MACKIE HDR24/96 - Manual (Page 196)

HDR 24/96
video signals. Unlike word clock however, video is not a “clock” signal at all. In fact, video signals
run much slower than sample clocks do. The three standard Video Field Rates in use today are:
50 Hz PAL and SECAM video.
59.94 Hz NTSC color video.
60 Hz NTSC black & white video and some HDTV.
Because there are two video fields for every video frame, the video frame rates correspond exactly to
25 Hz, 29.97 Hz, and 30 Hz, which are also standard Time Code Frame Rates. How convenient! In
order for a digital system to synchronize to video, it must “synthesize” its sample clock from the
much slower video signal. The relationship between the video field rate and the sample rate of the
box is determined by a simple mathematical relationship. For example, if you have a 50 Hz video
signal and want the device to run at a 48 kHz sample rate, then the relationship is determined by the
following formula:
(48,000 samples/second) / (50 frames/second) = 960 samples/frame
This relationship says that for every video frame exactly 960 samples are clocked. In the HDR24/96,
this is governed by the Sample Rate and Video Field Rate parameter settings. If the video signal
speeds up or slows down, the sample rate speeds up or slows down in the same proportion.
When a sample clock is locked to a video signal in this relationship, the sample clock is said to be
“resolved” to the video signal. In the HDR24/96, always set the Video Field Rate parameter to
correspond to the actual field rate of the video you are resolving to. If you don’t do this, then the
actual sample rate will be different from the Sample Rate setting. For example if the Video Field
Rate setting is 60 Hz, and the Sample Rate setting is 48 kHz, feeding a 59.94 Hz video signal into the
HDR24/96 will cause the sample clock to run at [(48,000 samples/sec)/(60 frames/sec)](59.94
frames/sec) = 47,952 Hz.
An example of how two HDRs can be synchronized to a VTR using video is shown below:
Video / Word Clock
Generator
Video Slave
Video Slave Video Slave
Blackburst Blackburst
Blackburst
VTR
HDR 24/96 196