MACKIE HDR24/96 - Manual (Page 203)

HDR 24/96
In practice, the most important thing to know about Sample Rate pull-ups and downs in digital audio
systems is that to maintain synchronization the Time Code Frame Rate must also be pulled up/down.
From our earlier discussion the reason why this is so should now be apparent. In a digital audio
system, sample clock time and time code time are really one and the same thing, simply expressed in
different units.
Because of the relationship established by the Sample Rate and Time Code Frame Rate settings, when
the Sample clock speeds up or slows down, the Time Code Frame Rate must also speed up or slow
down in proportion. This is no different than slowing down an analog multitrack tape that has time
code on it - as the tape speeds up or slows down, the time code frame rate speeds up or down along
with it.
Unlike a SMPTE-striped multitrack tape where the position and rate of the time code with respect to
the audio cannot change, digital systems can be set up to arbitrarily change this relationship at the
touch of a button. To use our previous example, if the HDR24/96 is set up to run at 48 kHz and 30
fps with the Sample Clock set to Internal, to apply an NTSC pull down set the Sample Rate to 47.952
kHz and the Time Code Frame Rate to 29.97 fps (48,000 / 30) = (47,952 / 29.97)
Below is a table that shows all of the valid combinations of Sample Rate and Time Code Frame Rate
for 48 kHz NTSC pull-ups and pull downs. Although not all of these combinations are useful, the
table illustrates whether a setting can be pulled up or down and the result of doing so. Similar charts
could be made for the 44.1 kHz Sample Rates as well as for 29.97/30 Drop-frame rates.
Sample
Rate
Time Code
Frame Rate
Pull-Up
Allowed
Pull-Down
Allowed
Result
29.97 fps No No
48.048 kHz
30 fps No Yes 48 kHz, 29.97 fps
29.97 fps Yes No 48.048 kHz, 30 fps
48 kHz
30 fps No Yes 47.952 kHz , 29.97 fps
29.97 fps Yes No 48 kHz, 30 fps
47.952 kHz
30 fps No No
Doing pull-ups and pull-downs with an external Sample Clock source is slightly trickier and requires
extra care in the setup process. Because word clock controls the actual rate at which the Sample
Clock runs, you can pull the HDR24/96 up or down without changing the Sample Rate and Time
Code Frame Rate settings by sending the HDR24/96 a pulled-up or pulled down word clock.
For example, sending a 47.952 kHz word clock to an HDR24/96 set to 48 kHz and 30 fps has the
same net effect as running on an Internal Sample Clock when set to 47.952 kHz and 29.97 fps. You
would actually get identical performance by sending 59.94 Hz video to an HDR24/96 set to a 60 Hz
Video Field Rate and 48 kHz at 30 fps.
The main reason that pull-ups and pull-downs cause so much confusion is that there is little
consistency way among manufacturers, or even among different products from the same manufacturer
for handling pull-ups and downs. These confusing and often contradictory methodologies in turn
make it difficult to understand how the equipment works, contributing further towards improper
equipment setup.
The HDR24/96 has been designed so there is never confusion as to what to expect based on how it is
set up. The key to setting up the HDR24/96 synchronization parameters is to remember that you
always get exactly what you specify out of the box as long as you put exactly what you specify into
Technical Reference 203