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Pullups/Pulldowns

Probably no aspect of synchronization causes more confusion than pull-ups and pull-downs. Simply put, a pull-up or pull-down is the raising or lowering of the speed of a device above or below the “standard” speed.

The only reason that pull-ups and downs exist is due to the Telecine process by which film is transferred to video.

Worldwide, films that are destined for theatrical release are shot on film that runs at 24 fps.  In the US, after a film is shot, it is typically transferred to NTSC color video and edited on a non-linear video editing system.  Once the film editing is complete, video work prints are sent out to the various parties involved in audio post production for the film.

Because NTSC color video runs at 29.97 fps and film runs at 24 fps, there is a non-integer ratio between the number of video and film frames.  To accomplish the transfer, the film is slowed down by .1% to 23.976 fps to establish an integer relationship of 5 video frames to every 4 film frames.  This slowing down of the film IS the pull-down.  The film and video frames are then transferred by alternating between using three and two video fields (not frames) per film frame in a process called 2:3 pull-down, as shown in the illustration below.

So what does this have to do with digital audio?  Well, audio recorded on location is synchronized to the camera that is rolling at 24 fps.  When the location recordings are brought in to the audio post production process, they need to be pulled-down by the film pull-down ratio to remain in sync with the picture (now in NTSC color video).  While this 0.1% pull-down does not perceptively (to most listeners) change the pitch of the sound, a 0.1% mismatch in rates will cause sync drift fairly quickly.  Therefore many digital audio devices support the ability to run at a pulled-down sample rate.

A similar Telecine pull-down process is sometimes used for 50 Hz video (PAL and SECAM) formats, but because of the existing integer relationship between video and film frames, the Telecine process is most often done as a real-time interleaving process (i.e.  no pull down is involved).  Thus PAL/SECAM pull-ups and downs are much less used than NTSC pull-ups and downs.

Occasionally location audio is pulled up so that after the Telecine it runs at a standard Sample Rate.  For example, when a music video is made, sometimes the director will have the artists lip-sync to the original song playing back at a pulled-up rate, so that when the film is transferred to video for television broadcast, the music will get pulled down to the original sample rate at which it was recorded.

Fortunately the need to use pull-ups or pull-downs only arises when working with location recordings for theatrical film, when synchronizing to certain digital video machines (that support digital audio), or when otherwise directed to do so by the technical director of a film for purposes of standardizing the post production process.  Most of you can breathe a sigh of relief.  For those of you who must use pull-ups and pull-downs, here is how they work.

For each standard Sample Rate, a pull-up and pull-down sample rate is defined for both NTSC and PAL/SECAM.  In version 1.1 the HDR24/96 will support both NTSC pull-up and pull-down sample rates, but not PAL/SECAM pull-ups/downs.  The standard Sample Rates and their corresponding pull-up and pull-down rates are as follows:

    50.0 kHz PAL/SECAM pull-up
    48.048 kHz NTSC pull-up
    48.0 kHz Standard rate
    47.952 kHz NTSC pull-down
    46.080 kHz PAL/SECAM pull-down
    45.9375 kHz PAL/SECAM pull-up
    44.1444 kHz NTSC pull-up
    44.1 kHz Standard rate
    44.056 kHz NTSC pull-down
    42.336 kHz PAL/SECAM pull-down

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. 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

48.048 kHz

29.97 fps

No

No

 

30 fps

No

Yes

48 kHz, 29.97 fps

48 kHz

29.97 fps

Yes

No

48.048 kHz, 30 fps

30 fps

No

Yes

47.952 kHz , 29.97 fps

47.952 kHz

29.97 fps

Yes

No

48 kHz, 30 fps

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 the box.  Using the above examples again, if the HDR24/96 is set to 60 Hz Video Field Rate, a 48 kHz Sample Rate, and a 30 fps Time Code Frame Rate, locking the HDR24/96 to a 60 Hz video signal produces a 48 kHz sample clock and 30 fps time code.

Enough said....

 

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