When two or more devices are intended to record or playback in sync, it is important to ensure that their word clock sources are locked together to prevent drift between the devices. For digital audio devices, audio sampling rate determines speed. For picture equipment, it is frame rate. Failure to lock audio devices can result in poor audio quality and/or lack of synchronization.
The 788T can slave to external sync sources or act as provider of master sync source for other devices. As a master device, the 788T outputs a rock stable, ultra-precise word clock (running at a rate equal to the sample rate) via BNC. Connecting this to a compatible slave device will ensure synchronous operation.
In slave mode, the 788T's internal sample clock circuitry has been designed to enable it to lock to a wide variety of synchronization reference sources including word clock, video sync and digital audio as well as its own internal crystal reference. Once locked, the 788T's digital audio processing and time code generating circuits are tightly synchronized to whatever source has been selected.
In the 788T, the sync reference is set in the [Rec: Sync Reference] menu. Lets have a closer look at the various options.
With this option selected, the 788T sample rate is derived from a highly stable, temperature compensated internal crystal circuit. Using an Ambient Clockit, it is also possible to tune this to within 0.2 ppm of an external source−significantly reducing the likelihood of drift.
Select Internal when running the 788T as a master clock source or when it is being used in isolation or when a suitable sync reference is not accessible. When set to Internal, the sample rate is determined by the setting in the [Rec: Sample Rate] menu.
Word Clock, W = Locked
Word clock is typically output by professional audio equipment or house clock generators. When set to lock to word clock, the 788T's sample rate setting is ignored since the sample rate is wholly determined by the incoming word clock rate.
Connect the word clock output from the external device to the sync input of the 788T. Once locked, the 788T LCD display shows the sample rate of the word clock source and the lock indication, W. Should the word clock source be disconnected, the 788T automatically reverts to its internal reference, but to make it clear that a sync problem has arisen, the front panel ring LED's flash yellow.
Video Sync, V = Locked
Video sync comes in many flavors – NTSC and PAL composite for Standard Definition video and a range of Tri-Level formats for High Definition. When set to video sync, the sample rate is determined by both the 788T's sample rate setting and the incoming video sync rate. Think of the sample rate setting as a coarse adjustment and the incoming video sync as a real time fine adjustment of sample rate.
Connect the video sync output from the external device to the sync input of the 788T. The 788T automatically detects any type of video sync and 'V' is displayed once lock is achieved. Should the video sync source be disconnected, the 788T automatically reverts to its internal reference, but to make it clear that a sync problem has arisen, the front panel ring LED's flash yellow.
Digital Inputs 1/2, 3/4, 5/6, 7/8; D = Locked
The 788T can lock to both Consumer and Professional digital audio formats, balanced and unbalanced. When set to lock to one of the digital input pairs, the 788T's sample rate setting is ignored - the 788T extracts the sample clock reference that is embedded within the digital audio stream.
Connect the digital audio output from the external device to the digital input of the 788T. Once locked, the 788T LCD display shows the sample rate of the digital audio source and the lock indication, D. Should the source be disconnected, the 788T automatically reverts to its Internal reference, but to make it clear that a sync problem has arisen, the front panel ring LED's flash yellow.
The 788T includes one other sync reference setting - Automatic. In this mode, the 788T automatically detects the type of sync source connected and locks to it. The 788T prioritizes one sync source type over another according to the following, in order from highest to lowest priority:
- Sync input (word, video)
- Digital inputs 1/2, 3/4, 5/6, 7/8
When set to Automatic mode, pay extra attention to the lock indication (D, W, V) in the LCD display. Should an external sync source become disconnected, the 788T will revert to internal but the ring LEDs will not flash since the 788T assumes Internal is a valid source.