+33 4 7642 9550 www.aaton.com Cantar-X User Manual v2.26 (r13) 2009 April 27
p.15
p.15
Sampling Rate, Camera fps, Audio-TC
These parameters are not correlated. The number of audio
samples per Earth second (Hertz), is called the sampling
rate, e.g. 48000 is 48kHz. The camera 'frames per
second' value should have no influence at all on the audio
sample rate frequency; unfortunately, some vintage NTSC
post-machines still impose the use of the disgraceful 48048
or 47952 sample rates to sync real-time audio with drifting-
time images. The frame-rate used to express the audio time
can be 30, while images are filmed at 24 or 25fps.
On the planet Earth
Timecode handling is simple at integer speeds such as 24,
25 and 30fps; at 29.97DF (Drop-Frame) the frame count
jumps over two images every minute except for every tenth
minute to keep the count in line with Earth's time. Stored in
the file metadata, the camera fps is a simple reminder and
can be changed later and then applied to the audio LTC
used in some audio-post sync operations.
'U' Universal : 24.00, 25.00, 30.00, 29.97DF
In a file digitized at 48kHz, the Format stamp indicates
the number of samples per real-second, i.e. 48000; the
Time-stamp carries the number of samples since midnight
using the sample rate value found in the Format stamp (the
same logic is used in the Barebone mode-B for 'slow-planet'
recordings).
On the 'slow-planet'
Unfortunately, NTSC-compatible 23.976fps HD cameras
cannot use the anti-drift drop frame system because nobody
found a way to emulate the counting drops invented for
29.97fps. In this environment the 'slow-second' (the time it
takes to shoot 30 frames of NTSC video, i.e. 1.001 Earth
second) imposes its rule.
As soon as you enter Camera fps = 23.98NDF or 29.97NDF,
you are on the 'slow-second' planet, with its lazzy 24 (aka
23.98fps on Earth) or lazzy 30 (aka 29.97fps on Earth).
The Cantar's TCXO clock switches to a slower beat and TC
separators change from ':' to '
*
', e.g.12
*
45
*
36.
Many nights of engineers' time have been spent trying to
find a universal method to sync sound and images on the
slow-second planet but nothing came out. In the U.S. you
must ask your post-facility which one of these three sample-
stamp modes is requested:
'A' Avid v11: '23.98NDF–A', '29.97NDF–A'
In a file digitized at 48kHz, the 'A' Format stamp indicates
the number of samples digitized in a slow-second, i.e.
48048; the Time-stamp is the number of samples since mid-
night, using the 48048 value of the Format stamp.
Introduced for the AvidMC v11.3.2, the 'A' mode is fading
away. See 'Avid-Recording-Rates' in Aaton tutorials h t t p : //
www.aaton.com/documentation/
'B' Barebone: '23.98NDF–B', '29.97NDF–B'
In a file digitized at 48kHz, the 'B' Format stamp indicates
the number of samples digitized in a real-second, i.e.
48000; the Time-stamp is the number of samples since mid-
night using the 48000 value of the Format stamp.
While images and timecode are beating the slow-second,
the audio remains on Earth! Since modern post-machines
(e.g. Indaw) perform high quality real-time sample rate con-
version, 'Barebone' and 48kHz should have been THE only
choice but unfortunately some other manuafacturers decided
differently (see below).
'C' Compensated (F) Faux: '23.98NDF–C', '29.97NDF–C'
The 'C' mode is made for challenged post-machines. In a
file digitized at 48kHz, the Format stamp value indicates the
real number of samples per real-second, i.e. 48000, and
the Time stamp is the number of samples since midnight as
if the digitization was done at 48048!
Hence the 'F' name (for Fake or Faux) given by Fostex and
FCP to this mode. This is the mode selected by most US
productions.
If it is 02h00m00s on Earth at the start of take, the Time-
stamp value is 345,945,600 samples (02h x 3600s x
48048).
Summary
Cantar at 23.976NDF or 29.97NDF
'A' mode. Recording: 48000 -Sample Rate written in the
files: 48048 -Timecode is stamped using: 48048.
'B' mode. Recording: 48000 -Sample Rate written in the
files: 48000 -Timecode is stamped using: 48000.
'C' mode. Recording: 48000 - Sample Rate written in the
files: 48000 -Timecode is stamped using 48048.
STOP 5. SPLITTING THE SECOND
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