HD24/96 Technical Reference

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before you can actually see a discrepancy between their time displays. This rate of sample clock drift is actually quite normal. Understanding the simple fact of clock drift puts you way ahead of most people in understanding how synchronization works.

Going back to the watch analogy, let’s suppose we were able to get two wrist watches to run at exactly the same speed. Then if the watches started running at exactly the same instant with the hours, minutes, and seconds hands perfectly lined up, we know that they would always read exactly the same time, forever and ever. Just like the watches, to synchronize two or more digital audio recorders to synchronize to each other, two things have to happen:

1.The Sample Clocks of all the devices have to run at exactly the same speed (i.e. Sample Rate), and

2.The transports need to start at exactly the same position at exactly the same time.

The first requirement is satisfied by distributing a common Sample Clock signal to every device. One device is configured as the Master and generates the Sample Clock, and all the others are configured as Slaves that lock their Sample Clocks to the Master. Word Clock is a standard Sample Clock signal format that many devices support, including the HDR24/96. Word Clock runs at a frequency exactly equal to the Sample Rate, and is transmitted using 75-ohm coaxial cable.

Several other clock signal formats exist, including AES Null clock and oversampled word clock (also called superclock), which runs at 128, 256, and 384 times the Sample Rate. Although the rest of our discussion about clocks will be limited to Word Clock, (the HDR24/94 supports Word Clock only), the principles apply to all of these clock formats.

The second requirement is satisfied by distributing a common time code signal to every device. The time code signal carries coded information about what time it is at any particular instant. One device is configured as the Master and generates time code, and all the others are configured as Slaves that lock their transport positions to time code from the Master.

There are several common time code signal formats, including LTC (longitudinal time code, sometimes called SMPTE LTC, or just SMPTE), MTC (MIDI time code), and VITC (vertical interval time code). The HDR24/96 supports both LTC and MTC, but not VITC. LTC is an audio-frequency signal and may be carried over standard audio cabling, routed through your console, and recorded just like any analog audio signal. MTC, unlike LTC, is not an audio- frequency signal and cannot be directly recorded; it’s transmitted exclusively over MIDI cables through MIDI devices. VITC is a time code signal that is embedded into a composite video signal, and must be encoded and decoded by special video signal processing devices.

The correct way to synchronize two HDR24/96 units together is shown below:

Time Code

Word Clock

Master HDR24/96

Slave HDR24/96

The master machine on the left generates both word clock and time code, and the slave machine on the right locks to word clock and time code from the master. As long as the