Video synchronization, Hdr 24/96 – MACKIE HDR24/96 User Manual
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HDR 24/96
fps, it still must count to 30, which actually takes longer than a second to accomplish. For this reason
it gradually gets behind in the count relative to the passage of “real” time. To correct this problem,
drop-frame code was invented. Drop-frame code periodically “skips ahead” a bit to make up for lost
time. The end result is that at the end of an hour, the slow counter has skipped ahead by 00:00:03:18
so that the clock reads 1:00:00 after an hour has passed. Drop-frame works by skipping ahead (in
count) by two frames every minute, except for the 10 minute, for a total of 108 frames per hour.
th
There are two drop-frame codes that are used in the industry:
29.97 fps Drop
Used for broadcast NTSC video applications in the US, Canada and Japan.
Drop-frame time code is typically only used in broadcast applications where program scheduling is
ruled by wall clock time (after all, you can’t broadcast the 12:00 news at 12:30). The great news is
that unless you are a broadcaster or have to deliver something that will be broadcast on air, you don’t
need to worry about drop-frame time code either.
In practice there are actually four different speeds (rates) at which time code runs (24, 25, 29.97, and
30 fps), and four different ways that frames are counted (24, 25, 30, and 30 Drop). When you shake
them all together and spill them on the table you get the six Time Code Frame Rates we described
above. Whereas almost all equipment supports 29.97 Drop, not all equipment supports 30 Drop. The
HDR24/96 supports all six standard fame rates, but don’t use 30 Drop unless you need to do so in
order to be compatible with something else. 30 Drop is essentially a corrective time code to
accommodate existing problems.
An important fact to know is that different manufacturers sometimes use different nomenclature for
the same Time Code Frame Rates, primarily because of a failure to differentiate between the frame
rate (how fast the code is running) and the frame count (how many frames get counted for each
second that is counted). This can be very confusing. The discrepancies always surround how 29.97
and 30 Drop and non-drop are represented. For example, one manufacturer may support 30, 30 drop,
and 29.97 (which in actuality is 30, 29.97 drop, and 29.97). If you see only 5 of the six rates
supported, you can bet that 30 Drop (as we have described it above) is not supported.
As a final note, the MTC specification defines only 24, 25, 30, and 30 Drop-frame rates (based on
frame count). However, 29.97 and 29.97 Drop MTC can be generated simply by slowing down the
rate of transmission slightly. Many devices, like the HDR24/96 do this. But because the Time Code
Frame Rate is actually coded into the MTC data, an MTC reader (as might be found on a sequencer)
might display that it is receiving 30 or 30 Drop-frame time code when in actuality it is receiving
29.97 or 29.97 Drop-frame. The only way to know what is actually happening is to know how your
equipment specifically deals with these situations.
Video Synchronization
Because analog systems do not have sample clocks, video is commonly used to synchronize the
transport speed of multiple analog systems, most notably video tape machines and synchronizers. In
a typical setup, a device called the “master house sync generator” generates a highly accurate and
stable blackburst video signal, which is distributed throughout the facility to every device that needs
it. Blackburst is simply a normal picture-carrying video signal (composite video) that contains
“black” as its picture. When a device is locked to house sync it is said to be “genlocked”.
Because analog systems do not have sample clocks, the only way to synchronize a digital audio
device like the HDR24/96 to an analog device like a VTR is through video. Many digital systems,
like the HDR24/96, support the ability to synchronize their sample clock to blackburst or composite
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