HD24/96 Technical Reference

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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 sequencer is to make the HDR24/96 the time code master and the sequencer the slave, as illustrated below.

Sequencer

Time Code

Time Code Master

Time Code Slave

More about Time Code

Just as the Sample Clock runs at a specific rate called the Sample Rate, the time code signal runs at a specific rate called the Time Code Frame Rate. Unlike a standard clock which displays time in hours, minutes, and seconds, time code further divides a second into frames. A display of a 30fps (frames per second) time code time might look like this:

02:18:57:28

This time code value reads 2 hours, 18 minutes, 57 seconds, and 28 frames. There are four standard Time Code Frame Rates, each of which has a specific application. These four rates are:

24 fps

Used for theatrical film applications worldwide.

25 fps

Used for music, broadcast and non-broadcast PAL and SECAM

 

video, and non-theatrical (broadcast) film applications in countries

 

which use 50 Hz power.

29.97 fps

Used for non-broadcast NTSC video and theatrical film post

 

production applications in the US, Canada and Japan.

30 fps

Used primarily for music applications in countries which use 60 Hz

 

power (primarily the US, Canada, and Japan).

All of this is very straightforward except for the case of 29.97 fps. Why do we have a 29.97 fps Time Code Frame Rate when all the other frame rates are nice whole numbers? Well, long story. Back before color television was around in the U.S., black and white TV ran at

30 frames/sec. In order to get the extra color information into the picture, the frame rate was slowed down to 29.97 frames/sec. 29.97 fps time code was invented to run in perfect sync to the NTSC color video so that each video frame could have a unique time code address.