Specifications

Chapter 9: Time Code and Synchronization Features 57
Audio Pull Down Factors and QuickTime Movies
Audio “pulls” are an industry standard method
of adjusting the speed of audio playback to line
up with off-speed picture.
In normal industry standard workflows, when
audio is pulled down, video and time code
maintain their current speed, and only the au-
dio and MIDI are pulled down, in relationship
to the video and time code.
For example, when 24 frame film to 29.97 NTSC
video transfers are made, it is necessary to slow
the film playback speed during the transfer pro-
cess by 0.1%.
In order to make the same speed change for pro-
duction audio to match the speed of this new
slower picture, Pro Tools audio needs to also be
“pulled down” by 0.1%.
To slow audio in Pro Tools, the audio can either
be sample rate converted on import or the in-
coming word clock sample rate source must be
“pulled down” by 0.1% (resulting in a 48,000 Hz
session speed to be pulled down to play in real
time at 47,952 Hz). The pull down setting in
Pro Tools adjusts the visual representation of re-
gions on the timeline to equal the word clock
pull up/down change.
When the clock is reset to the non-pulled down
sample rate, the audio plays at the original film
speed.
Pro Tools offers limited pull capabilities, primar-
ily for Pro Tools HD users who have a secondary
Pro Tools system that they use to perform
smaller tasks or for remote work.
Although Pro Tools offers some pull down op-
tions, it does not have the ability to adjust the
internal sample clock the same way
Pro Tools|HD and Pro Tools|HD Native systems
do in tandem with a SYNC peripheral (such as
SYNC HD).
Since Pro Tools does not have any means to ad-
just the Pro Tools sample clock, it instead simu-
lates the audio/video pull relationship by pull-
ing up the video instead of pulling down the
audio. This maintains the proper video/audio
speed relationship when using the fixed
Pro Tools internal clock. The audio sample clock
is not pulled down, so the digital outputs are not
outputting a pulled down sample clock.
If you are using an external clocking source
(such as S/PDIF or ADAT Optical), Pro Tools au-
dio and MIDI are resolved to the digital input’s
clock speed. If you select audio pull down, the
audio still doesn’t pull down in Pro Tools LE;
the video still pulls up to maintain the proper re-
lationship, and the actual Pro Tools sample rate
is now dependent on the digital clock source.
However, if the incoming digital clock speed is
pulled down, Pro Tools audio and MIDI will run
at a pulled down rate, and the video will play
back at its normal speed.
When pull down is selected in Pro Tools, the fol-
lowing occurs:
• If the external clock is not pulled down, the
actual sample rate of Pro Tools audio and
MIDI is not pulled down, but the video is
pulled up.
• If the external clock is pulled down, the ac-
tual sample rate of Pro Tools audio and
MIDI is pulled down, and the video plays at
its original non-pulled speed.
MIDI is always pulled in line with audio
pulls.