MACKIE HDR24/96 - Manual (Page 193)

HDR 24/96
Jam Sync
Jam Sync is the method used in the HDR24/96, as well as every digital tape machine and most
digital audio workstations. There are two variations of Jam Sync: Jam Once, and Jam
Continuous. In both methodologies, once lock is acquired the transport position of the device is
completely governed by the playback speed (i.e. Sample Rate) of the device without further
reference to external time code. If drift occurs between the time code source and the slave,
nothing is done to correct the drift. In other words, once the transport is locked and the position
is set it, external time code is essentially ignored (“set it and forget it”). Jam Sync assumes that
the clocks of all concerned devices are synchronized to the to the same clock source.
In Jam Continuous operation, the slave monitors the incoming time code signal for unexpected
events (dropouts, garbled frames, abrupt changes in position). If nothing unexpected happens, the
slave transport continues to play, even if drift accumulates. If time code stops or jumps suddenly
to a new position, the slave either drops lock and stops or re-locks to the new position.
Temporary anomalies are always overridden. This is the normal mode of time code sync
operation for the HDR24/96.
In Jam Once operation, once the slave is locked, external time code is completely ignored and the
transport continues to run until stopped by the user. Jam Once operation is most useful when
recording or playback disruptions due to faulty time code cannot be tolerated (for example during
live remote recording). To put the HDR24/96 in Jam Once operation, disable TC Chase after
lock is acquired.
Trigger Sync
Trigger Sync is a method used by some non-linear digital audio devices (digital audio sequencers,
workstations and disk recorders) and all non-audio (non-DAS) MIDI sequencers. Trigger Sync
by its nature is never used on linear digital audio devices (DATs, MDMs). In Trigger Sync
operation, the start or playback of every event on the slave is triggered based on incoming time
code time rather than the slave unit’s internal time.
In a non-linear digital audio device, the event being triggered is actually the playback of recorded
audio. Unlike Jam Sync where the playback position of the master and slave are aligned only
once at lock, in Trigger Sync operation the slave unit’s position is continually realigned to the
position of the master, and audio region playback always starts in sync with the master.
However, once region playback starts, playback continues uninterrupted, even if the slave unit’s
position is readjusted due to drift with respect to the master.
Trigger Sync operation can compensate for not having sample clock synchronization in projects
containing audio regions shorter than 30 to 40s. In projects containing longer regions, the
possibility exists that regions on the same unit can get out of sync with each other when slaved to
time code, but play back in perfect sync offline, leaving you and your equally-bewildered support
technician scratching your heads.
In a MIDI sequencer, the event being triggered is the transmission of one or more MIDI
commands. Because MIDI commands are merely small data packets, there is no requirement for
sample clock synchronization. MIDI data can be transmitted as soon as an external device tells it
that it is time to go. If the time code source speeds up or slows down, the MIDI sequencer
follows it. As long as the sequencer is a time code slave, there is no possibility for drift to occur.
Because of the way Trigger Sync works in MIDI sequencers, it is never possible to properly
synchronize a Jam Sync-based digital audio device like the HDR24/96 by making the HDR24/96
the time code slave. This is because the clock that generates MTC or LTC time code from the
sequencer is derived from a hardware timer on the computer that cannot be synchronized to an
external source. The correct way to make the HDR24/96 synchronize to a computer-based MIDI
Technical Reference 193