The Advantage of a Word Clock

What is a Word Clock?

Every digital device, from a simple portable CD player to a Pro Tools HD rig, has a word clock. Not to be confused with sync clock (like MTC or SMPTE), word clock is what tells a A/D or D/A converter when to take the sample. It fires an impulse to the converter 44.1k (or 48k, or 96k, etc) times per second. The reliability of this clock, how evenly spaced those pulses are, determines the accuracy of the conversion process.

If you have digital connections in your studio (S/PDif, lightpipe, etc), you MUST have a single clock source. If multiple devices are each trying to be the master clock, you will probably experience pops, clicks, or sometimes chirps in your audio. The solution is usually to make sure that you only have one master clock, and that all slave units are getting a clean clocking signal.

But even without obvious problems like pops and clicks, you may still have more subtle word clock problems. Think of it like this…

Ever seen clips from the very early days of film? Back in the days of hand-cranked cameras, it was extremely difficult to get the shutter speed to be perfectly even. The result was a moving image that, when played back at a constant shutter speed, seemed “jittery.” the motion was often jerky an unnatural. This was due to the imperfections of the original film speed.

The same thing happens in digital audio. The more “jitter” present in the clock source, the less accurate the resulting conversion will be. Jitter is manifested in audio as phase distortions and discrepancies, particularly in the higher frequencies. They can lose some of their sparkle, transients can lose some attack, and stereo images aren’t as vivid.

Improving the quality of your clock is an easy way to upgrade every A/D and D/A converter in your studio. In most cases, adding a high-quality master clock, such as Apogee’s Big Ben, will result in a marked improvement in the overall quality and clarity of your finished product.   (Source:   Jeff Barnett http://www.sweetwater.com/jeff_barnett)

Technical Review –

published by SOS (http://www.soundonsound.com/sos/aug05/articles/apogeebigben.htm)

The Big Ben is fundamentally a Grade 1 master clock generator able to operate at any of the standard rates from 44.1kHz to 192kHz. It provides six word-clock outlets, two AES outputs, a coaxial S/PDIF socket and a configurable optical port for S/PDIF or ADAT interfaces. Two of the word-clock outputs can be configured to provide DSD, Super Clock, or various other multiple-rate references. There is also an option-card slot which can be used to extend the unit’s compatibility with future interface standards. Firewire compatibility is already available, and a video generator card is expected shortly.

Many master clocks are designed to run independently, but the Big Ben can be slaved to an incoming clock reference signal (including video), if required. Apogee claim that any jitter on an external reference clock is removed to such an extent that it is usually impossible to tell the difference between Big Ben’s operation slaved to an external clock or running from its internal clock generator. An extension of this de-jittering functionality is that, when using a reference input carrying audio (in other words AES, S/PDIF, ADAT, or Firewire), the data is automatically de-jittered before distribution to all the other audio-capable clock outputs.

The Big Ben carries a lot of socketry on the rear panel to enable easy connection with a wide variety of equipment, although, unlike many master clocks, there are no front-panel interfaces to make it easier to accommodate temporary equipment. Six BNC output sockets to the right-hand side of the back plate carry separately buffered standard word-clock outputs. The first four operate at the unit’s selected clock frequency, while the last two can be configured independently to output clocks at some multiple of the clock rate (quadruple, double, half, or quarter the selected sample rate, plus Super Clock or DSD references). To the left of these outputs is a seventh BNC socket which accepts an external word clock or a video reference input signal. The socket is permanently terminated with 75(omega), and the input format is determined by a front-panel menu selection.

Moving further left, there is blanking panel for the slot-in option card. This is a nice idea and in theory enables the Big Ben to provide reference clock signals for any future format. The X-Firewire card was the first to become available, potentially enabling keyboards, synth modules, and other Firewire-based devices to provide stable clock references. A forthcoming X-Video card will allow Big Ben to function as a master video-sync generator in sync with its digital word-clock outputs.

The remaining interface sockets can all carry digital audio in various formats, and are provided as both inputs and corresponding outputs. S/PDIF is catered for with both coaxial and optical connections for input and output, but the Toslink connectors can also be configured for the ADAT format. The AES interface is equipped with dual XLRs for both inputs and outputs, allowing a choice of AES references as well as accommodating a high-sample-rate stereo signal in the double-wire mode.

It is unusual for a reference clock to distribute audio along with the embedded clocks (because of concerns that the audio data can increase cable-induced jitter artefacts), but that is precisely what Big Ben does by default. If the idea of distributing audio with your embedded clocks worries you, there is an internal jumper option to defeat the audio distribution mode and send ‘digital black’ (in other words a mute signal) to all digital audio outputs instead.

However, by default the audio carried on a selected external clock reference is distributed automatically to all the outputs capable of carrying audio, complete with the necessary format conversions. The situation is complicated slightly since the ADAT interface is able to carry up to eight channels while the rest are only stereo, but the channel routing is entirely logical given the inherent restrictions. So, for example, if an eight-channel ADAT signal is chosen as the reference, all eight channels will appear at the optical output (still in ADAT format), the first two channels will also appear on the coaxial S/PDIF output and the first AES output, and channels 3+4 will be presented on the second AES output. Likewise, a stereo input is copied to all the other stereo outputs, and repeated across each pair of available ADAT outputs. The only disappointing limitation is that it is not possible to convert between single-wire and double-wire AES signals — although I doubt that will be a concern to many.

The final back plate connector is the usual IEC mains inlet. The internal power supply is a switched-mode design that accepts 100-240V AC, at 50Hz or 60Hz.

The Big Ben shares its front-panel styling with other Apogee products: purple and silver background colours with clear black and silver labels. Controls are kept to a minimum with just a quartet of cursor keys and a power/standby button, but the panel also carries a comprehensive set of LEDs to indicate every aspect of the unit’s status. An alphanumeric LED display is included to show the current sample rate, various configuration messages, and the lock status. Turning the unit on results in an entertaining light show as every LED is tested (several times!) and the unit’s name is flashed on the LED display.

The four cursor buttons are used to adjust the Big Ben’s configuration in a pretty intuitive way, and there is a degree of ‘intelligence’ such that inappropriate functions and settings are automatically hidden from or added to the available options as the machine’s configuration is changed. For example, if an external clock source is selected, the facility to change the sample rate is removed, since that parameter is defined by the external clock source itself.Unusually, the powering arrangements can be configured with a pair of internal jumpers. The default condition is for the front-panel button to toggle the power mode as you would expect (although this is not a traditional mains isolation switch — it is a control input to the power supply). However, this button can be disabled completely if required to prevent the unit being switched off accidentally, and there is also an ‘auto-on’ option in which the unit will power on automatically whenever a mains supply is connected (but it can still be switched between the On and Standby modes). This is a useful set of options that will appeal particularly to installers using the Big Ben as the reference for a large system.

Clock Sources

On either side of the sample-rate display are two columns of legends used to indicate the current clock source — and it is a comprehensive list! On the left are options for the internal mode and the external reference inputs: the two AES inputs, the S/PDIF coaxial connector, the optical port, and the option card. On the right are legends for the external word-clock and video-input options, and if Video is selected then the detected video format is also displayed (NTSC, PAL, or B&W). There is a warning in the handbook that it can take up to 30 seconds for the unit to fully stabilise to a video reference, and if the AES mode is configured for double-wire operation, then selecting AES 1 as the reference input automatically illuminates the AES 2 flag as well.

The four-digit sample-rate display shows the selected clock frequency if operating from the internal clock generator, with all the standard rates between 44.1kHz and 192kHz. If the Big Ben is locked to an external source, the display shows the actual (measured) input sample rate. Immediately below the numerical read-out is an indication of the unit’s lock status, using a green arrow and a blue LED, along with Lock Wide and Lock Narrow legends. When the unit is solidly locked to the selected reference both the green arrow and the blue LED light steadily, along with the Lock Narrow legend.

There are two situations where the Lock Narrow indicator may go out. One is if an external clock source is jittery or unstable in some way. In this case the blue light goes out and the Lock Narrow legend is replaced with Lock Wide — meaning that the unit is still synchronised to the external reference and the derived clock outputs are stable, but that the external source is dubious in some way and Big Ben is struggling to remove all of the instability or jitter.

The other situation is when the unit loses lock completely — such as if the reference input cable becomes disconnected. In this case the Lock Narrow legend goes out immediately and the green arrow starts blinking — but the blue light remains on to indicate that Apogee’s Sure Lock function is active. Essentially this means that the Big Ben is continuing to generate clock outputs at the last valid sample frequency from its internal clock generator. When the external reference is reinstated the internal clock will slew back into synchronisation with the external feed (which is indicated when the arrow stops blinking and the Narrow Lock legends illuminate). However, depending on the attached equipment and the relative local/reference drift, some sync disturbances may occur as the Big Ben re-establishes lock. For situations where it would be better to be aware of a missing reference clock, an internal jumper is provided to defeat the Sure Lock mode if required.

Sure Lock is a good idea and seemed to work well… when it worked. However, I found that the review model appeared to disable the Sure Lock mode automatically! Resetting the unit to the factory default (by holding the Down key while turning the power off and on again) activated the Sure Lock feature which worked as advertised the first time I removed an external reference clock.

Changing any of the machine’s parameters or configurations is a simple case of navigating to the required setting using the Prev/Next buttons, and then changing the value with the Up/Down buttons. Pressing any of these four keys causes the unit to enter Setup mode, indicated by a small LED in the centre of the quartet of buttons. The most recently adjusted parameter then flashes and it can either be adjusted further or a different parameter can be selected and then adjusted as necessary.However, subsequent removal of the clock resulted in no clock outputs and ‘0000’ displayed on the LED sample-rate counter. Only resetting the unit would temporarily reactivate the Sure Lock mode and I could find no explanation for this odd behaviour in the handbook. After checking with Apogee, it turns out that this is a known bug that strikes at random, and which they are hoping to address in a forthcoming firmware release.

Repeated presses of the Prev/Next buttons cycle around the various options. The complete set comprises the clock reference source, the sample rate, the optical interface format, the AES format, the video pull-up/down options, and finally the multiplier rates for the fifth and six word-clock outputs. Not all of these are available at all times, depending on the underlying configuration. The unit drops out of Setup mode automatically two seconds after the last button press and the displayed parameter stops flashing to indicate that the new setting has been activated. All settings are stored in non-volatile memory and are recalled when the unit is powered up. If required, the default factory settings can be restored by holding the Down key depressed while powering the unit off and on, and if the Up key is held when the power is cycled word-clock outputs five and six provide DSD reference clocks.

I have already explained the clock-reference and sample-rate options. The following adjustable parameters concern the optical and AES interface formats. The optical port can be switched between S/PDIF and ADAT modes, but the latter is extended with the associated SMux 2 and SMux 4 formats. SMux 2 conveys four channels at 88.2kHz or 96kHz over the standard ADAT interface, while SMux 4 provides two channels at 176.4kHz or 192kHz. The AES modes are simply single-wire or double-wire configurations — the former supporting two channels at all rates between 44.1kHz and 192kHz, and the latter using two AES pairs to convey stereo signals at rates from 88.2kHz to 192kHz (but operating each cable at half the sample rate).

The next set of options affects the pull-up/down multipliers that are often required when working with film or NTSC video — but these are only available when the Big Ben is switched to internal clock or external video references. By default, the LED array shows None, meaning that the sample rate is as shown on the numeric display. However, if any of the pull-up/down options are employed, the appropriate offset is indicated by an associated LED: ±4 percent or ±0.1 percent. The first is used to translate between 24fps (frames per second) film and 25fps PAL video, and the second for working with 29.97fps colour NTSC and either 3:2 converted film or B&W (non-drop frame) NTSC. Clearly, these are specialised functions, and a detailed explanation of their use is beyond the scope (and space) of this review — but their inclusion certainly broadens the usefulness and appeal of the Big Ben as a master clock in video-related audio areas.

Another option in this section is labelled VSO — Variable Speed Override. When operating on the internal clock, entering this mode allows the user to change the sample rate to any desired value. The varispeed display can be given as sample rate, percentage change, or musical cents (the display mode being changed by holding the Prev/Next buttons).

As mentioned earlier, if the Up button is held while the unit is powered off and on again, outputs five and six are re-configured to supply DSD reference clocks. In this mode output five provides a clock at x64 the base rate (around 3MHz), while output six provides a signal at x128 the base rate (around 6MHz) — and in this case the base rate means the lowest denominator of the selected sample rate (so either 44.1kHz or 48kHz). The front-panel display doesn’t highlight this special DSD mode in any way, although if you try to adjust the multiplier value for these two clock outputs you find they are inaccessible. Turning the unit off and on again resumes the standard operating mode.The final parameter sets determine the independent multipliers for the outputs from word-clock outputs five and six. The options are x1, x2, x4, and x256 (Super Clock), plus /2 and /4 (in other words division by two or four), although not all options are valid with all base sample rates. The x4 and x256 modes are sensibly disabled for sample rates above 88kHz, and the x2 mode for rates above 176kHz. Likewise, the /4 mode is inactive for rates below 176kHz, and the /2 mode for rates below 88kHz.

The final array of LEDs on the right of the front panel comprises pairs of red and green lamps for each of the six word-clock outputs. Green LEDs illuminate when a correct 75(omega) termination is sensed, and red LEDs shine if the detected load is less than 75(omega) (for example an incorrect 50(omega) termination, a double termination, or a cable short-circuit). Unterminated or open-circuit cables are indicated by extinguishing both LEDs. This facility reassures that the system is wired correctly, and is also useful in confirming that clock connections are correctly and precisely terminated — something which is always important, but which becomes critical with elevated clock rates.

My own digital studio equipment is generally clocked from an Aardvark Aardsync II, supplemented by a Drawmer M-Clock for the occasions when I need to operate separate sections at different sample rates. Consequently, plumbing in the Big Ben was a straightforward task.

Everything worked exactly as expected, and I found configuring the Big Ben very fast and easy once the functionality had become familiar — although there do seem to be an awful lot of button presses involved when trying to change some settings. In most cases, though, once the system is configured only the sample rate would need to be changed regularly, and since the Setup mode remembers which parameter you last changed, changing the rate again becomes much faster.

I was unable to hear any difference from the connected equipment when switching between the Aardsync II and Big Ben as master references, but in itself that suggests that the Apogee box is at least as good as the Aardvark — one of the best clocks available in my opinion — and quite possibly better. Given the moderate cost of the Big Ben and its far more extensive functionality, this is a very impressive result.

Using the Aardvark as an external reference for the Big Ben, I was able to check that its sample rate display gave accurate values for a range of non-standard reference sample rates. The Aardvark can generate numerous pull-up/down rates, but the Big Ben recognised them all without any problems. It even locked happily to 32kHz despite this being outside its declared operating range.

One aspect I was interested in investigating further was the Big Ben’s ability to reduce the jitter of poor-quality sources, so I hooked up a cheap domestic portable CD player, using its S/PDIF output as the embedded clock reference and using an ordinary 1m audio phono cable (not a proper 75(omega) digital cable) to make life as difficult as possible, along with a very simple old stand-alone ‘hi-fi’ D-A converter without any sophisticated de-jittering functions of its own. It didn’t take much critical listening before it became clear that the soundstage was flat and lifeless, and the acoustic ambience did not have the spaciousness that I knew was encoded in the source material. I then re-plugged the audio phono cable carrying the S/PDIF signal to the Big Ben and used a pukka 75(omega) cable from the Big Ben back to the converter. The difference was quite marked, with the stereo image becoming far more solid and three-dimensional, and the subtle ambience cues becoming far more lifelike. An impressive result indeed, if entirely subjective, suggesting that the de-jittering capabilities of the Big Ben are very powerful.

Overall, the Big Ben is an impressive machine with capabilities that reach well beyond those normally associated with master clocks. As a result, it is a very cost-effective and pragmatic device for a wide range of applications, and will serve as a flexible problem-solver in many more. Equally at home in a modest home studio, an audio-video post-production suite, or a high-end digital-audio production or mastering studio, the Big Ben’s unique feature set makes it worthy of very serious consideration