The Sony HDW-F900 HD camera, with the ability to acquire images in 1080p/24, is becoming increasingly popular, as more and more users look to integrate a 24P workflow into their existing work environment. For the most part, the 24P workflow replicates many of the well-known and well-documented processes used in film, as both share the same frame rate and progressive nature.
While getting to the ITU-R601 submaster (PAL or NTSC) needed for offline and online finishing systems requires processesrequires processes slightly different from film, once the tapes are made, the postproduction process is nearly identical.
This document outlines the process by which the Sony HDW-F500 creates these submasters, as well as reported problems and workarounds. Most issues are related to NTSC down-converts while the PAL process to date seems relatively trouble free.
Note that material is acquired at 24 frames per second with a segmented frame method that splits the progressive image into two parts storing each half on a field of tape. It is important to understand that the frames are still progressive, since there is no motion between these segmented frames. Using segmented frames is a unique and efficient way to acquire and store progressive material on existing field-based technologies.
This type of segmented frame has existed for quite some time. In PAL for example, when the 24fps film is transferred to video, it is sped up 4.1% to create a 1:1 relationship between film and video. Although the resulting video has two fields for every film frame, both were created from a single film frame. This approach is the same used by the Sony HDW-F900 HDCAM. A more technical and detailed description of the segmented frame format can be requested from Sony.
Avid Editing/Finishing Systems and 1080p/24
Avid's SOFTIMAGE|DS HD system, which will ship in Q4 2000, will enable users to digitize and edit with native 1080p/24 sources. Not only does it work with 1080p/24, but also 1080i/30. 1080i/25 and 720p. Many other HD systems are coming to market, but this document deals with Avid sysytems.
Unless the project can be edited in native HD resolutions for both the offline and the online, the workflow will require either a standard definition NTSC or PAL down-convert from the HD masters. For this deliverable, Avid provides Media Composer with the Film or Universal Offline option, which can edit in native 24P and Symphony Universal, which edits and masters in native 24P (uncompressed ITU-R601).
For Media Composer with the film or Universal Offline option, or Symphony Universal, the following statements apply:
HDW-F500 to Standard Definition NTSC
The HDW-F500 VTR emulates the same process as a film-to-tape transfer when creating HD down- conversions. The original 24 frame progressive material is played out through the serial digital output with 2:3 pulldown inserted to create 60 field NTSC. This is done by adding an optional NTSC down converter card and an optional 2:3 insertion card into the VTR. Please contact your Sony dealer for exact part numbers and ordering.
Avid customers have encountered some issues in the field when using Avid systems with this VTR. Explanations of these issues and workarounds [when available] are provided below. Again, for clarity, the issues below relate to a NTSC workflow. At this time, no issues using the HDW-F500 VTR have been reported using a PAL workflow.
It should also be noted that Avid 24 frame editing systems truly work at 24 frames per second. This means that all shooting should be done with the camera at 24fps and since 23.976 will cause the audio to be treated as analog for all IO. .When shooting at 24fps, audio can be digitized digitally via AES/EBU and be synced within the editing system using a common timecode. The 24 frame editing systems assumes 24 frames = 60Hz = 44100/48000. If a 23.976 frame rate is used, then audio digitize will need to be done with analog input and the pulldown switch in the .99 position. See the Avid editing User's Manual for more detailed information.
Deck control issues mentioned in this document pertain to NTSC project types only.
|
Can the Avid system control HDW-F500 directly to digitize material into the system? | Even while a NTSC down converter and 2:3 inserter card is installed on the HDW-F500, the timecode that is sent down the RS-422 (9 pin) is still 24fps. Media Composer and Symphony Universal expect the timecode to be 25 or 30 (drop/non-drop) frames from which the 24 frames are recreated. Direct control of the HDW-F500 is scheduled for a future Media Composer/Symphony release. |
|
How does one digitize the material into the Avid system? | Users must down-convert to any of the standard Avid-supported NTSC tape and deck. The easiest way is to use the serial digital output from the down converter and create NTSC submasters. |
| Are there any issues involved with the down conversion? |
Yes, users need to be aware of several issues when creating the NTSC down conversions:
|
| When using the 2:3 inserter card, the playback of the VTR is actually 23.976 and not the original 24 frames. The VTR uses the same process as telecine. The difference with the VTR process is that during the film-to-tape transfer with sync audio, both picture and audio are referenced to video, thus slowing all sources by .1%. In the case of the HDW-F500, only the picture gets slowed down to 23.976 in order to create a perfect 2:3 pulldown, but the audio still remains at its original rate. This process results in a cumulative error where picture and audio will drift out of sync. .1% will be a cumulative 1 frame out of sync every ~40 seconds. Again, the solution to the audio sync issue is to use a demux, as suggested above or decide on the audio workflow beforehand. Another way to deal with this issue is to digitize the audio separately from the picture, and sync the picture and sound within the Avid system using the common timecode with AutoSync™. Please refer to the [Media Composer/Symphony] user’s manual for instructions on this process. | |
| Is there a FLEx file or other file that I can import to batch digitize? How does the system know how to digitize 24fps? | FLEx files and otherfilm transfer logging files are usually created during the film to tape transfer. During the transfer, systems can automatically create the timecode, KeyKode&trade, and pulldown relationship. These files are created whether the transfer is going to 1080p/24 or to standard definition NTSC. The Aaton Keylinker system is one such system that can create -- during the film transfer to1080p/24 -- a FLEx file that reflects the down convert. Please visit Aaton or Evertz for more information on these systems. In the case of HD acquisition at 1080p/24sF (segmented frame), the down conversion process will not create a FLEx file for importing. The logging process will be the same as for any other video-based project. In this case though, there is a basic assumption that during the down conversion using 30-frame non-drop timecode. Every timecode ending in :x0 and :x5 will be an "A" frame. The user should log accordingly. The logging relationship should be as follows:
It has been noted in the field that this process [always/sometimes] produces a one-frame delay. This occurs regardless of whether users are working with the NTSC 2:3 inserter card or a demux. A firmware upgrade is expected. Please contact Sony for more information. Correcting this error can be accomplished within the Avid editing systems. Users can select all problem clips, and change the pulldown relationship of the clips via the "clip modify" command in the "clip" menu. Please refer to the [Media Composer/Symphony] user’s manual for instruction on this process. |
| How should one create a "selects" reel from a 1080p/24 master? | Avid recommends users make a down convert from the 1080p/24 master first to a NTSC source, then a selects reel made from the NTSC down conversion. This way the timecode conversion is not interrupted, guaranteeing correct tracking of the original 24 frame sources. |