelope Isochrone OCX Master Clock


Without a clock signal generated somewhere in your CD player, streaming device, D/A converter or digital video player you would not be able to get any sound or picture out of the box. Most clocks are very simple devices that generate a waveform from a quartz oscillator with some simple parts to get the clock going. Except for size these are no different to the quartz movement in a watch. More sophisticated players use advanced clocks with low jitter, low noise and a dedicated power regulator. Still improvements can be made and that is why companies like Trichord, Tentlabs and LC Audio Technology became popular. They offered clocks as modules to replace the circuit within the player and even dedicated power supplies became available. All in an effort only to make sure the ones and zeroes arrive at the right time.

In professional recording studios they don’t depend on the clocks in each device, jitter levels would become unacceptable because of frequency deviations, so master clocks or word clocks as they are called came along to make sure that every bit arrives exactly at the same time all over the studio. Some companies like Antelope Audio have made their name manufacturing clocks to the highest level. The ultimate clock uses Rubidium atomic as a reference generator to get a clock signal 100,000 times more accurate than any crystal oscillator could provide. The next level down from Rubidium is a temperature controlled crystal, often called oven controlled, where the crystal is contained in its own closed box and kept at a constant temperature. The lowest level is a voltage controlled oscillator circuit found in many non-professional devices. In a large studio the master is often a Rubidium clock, with OCX (Oven Controlled Oscillator) word clocks working as slaves and connected by a separate wiring system to make sure the clock is not dependent of the data stream.

ocx fr single straight
Recognizing the importance of a clean, low noise, phase correct clock signal, companies like CEC, M2Tech, dCS, Aurender and Esoteric offer word clock inputs (and sometimes outputs too) on CD players, streaming devices, sample rate converters and digital to analogue converters. Some offer clocks themselves from a voltage controlled oscillator (M2Tech) up to Rubidium ones (Esoteric). Not only to make sure that your player and DAC get the same time based waveform but also to improve the sound quality of the individual parts. As is often the case within the audio industry these specialties form a niche market. Small quantities and high development costs lead to high prices, while master clocks for professional use are more common and are available at more reasonable price levels. That is why for instance Antelope Audio’s pro models are of interest.

The Antelope Isochrone OCX master clock is a 19 inch studio model with a faceplate of 482 x 44 mm, while the electronics compartment behind it is only 420 x 145 x 42 mm. On the front is a blue LED indicating that the oven temperature is at the correct value and two frequency switches for sample rates from 32 kHz to 192 kHz and all the usual points between. Next we have a red display (with variable brightness) plus two knobs that send a clock signal to BNC outlets 7 and 8. These can be a quarter, a half, once, twice and four times the selected clock rate or 256 Fs for ProTools. On the back are atomic clock in/thru connectors, eight BNC out connectors, two XLR and two RCA outputs. Inside is an oven controlled crystal oscillator with an accuracy of 1 ppm and an Antelope developed anti  jitter circuit, since low jitter operation is critical in the modern studio environments and at home. They use adaptive loop filtering with a sophisticated DSP based multistage IIR filter.

A few months ago a friend lent me a €2,400 Grimm Audio master clock for a couple of days and it produced an improvement in sound quality with my Esoteric D-07 D/A converter. That inspired me to do some research on clocks and to try and get hold of one for myself. Esoteric was the first choice, but even the least expensive one costs almost €4,000 and has now been replaced with an even costlier model. Having recently reviewed an Antelope Audio DAC, I asked the distributor for the Isochrone OCX to try with the Esoteric, a DAC that converts PCM data into DSD before the actual digital to analogue conversion takes place. Its main purpose is to convert data coming from a NAD M50 digital music player, with FLAC, WAV and ALAC files stored on a local M52 music vault. The clock does not come in between player and DAC, it only connects to the DAC’s word input, that way it is impossible to alter the data stream coming over the Apogee Wide-Eye balanced cable. A BNC terminated Stereovox XV2 digital cable feeds the clock signal into the converter. A remote control comes in handy to switch between the internal clock and the external master clock from the listening position.


ocx ph angle

In between the Grimm and the Antelope I changed from PMC fact.8 speakers to fact.12 and with the transparency of the fact.12 I had to get used to the change in sound that the Antelope brought. This speaker is extremely revealing and works like a magnifying glass on any change you make to the system, it’s the reason why I kept switching from the internal clock to the external one and back, wondering which I liked best. My system moves the solo singer or instrument forward to the listener, while the band is spread across the room near the back wall. With the Antelope the whole picture is set back, wider than it was, but because of this the feeling of ‘being there’ is less prominent. Voices, male and female, are intimate with the D-07 internal clock, warm and cuddly at times, tender and a little softened. The Antelope does the opposite enhancing every detail, every breath of air, it almost shows the singers mouth. A voice like Alison Krauss’ becomes clear as a crystal, almost at the edge of too bright. But as soon as you go back to the internal clock you notice that her voice is a little rough around the edges, even slightly distorted and muffled. This clearing up is not to everyone’s taste, my wife for instance prefers the warmer and more muffled sound, which she feels comes closer to a live performance. She and I often agree on what we prefer, the Antelope divided us. Other listeners agree on the improvement in detail and window cleaning effect, but again some preferred the old sound. It took me some time to get used to the new situation, only to find that after extended listening to the Antelope there is no way of going back. It seems strange that a clock alters the sound that much, it’s like having a completely new DAC, maybe even from a different manufacturer. Being a Trichord and Tentlabs adept in the past I cannot remember these sorts of large influences from a clock change (the Trichord clock turned CD into a listenable format for me – Ed).

A step toward to audio nirvana
Moving away from the solo artist and listening to the band a considerable improvement is made with the external clock. Small details were always there, now they are highlighted and brought forward. Probably due to a cleaned up low end where bass tones are tighter, better defined and show more of the instrument’s character. A run of wavy piano notes tight together on Rachelle Ferrell’s live recording of My Funny Valentine changes into a string of individual piano notes. It makes it more clear how rapidly the pianist’s fingers move over the keyboard. Drums sound more dynamic, on guitars the difference between types of strings is easier to recognize. Brass instruments gain in delicacy. The funny thing is that due to improved dynamics it seems like the music is louder, but I made sure the volume control was never touched, neither on the preamp nor on the DAC. One listener complained that the music became so clean it almost changed from reality into hi-fi. Another mentioned a demo with dCS electronics where they turned off the master clock and the sound collapsed. This guy and I also fell in love with the way a Bach piece was played on harpsichord. So tender, fragile, clear and yet so intensely musical. Yet the Antelope clock must be tried at home before you buy, you might be happier with the sound that inspired you to buy system, instead of this new realism with all its exposed details. For me, this word clock is an important step toward to audio nirvana. Maybe because I am the most familiar with my system and had the longest period to listen to the Isochrone OCX I clearly noticed the improvement.

I was ready to order the Isochrone OXC when a used Esoteric G-03X came up, as it was in excellent condition and at a fair price I bought it immediately and compared it at home with the Antelope. The improvement in sound is about the same with both devices, the Antelope is a bit clearer, the Esoteric a bit more musical. Edgy compared to round. But the differences are so small that I would choose the Antelope over a new Esoteric. The latter does however have the advantage of being able to output a 100 kHz clock signal. The D-07 is able to transform the 100 kHz into 44.1 and 48 kHz clock frequencies or their duplicates of two and four times the basic rate. Which is almost a must if your music collection is in all kinds of sample rates. The Antelope and most others need to be changed from 44.1 to 48 kHz by hand (or 88.2 to 96, or 172.4 to 192), otherwise your DAC shows a word clock error and keeps quiet. If you only play CDs or don’t mind pushing a button when required, you could save a lot of money. In the end I can only reach one conclusion, if your player, streamer, DAC or whatever device has a word clock input, give it a try. You might become as addicted as I am now to this sort of technology. Digital audio still has many hidden secrets to unveil. This one is clear, the influence of the clock signal is and has always been of major importance to sound quality.


Oven Controlled Discrete Transistor Crystal Oscillator for lowest jitter
Atomic Clock Input resolves to Rubidium Clock for perfect stability
Jitter Management Module improves sonic performance
Sample Rates 32-192 kHz
Factory calibrated to better than +/- 0.02PPM accuracy via Atomic Clock
Can be recalibrated in the field to an Atomic Clock for better than +/- 0.02PPM accuracy
W/Cx256 "Superclock" Output for Digidesign Pro-Tools systems
Outputs multiple, divisible sample rates
8 Word Clock Outputs
2 AES/EBU Outputs
2 S/PDIF Outputs


Price when tested:
Manufacturer Details:

Antelope Audio
tel: +44 20 81 33 8355



word clock


René van Es

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