Hi, some random data on pullups and pulldowns... I don't remember where I found this, but, I found it interesting...
When some engineer had the bright idea of operating the NTSC television system at 59.94 fields per second rather than a nice simple 60, life became more complicated for the sound guys. Telecine is the culprit!! 24 pictures in one second on film do not fit elegantly into 30 available video frames in one second. To complicate things further the video system does not use real seconds. Consequently the film on a Telecine machine is being played at 23.9 frames per second NOT 24, as it was shot. As we discovered when we introduced Time Code to the 1/4" analog tracks, the production mixer had to use 30 FPS time code during recording so that the post house could slow the tape down 0.1% by synchronizing it with a 29.94 time code. That made picture and sound synchronous, if not on speed. Good enough for TV! The term PULL DOWN was born.
Now we have those cool little DAT machines. They operate with rotary scanning heads and internal clock frequencies and other things reminiscent of Video Recorders. Scary `eh what?
Because of the way DAT machines work, they are so stable that no resolving is necessary to achieve synch. That is, the DAT machine will provide a constant speed playback exceeding that of an analog 1/4" tape with crystal sync. Sync requires that the picture also have the same accuracy in speed or that both picture and sound have the same inaccuracy in speed.
In traditional film production the DAT tape needs only to be played back and transferred to mag film without any resolving, providing that the mag film recorder is running from a crystal controlled motor and the camera was, likewise, "Crystal Sync".
Professional DAT machines provide analog and AES/EBU digital outputs. They also usually operate at one of two sampling frequencies 48KHz and 44.1 KHz. Most dialog recording for motion Pictures and Television is done at 48 KHz. The music industry uses 44.1 KHz. to match their CDs.
Which sampling rate to use should be discussed with the production (particularly the post-production) people before beginning the shoot.
In MOST situations the Time Code DAT machine will behave in much the same way as the Time Code 1/4" analog machine did as long as only the analog output is considered. That is, record with 30FPS TC on the set and the Telecine guys will "PULL DOWN" by synchronizing the playback with 29.94XX Time Code. Picture and Sound match...everyone is happy!!
Some engineer will say "We have all this neat digital sound on the tape, why convert it to analog so soon"? He has a point...
Digital audio signals are at a specific sample rate or frequency. If they vary from their specified frequency nothing comes out. Changing the sampling rate from the nominal 48 or 44.1 should only be attempted by professional drivers under controlled conditions. We equipment manufacturers provide adequate horsepower to get you into trouble....you have to know how to stay safe!
The D2 DIGITAL video recorder has a 48KHz. audio track. It must have 48 KHz. or it accepts nothing. If the production mixer records his track with a 48KHz. sampling rate and the Telecine PULLS it DOWN to 47.952 KHz. there is no audio to record onto the videotape. Conventional practice is to convert the digital to analog, transfer the analog to the D2 and then reconvert the analog back to digital. It works.
A more daring solution would be for the production mixer to "PULL UP" on location, and record at 48.048 KHz. (with a 30FPS time code). Then the "PULL DOWN" in telecine would produce a digital audio signal of 48 KHz. making the D2 machine very happy. Remember that this makes the DAT tape totally useless for normal playback.
Many productions are being edited on digital workstations (AVID, LIGHTWORKS etc.) They accept the picture from videotape that is already "PULLED DOWN" by the Telecine process. The sound needs to be "PULLED DOWN" as well to match the picture. These machines are all digital so they are happiest with a digital audio source. A tape recorded at 48 KHz. and then "PULLED DOWN" to 47.952 is useless to them. They have to go through the digital to analog and analog to digital conversion to get the 48 KHz. they need. Had the production mixer "PULLED UP" and recorded at 48.048 the editor could directly down load the digital audio "PULLED DOWN" to 48 KHz. Remember, the DAT cassette will not provide a normal digital signal at nominal speed when recorded "PULLED UP". For a production where the final product is a videotape, this should work. If it is a film production which will be released in film the technique needs more thought...
The picture is "PULLED DOWN" by 0.1% to edit as video. The Key Code cut list for the negative cutters effectively provides the "PULL UP" to bring the film back to 24FPS. If the sound has been recorded with a "PULL UP" to accommodate the editors, it will be impossible to provide a digital signal at nominal speed to lay back to the film. At this point an analog signal will be all that is required and all is well. If, however, the post guys want to stay in digital (I refuse to use the term "DIGITAL DOMAIN") it would have been prudent to have made a nominal speed (48 KHz.) digital dub during the "PULL DOWN" for the editing process.
Music video is another "can of worms", probably on the set as well as on the sound cart. The music production company gives you a DAT cassette for playback with Time Code at 29.97. You know that the film, shot at 24FPS will be "PULLED DOWN" to 23.9 in telecine, but not the sound (it's already made with 29.97 time code) What to do? "PULL UP" the DAT on playback so the sound is 0.1% faster than original. The actors will move 0.1% faster. In telecine the "PULL DOWN" on the film will make the action correct and properly match the sound. The "PULL UP" on playback also makes the 29.97 Time Code into 30 for the electronic slate. Be sure to use a cable or radio link to the electronic slate as "jamming" the generator will not be accurate.
Perhaps it would be worth putting up with the flicker and the eight field color phase of PAL to be able to work at 25FPS for everything.
Maybe simply changing NTSC to a true 60FPS and shooting film at 30FPS with 3 perf pull down would do it ... anyone ever thought of that.....?
Simply put .... film runs at 24 frames per second through a film camera. (That is 90 feet per minute for 35mm) Film runs at 23.976 frames per second through an NTSC Telecine transfer machine. That is 0.1% slow. To make the sound fit the slowed down picture the sound must also be slowed down by 0.1%. Pretty simple eh? It works with no problem in the analog world. The sound is just a bit slow and slightly lower pitched. No big deal. (unless you are a musician with "Golden Ears") However, in the digital world we should keep our sampling rates constant if we intend to use the AES/EBU digital signal for anything practical. That means that the DAT played back at 0.1% slow will have a digital sampling rate also 0.1% slow. Not Good! The simple solution is .... RECORD the material at 0.1% FAST (48.048KHz. instead of 48.00KHz). When the Telecine transfer process slows it down by 0.1% to match the picture the DAT plays back with a 48.00KHz sampling rate... the AES/EBU standard. In fact the cassette recorded at 48.048 will play back at 48.00 on a standard DAT machine ... great for downloading into a Digital Editing system. No bit rate conversion ... higher quality ... simpler.
What about Time Code? If you use 30 Non Drop code at 48.048 it becomes 29.97 Non Drop code when played back at 48.00. If you use 30 Drop Frame code at 48.048 it becomes 29.97 Drop Frame when played at 48.00.
Sync playbacks for Music Videos are basically the opposite, but, usually using 44.1KHz sampling rates. Some people suggest that the music suppliers should produce a special cassette at 44.056 with 29.97 time code. This, played at 44.1 will produce a playback 0.1% fast with 30 fps time code. It will work but a second DAT at 44.1 will be needed for Telecine or other transfers. It is much simpler to have the music suppliers simply supply a standard 44.1KHz 29.97fps (Drop or Non Drop) time code cassette. They need do nothing special or different; then play it back on the set at 44.144KHz. This speeds it up the necessary 0.1% and provides the 30fps time code for the slate. No one, other than the playback mixer has to do anything special and he is, after all, the "specialist" anyway.
cheers
geo