[georgia]

Three commonly used workflows, as well as a fourth that is growing in popularity with the availability of hard disk recorders and the BWF file format, provide a good means for identifying all of the considerations.
1. Feature film – double system at 24fps and 48kHz audio recording for 24fps postproduction.
2. Film-based television providing sync dailies on DigiBeta (23.976 and 48kHz) for 23.976 postproduction.
3.Film or HD Production at 23.976 with single or double system audio recording for 23.976 postproduction.
4.Feature film and film-based television production at 24fps with hard disk recording at 48.048kHz for 23.976 postproduction.

1. Feature Film Double System
Most, if not all, feature film production intended for theatrical shoots film at 24fps while recording audio digitally at 48kHz. Because the audio elements tend to move to audio post via OMF, capturing the production audio digitally and at the highest quality and digitally is a requirement.

Picture: The camera negative is transferred to video directly or from a workprint, but the film is now running at 23.976fps during the telecine process in order to create a known 2:3 pulldown cadence to the 29.97fps video rate. Once digitized into the Avid in a 24p project, the frames are “stamped” as 24fps in order to play back in sync with audio captured directly via AES/EBU or Broadcast WAV files recorded at 48kHz.

Audio: Because the audio was captured digitally – either synced to work clock or imported as 48kHz – it expects to be in sync with the picture as it was originally captured – 24fps. The native sample rate of a 24p project is 48kHz and all other rates are resolved to that during capture. An example of this is capturing analog audio with the .99 setting active. This tells the Media Composer to “count” samples .1% slower and stamp the captured files as 48kHz.

When playing back at 48kHz, the audio plays back .1% faster creating a true 24fps playback from 23.976 sync sources. When capturing digitally at 48kHz, no samples are converted. It is a digital clone.

2. Film-Based Television with Sync Dailies

Due to the tight schedules in television programming, dailies are already synced when delivered to the editing room. The transfer facility has already resolved the original shooting rate of 24fps to 23.976 and has sample-rate-converted the digital audio sources to be in sync in the source tapes. Because some tapes expects only 48 kHz for 29.97 NTSC, the audio must be sample rate converted when going from 24fps to 23.976 on the video. The path looks like this:

Picture: 24 -> 23.976 to 29.97 video creating 2:3 pulldown
Audio: 48kHz -> 47.952 slow down (.1%) sample corrected -> 48kHz to 29.97 video

In this case, the postproduction should be done in a 23.976 project type, since it assumes that the 48kHz audio sample rate is in sync with picture playing back at 23.976fps from the DigiBeta captured sources. It has the same result than that of a film-to-tape transfer to tape. But since there is no need to speed up to true 24fps in this project, audio samples remain untouched at 48Khz throughout the postproduction process, through the audio mix and back to the NTSC broadcast master. Using this project type for this workflow will only go through one sample rate conversion during the film to tape transfer.

3. Film or HD Production at 23.976

Film cameras have been able to shoot at 23.976fps for many years, but it was not until HD acquisition became popular that people started using the frame rate regularly in production. Although HD cameras can shoot at true 24fps, the preferred shooting rate is 23.976fps because of the audio consideration when down converting to NTSC. No one wanted to deal with a sample rate conversion in the audio when working in a fully digital environment. In a double system environment, the DAT or hard disc recorder records at 48kHz. So shooting at 23.976fps eliminates the need to do a sample rate conversion or an analog audio capture with the .99 setting.

The resulting NTSC down convert is now the same as in the previous example where 23.976 video with 2:3 pulldown is in a DigiBeta tape with sync 48kHz audio. Keep in mind that when capturing directly from the Sony HDW-F500, the down conversion and 2:3 insertion processes cause the picture to lag 2 frames behind the audio. To address this condition, Avid Media Composer Adrenaline and Avid Xpress Pro systems include an audio delay feature in the Capture Window to delay audio by 1-5 frames as needed to resync picture and sound during the digitizing process.

If working double system, the DAT or BWF files from the hard disk recorder, the 48kHz recording will come straight in with no sample rate conversion or speed change to sync with the 23.976 picture.

4. Feature Film with 48.048kHz Audio Recording

Even though film cameras can run at 23.976, using them in that way never caught on for a variety of reasons. However, the audio workflow can change to allow a 23.976 postproduction workflow despite the film running at 24fps. This workflow is only for picture capture frame rate of true 24fps and a NTSC postproduction workflow. DAT, and more common to this workflow, hard disk recorders, can record at 48.048 kHz – which is really just 48kHz with a .1% speed up as part of the capture.

- 0.1% 47.953 Khz
48.000 Khz normal
+0.1% 48.048 khz

If a BWF file is detected as having been recorded as 48.048, you will be presented with a dialog box asking whether the import should perform a sample rate conversion or import as 48 kHz. If no sample rate conversion is chosen, the imported files are stamped as 48kHz, thus slowing them down by .1%; the same amount that the film is slowed down during the film to tape transfer. This way no sample rate conversion is performed, and a digital audio pipeline is maintained for the postproduction process.

To support the double system workflow used in feature film, it was necessary to create a true 24fps environment. This allowed audio dailies from the set to be directly captured into the Film Composer and remain in sync with the picture. This capability became a requirement when audio recording moved to digital formats such as DAT. Productions wanting to maintain a pure digital pipeline for audio needed to capture as AES/EBU with no sample rate conversion in order to maintain the highest quality. Audio sample rate and digital audio workflow are the main decision drivers of working at 23.976 or 24 in the Avid.
Project Window.


Digital Cut Window

The same can be done during the film-to-tape process from DAT or BWF recorded at 48.048kHz to create sync digital dailies directly to disc via a MediaStation or directly to a DigiBeta tape.

Capture, Edit, Digital Cut

Capture: The project type determines the native capture rate of the project, either 23.976 or 24p. It also determines the native audio sample rate of that project that will not have a sample rate conversion or analog process involved when capturing, playing, or digital cut.

Edit: In the Film/24p settings you will see the “Edit Play Rate” as either 23.976 or 24. This control sets the play rate of the timeline. It does not affect any of the digital cut output settings. This control lets you set a default state of frame rate for outputs that are made directly to tape, such as a crash record.

Digital Cut: Here you can output the timeline as 23.976, 24, or 29.97. The important thing to remember is that this is the playback speed of the Avid timeline, not the source tape destination. The NTSC frame rate of 29.97 cannot be changed. What is changing is the frame rate of the picture within the NTSC signal.

1. 23.976. This creates a continuous 2:3 cadence from beginning to end of a sequence and is the expected frame rate of a broadcast NTSC master from 24 frame sources.

2. 24: This is used for feature film production to create a true “film projected” speed from an Avid timeline on NTSC video. It is also the output type to use when using picture reference in a Digidesign Pro Tools system using OMF media from a 24p project type. Note that this is not a continuous 2:3 cadence. Adjustments are made over 1000 frames with the pulldown cadence. No frames are dropped, just the field ordering with the 2:3 cadence.

3. 29.97: Timeline will play back 25% faster to create a 1:1 film frame to video frame relationship. This can be considered a 2:2:2:2 pulldown cadence. This output is useful for animation workflow or low cost kinescope transfers where a 2:3 pulldown cannot be properly handled.

The above workflows are intended to maintain a digital audio pipeline from capture through post and into the final mix. It is possible to cross frame rates with sample rates using analog capture instead of digital or using a digital audio sample rate converter at time of capture.

Here are 4 rules of thumb that determine whether or not 48.048 kHz workflow is possible in your project, and how it must be used. These are:

Rule # 1: That 48.048 kHz workflow only works if picture is being shot at true 24 fps, and picture and sound editorial is being done in NTSC 29.97 or 23.98 HD video.

Rule # 2: That 48.048 kHz workflow only works when the project is finishing and releasing on video (not film).

Rule # 3: That 48.048 files are only useful if they are stamped using –F mode.

Rule # 4: Sound Editorial must agree that it’s a good idea to use 48.048 kHz sound, and the producers and post supervisor must agree with them

cheers
geo