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Digital Audio Clocking In Pro Tools

Pro Tools Tips & Techniques By Simon Price
Published July 2003

Switching Pro Tools' clock source to external digital (in this case S/PDIF) is necessary to ensure clean transfers from DAT.Switching Pro Tools' clock source to external digital (in this case S/PDIF) is necessary to ensure clean transfers from DAT.

The subject of clocking in digital audio systems is often a hazy area of understanding among us users, perhaps because we've been turned off by propellerheads who love arguing about it. Here's what you need to know in Pro Tools from a practical angle.

No matter what kind of Pro Tools system you use, somewhere or other in your studio there's a piece of hardware whose contribution is to mark the passage of time. This might be the timing crystal in your Mix system's 888/24 interface, or your computer's internal clock in a Pro Tools Free setup. Obviously, if Pro Tools is recording or playing back audio at, say, 44100 samples per second, it has to have a way of telling what a second is, or more to the point, what 1/44100th of a second is. For a single isolated audio system, such as one Pro Tools rig with one audio interface, the clock needs to be reasonably accurate and consistent so that the music plays evenly and at the right speed (and therefore the right pitch). The situation becomes complicated when there's more than one digital device involved. This is the case when transmitting digital signals between two devices (such as Pro Tools and a DAT machine), but also when you have more than one Pro Tools audio interface. The difficulty lies in the fact that inevitably there will be slight variations in the devices' clocks. At everyday human timescales, you might notice two watches varying by a few seconds from each other over a week, which at worst might make two extremely punctual people arrive at the pub a few seconds apart. However, when you're measuring time in units of 1/44100th, or even 1/192000th of a second, clock variations become apparent very quickly, and in digital audio the consequences are more severe: loud clicks in the audio signal. The answer is very simple: any piece of digital audio gear worth its salt should have the option of switching to an external clock source so that all your gear can sing from the same hymn sheet.

Multiple Audio Interfaces

Even if you never normally have to think about clocking because your stuff's all analogue outside of Pro Tools, you may have had an encounter with it when you put your TDM system together (LE systems only ever have one interface). This is because when you have more than one audio interface (such as two 888/24s or 96I/Os), they must be configured to lock to the same clock source. On TDM systems other than the new HD generation, Pro Tools interfaces have two BNC connectors — 'Superclock In' and 'Superclock Out' — for sending and receiving clock signals. Superclock is a very fast clock pulse signal used by Digi instead of the more common word clock found on most digital audio gear (see box). The first interface connected to your Pro Tools PCI cards is considered the 'clock master' for the whole system, and should provide the clock to all your interfaces and PCI cards. You set this up by 'daisy-chaining' together your interfaces with Superclock connections. As well as its main connection to the computer via the PCI card, each interface should have a BNC cable running to its Superclock In port from the previous interface's Superclock Out connector.

 Current Versions 
 Mac OS X
  • Pro Tools HD, Mix, 24, 001, M Box: 6.0.1.
Mac OS 9
  • Pro Tools HD: 5.3.1cs5.
  • Pro Tools Mix, 24: 5.3.1cs10.
  • Digi 001: 5.1.1.
  • Digi 002: 5.3.2.
  • M Box: 5.2.
  • Pro Tools HD: 5.3.3.
  • Pro Tools Mix, 24, Digi 001: 5.3.1.
  • M Box: 5.3.3.

If you have Digi's Universal Slave Driver box, or another sync box that generates Superclock (such as MOTU's MTP/AV), this takes the place of the first interface at the top of chain and becomes clock master. This is how sync boxes keep Pro Tools running in time with the sync source: the USD reads incoming timecode and varies the Superclock rate to speed up or slow down Pro Tools accordingly. With the new Pro Tools HD hardware, Digi have changed the way clock is distributed throughout the system. The main difference when configuring the new Loop Sync is that the clock out connector on the last interface is cabled back to the clock input of the first device, creating a closed loop.

In The Field

Whatever kind of Pro Tools system you have, you can use your clock skills to make sure that digital connections between Pro Tools and your other gear are solid and click-free. This can be a permanent arrangment, such as a connection to a digital mixer, or a temporary change such as connecting up a hired DAT machine. Without any sync devices like a USD, any Pro Tools system (except PT Free) can be set to Internal or Digital sync mode. There is a pop-up toggle for this in the Session Setup window. In Digital sync mode, Pro Tools will switch to using whatever device is currently connected to your first digital input. It can do this because digital audio signals contain clocking information that any device on the receiving end can use. Remember, this is purely a

data-rate locking system, not a sync system like timecode (see box for more on this). The basic rule of thumb is to make the source device the clock master and have the destination set to digital/external sync. If you want to record something into Pro Tools from DAT, for instance, you should plug it into digital input 1-2 on your first interface, and switch Pro Tools to Digital sync mode. On some systems you need to select whether you're using the AES, S/PDIF, or optical input type. On HD systems the Session Setup box will let you choose any of the digital inputs as the clock source, a benefit of the new Loop Sync system. Either way, your whole Pro Tools system will be telling the time exactly the same as the DAT, resulting in a clean digital audio transfer. Alternatively, you might be going the other way, with Pro Tools sending audio to a digital mixer and a DAT or other master recorder. In this case you should poke buttons on your mixer until it goes into external clock mode, or you could read the manual to save time!

Pro Tools HD systems can set any of their digital inputs as the master clock.Pro Tools HD systems can set any of their digital inputs as the master clock.

Knowing this much should get you by in most circumstances, but there is another common scenario in larger studios when you have a USD or Sync I/O unit in your setup. Digi's sync boxes can be used to lock Pro Tools to just about any known type of clock signal, including the standard word clock and Superclock. As I've already hinted, they can also generate a varispeed clock for Pro Tools based on incoming timecode. Most commonly, though, you see them connected via a BNC plug labelled Video Reference (back to that in a second). All of these options are selected from within Pro Tools' Session Setup window under the Clock Reference pop-up. This is in contrast to the Positional Reference selector, which chooses a source of timecode when required.

Anyway, video reference is used universally in TV/Film post production, and increasingly in music studios, and is also known as 'black and burst' or sometimes 'house sync'. The idea is to take a standard timing (clock) reference from a dedicated box, and distribute it to all your equipment, so that you can forget about clocking most of the time. There may be a 'black and burst generator' for the room, or for the whole studio facility, allowing tie-lines to run digital audio between rooms. While a standard black video signal is the most common form of 'house sync', the idea could stretch to other formats. For example, some will buy a dedicated Superclock generator to run their Pro Tools interfaces, rather than daisy-chaining the built-in clock. The stability of a system's clock source has a bearing on the audio quality, which explains why arguing about which clock source to use is one of those Internet discussion forum perennials.

Quick Tips

  • While the audible signs of incorrectly configured clock are clicks, static, or grunge in the audio, you can often see there's a problem before any audio is running because the meters on your interfaces will randomly spike.
  • The left and right cursor keys re-centre the screen on the start and end of a selection respectively. If you centre on the end like this, the zoom keys switch to zooming in and out on this point, whereas in all other circumstances they would move the view to centre on the selection start.
  • Apple Core Audio Drivers for OS X are now available for download from, allowing you to use your Digi interfaces with other software in OS X, including Reason 2.5 (hooray!). You'll need PT6 installed because it uses some of the components, but there'll be a download soon which gets past this requirement. A down side is that iTunes 4 won't work, but there's an update on the way.

Superclock & Loop Sync: What's The Difference?

So what's with all these clock formats, and what exactly are they? If you're sitting comfortably, I'll clip on my beard. The basic digital clock type is word clock, which is embedded in digital audio signals, but can be sent on its own via a dedicated word clock connection. This is a code within the digital stream of ones and zeros that says 'the next sample (or bunch of numbers that represents the sample) starts here', allowing the destination machine to 'slot in' the received numbers at the right time position. It's the same concept as the squiggle in a video signal that says 'the next frame starts here' so the picture can be lined up at the top of the screen.

Normal word clock runs at the same rate as the sample rate, so is adequate for transferring digital signals from place to place. Pro Tools' TDM mix environment can handle 256 digital audio streams, busses or connections, so it needs a master clock that runs at 256 times the speed of normal word clock. This is where the requirement for Superclock comes from, and explains why it needs to run at 256x. ADAT optical connections, on the other hand, send eight channels of audio down one cable, so need another format again. Pro Tools HD and TDM II feature high bit-depth samples and 512 possible streams in the mixer, meaning that Superclock is no longer adequate. Rather than make an even faster clock, which would be more prone to timing innaccuracies (called 'jitter'), Digi rethought the design and came up with Loop Sync, which allows the system to self-monitor and stabilise its own clock.

One thing that clock is not is timecode, as it only indicates speed, rather than any particular time. Pro Tools differentiates between timecode and clock by calling them positional reference and clock reference respectively. This could cause confusion in versions previous to PT6, because the internal/digital reference mode selector is labelled 'sync mode'. It's confusing because when you're chasing timecode you will most likely be using clock from your sync interface, which means that the sync mode setting should be set to internal. Anyway, they've relabelled it in v6 so I'll shut up now!

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