MOTU Digital Timepiece User's Guide download pdf (Page 10)

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DEGREES OF ACCURACY

with the master digital clock during playback or

recording, sample for sample, to prevent phasing

(which causes distortion) or drifting (which causes

sync problems). But the highest resolution at which

Pro Tools can locate — and begin playing or

recording — is one 120th of a second (quarter-

frame resolution). If you transferred a stereo track

pair in two separate record passes into Pro Tools,

the stereo image would not be transferred in

perfect phase. In other words, Pro Tools cannot

start at exactly the same sample as other digital

audio devices, like ADATs can. (Future versions of

Pro Tools may provide sample accurate synchroni-

zation capability.)

FRAME-ACCURATE SYNC

When two devices achieve frame-accurate sync

without phase lock, master and slave remain in

sync with each other, but their digital audio clocks

are not kept in phase. Instead, they stay in

continuous sync via time code, which has a

resolution of a thirtieth of a second (or one of the

other standard SMPTE time code frame rates).

This form of synchronization inevitably causes two

digital audio devices to phase with one another as

they play, since the timing reference (30 frame per

second time code) has such a lower resolution than

their internal sample clocks.

An example of this type of sync would be a stand-

alone hard disk recorder slaved to the Digital

Timepiece via SMPTE time code only, with no

word clock connection between the devices. The

hard disk recorder would read the incoming time

code and continually adjust its digital audio output

to stay in sync with the time code.

FRAME-ACCURATE TRIGGERING

With frame-accurate triggering, unlike any of the

continuous forms of sync already discussed, the

master device only tells the slave device where to

locate (at a speciﬁc time code location). But when

the slave begins playing or recording, it runs under

its own internal clock, inevitably drifting out of

sync with the master, given enough time. The time

it takes for drift to become noticeable depends on

the devices involved and the situation in which

they are being used. Timing resolution is

equivalent to frame rate being used (e.g. 30 frames

per second).

Most devices today use one of the continuous

forms of sync described earlier. You probably won’t

encounter a device of this type in your work with

the Digital Timepiece.

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MOTU Digital Timepiece User's Guide Page 10

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