Synchronization—getting two or more devices to record or play back together—is a fundamental factor in making a studio work. Sync issues are similar for analog and digital devices. In fact, synching digital devices is often simpler than synching their analog counterparts. There are some different methods in the digital synchronization toolkit, which may be unfamiliar to you. In this article, we''ll look at the basic concepts of synchronization and how they apply in the digital domain.
First, it''s important to realize that synchronization has two components: one is starting at the same time, and the other is proceeding at the same speed. For the sake of convenience, we''ll call the first “start sync” and the second “continuous sync.” In the analog world, these are often two facets of a single synchronization method, such as SMPTE time code. With digital devices, on the other hand, they may be handled separately.
Let''s start with the analog case first. With analog tape, both start sync and continuous sync are generally handled by SMPTE (Society of Motion Picture and Television Engineers) time code. For instance, to synchronize two analog 24-track tape machines, you would stripe both tapes with SMPTE (that is, record time code onto a track) and connect the machines using an analog tape synchronizer.
The synchronizer listens to SMPTE from both machines and directly controls their motors, and also provides simple transport. When you press Play on the master machine, the synchronizer shuttles and varispeeds the tapes until both machines are playing back from the same point in the music: at the beginning of the second chorus, for example. Once this happens (and it can sometimes take a while), start sync is achieved.
If that were all that the synchronizer did, the tapes would start together but then gradually drift apart. This is because the machines'' motors run at slightly different speeds, even though both are ostensibly moving the tape at 30 inches per second. To solve this problem, the synchronizer listens to the two SMPTE streams throughout playback, adjusting the tape speed as necessary so that the machines keep playing together.
Synching an analog system to a digital system, either tape or a DAW, is conceptually the same as synching two analog machines. The two systems need to start at the same point, and they must agree on the playback speed.
FIG. 1: When synching a computer-based digital audio system to an analog tape deck, a synchronizer translates SMPTE from the analog deck into word clock for the computer hardware.
As with analog machines, time code, such as SMPTE or MIDI Time Code (MTC), is used for communicating the start point. Continuous sync, however, is achieved differently, because DAWs have no tape motors to control. Setting the playback speed must be accomplished with word clock.
A digital audio recording consists of a long stream of individual samples. You might picture each sample occupying a very small area on the surface of an analog tape. Just as analog tape moves across the tape heads at a constant speed (such as 30 inches per second), the samples within a digital audio recording flow by at a specific sample rate (such as 44.1 or 48 kHz). The word clock controls this flow by setting the precise sample rate of the digital system. Each time the clock “ticks,” the system sends or receives another sample. If the device''s sample rate is 48 kHz, for instance, the clock will tick once each 48,000th of a second. The word clock thereby controls the “tape speed” of a digital system. Even on common digital tape systems, in fact, the word clock is what controls the speed of the physical tape transport; the tape speed is adjusted to match the word clock.
Word clock can be generated internally or received from an external source. Most digital audio connection formats, including S/PDIF, TDIF, AES/EBU, and ADAT Optical, include the word clock signal. Word clock can also be transmitted separately, without audio data. To sync a digital system to an analog one, you need to translate the analog tape speed into the digital word clock. A number of synchronizers (such as the Mark of the Unicorn MIDI Timepiece AV, Opcode Studio 64 XTC, and Digidesign Universal Slave Driver) do exactly that: they read the SMPTE or MTC signal from the analog tape and use the rate of the time code to generate a word clock signal (see Fig. 1).
For instance, if SMPTE is running at 30 frames per second, and the audio sample rate is 48 kHz, each SMPTE frame corresponds to 1,600 ticks of the word clock. Synchronization ensures that when the SMPTE from the analog tape runs faster or slower, the word clock, and thus the “tape speed” of the digital audio system, speeds up or slows down accordingly. Some digital audio software and hardware does this internally, without changing the audio hardware''s word clock rate. Instead, they measure the incoming SMPTE or MTC stream, varispeed the audio in software, and then send out the processed audio at a constant sample rate.
Digital to Digital
As with the all-analog and hybrid analog/digital cases above, all-digital setups also use time code for start sync. Depending on the equipment, they may use the familiar MTC or SMPTE time codes or new proprietary, sample-accurate time codes (as discussed later).
Continuous sync, happily, is much easier to achieve with all-digital setups. You no longer need to directly control physical tape speeds or varispeed word clock based on SMPTE or MTC. Instead, you connect the word clock ports of all the devices, so that the units play and record at exactly the same sample rate; thus, all will have the same “tape speed.”
FIG. 2: Synchronizers can translate an MDM''s sample-accurate time code into standard MTC. In this case, the time code and word clock from the ADAT follow separate paths.
You might think that, with the precision of digital equipment, 48 kHz would be the same from one machine to another, making word clock connections unnecessary. Unfortunately, this isn''t the case. Just as analog tape decks differ on what 30 inches per second means, so digital devices interpret sample rates differently. With two machines set to 48 kHz, for example, one might really run at 47.998 kHz, while the other might run at 48.001 kHz.
The solution to this dilemma is a master-slave setup similar in concept to analog time-code configurations. The master sets the sample rate, and the slave(s) ignore their own internal clocks, using the master''s clock instead. A proper word clock arrangement will ensure perfect, driftless, continuous sync between all connected devices.
You can think of each device''s word clock as a mechanical gear, with every tooth in the gear representing a single sample. Spinning by themselves, the gears can move at any rate they like. But when two or more gears are fitted together, they move in lock-step precision. Every time one gear advances by a single tooth, the others turn exactly one tooth as well—no more, no less.
When the word clocks of digital audio devices are synchronized, they act in the same way. Each time the master plays a single sample, the slaves play one as well. If two devices are playing the same digital audio file, they will take exactly the same amount of time to play that file.
It''s important to note that word clock synchronization is critical not only for achieving continuous sync but also for transmitting digital audio data accurately under any circumstances. Without proper word clock sync, you''ll be plagued by audio artifacts, including pops, clicks, and distortion.
As a side note, standard word clock signals tick once per sample. In contrast, Digidesign''s systems use a “superclock,” which ticks 256 times per sample. The two clocks are not directly compatible, although some synchronizers offer both options. Some may even offer a single set of word clock I/O that is switchable between the two, so make sure that you''ve selected the correct option for your gear.
Setting Up Word Clock
Setting up a studio''s word clock synchronization is usually done in two steps. First, the word clocks must be physically connected. You can do this by making a digital audio connection from the master to the slave, such as S/PDIF out to S/PDIF in, ADAT optical out to ADAT optical in, and so on.
Sometimes, you''ll also need to use dedicated word clock cables, such as with complex setups or with devices that have word clock I/O but not digital audio I/O. With larger systems, you can also mix and match formats as required by the I/O available on each device. The second step is configuring the slave devices to use the word clock from the master device. In digital audio software/hardware combinations, this is usually referred to as something like “sync source” or “audio clock source.” On hardware devices, there may be an obvious front-panel selection (such as Clock Source on the ADAT XT), or a hidden key combination (as with original ADATs). Some older or simpler devices, such as DATs, may simply switch automatically as digital I/O is enabled or disabled, which can limit your options. Fortunately, most new devices are more flexible.
If your digital audio software supports software-based continuous sync, disable this feature; the word clock connection will take care of this via your hardware. In some cases, in fact, software-based continuous sync can interfere with hardware-based sync, causing audio artifacts or even total sync failure. Names for this parameter vary among software: in BIAS Deck II, for instance, you should enable Trigger Sync, while in Cakewalk Pro Audio, you should set the Audio Options>Advanced SMPTE/MTC Sync to Freewheel.
The Final Frontier
With a proper word clock setup in an all-digital system, it''s easy to achieve perfect, sample-accurate continuous synchronization. The accuracy of the start point, on the other hand, depends upon the type of time code being used.
FIG. 3: With hardware and software that directly support sample-accurate time code, you can connect the MDM directly to your computer hardware.
MTC, for instance, offers resolution of a quarter frame at 30 frames per second, or a 120th of a second—equivalent to about 400 samples per second at 48 kHz. SMPTE may offer greater resolution in some cases; however, if you''re using a digital audio program, chances are that SMPTE must be converted to MTC before reaching the software, which means that you''re still operating at MTC resolution.
ADAT and DA-88 time codes, in contrast, are based on the word clock. Each tick of the clock is also a tick of the time code, in a convergence of time code and “tape speed,” which brings us full circle from the dual-function SMPTE of all-analog sync (see Fig. 2). Both systems offer single-sample accuracy for start sync, with a resolution of a 48,000th of a second at 48 kHz; hence the term sample-accurate sync.
Several combinations of computer-based audio software and hardware also offer direct support for sample-accurate time code (see Fig. 3). Due to the slight indeterminacy of computer operating systems, the sync may be perfect or it may vary slightly by a sample or so. Even with a bit of slack, however, this is much more accurate than MTC.
Resolution and Results
So, what is the advantage of that increased start-point accuracy? The most compelling use is in computer editing of audio tracks from a digital tape recorder. With sample-accurate sync, the audio can be transferred into the computer and then laid back to tape with extreme precision.
To test the difference for yourself, record a track from an MDM into digital audio software, first using sample-accurate sync and then using MTC. Simultaneously play back both the computer and MDM tracks, again using sample-accurate sync for playback of the first and MTC for playback of the second, and compare the two. You''ll notice that the sample-accurate tracks sound louder, with little or no phasing, while the MTC tracks sound muted and noticeably phased.
Similarly, with MTC, tracks transferred in different record passes may have noticeable differences in phase. For best results when transferring phase-correlated audio (such as a drum kit recorded with multiple mics), it''s best to transfer all tracks in a single pass. Sample-accurate time code has certainly upped the ante in sample resolution. Remember, though, that almost every CD you own was recorded using SMPTE/MTC for synchronization (if the project used sync at all). They sound just fine, don''t they? So, if SMPTE/MTC sync is all you have available, don''t be too concerned.
Also, remember that regardless of start-point resolution, a proper word clock setup always ensures drift-free continuous sync. So, keep your eye on those word clocks, and be safe synchers.
Dan Phillips is a singer, songwriter, and producer and is part of the team at Korg Research and Development.