File Background

This file was digitized from a miniDV by the media archive at Democracy Now! and represents a rare but problematic occurrance in the digitization process. Within this study, this was the only file identified with this particular error.

DV Structual Integrity

As a deck reads data across a DV tape it reads sections of data for subcode (timecode), audio, video, and identification information. The tape stores this information is separate locations to allow for insert editing or separate handling of audio, video and subcode. As the decks reads the tape to output a DV stream that may be written to a file, these seperate sections of data are combined into a new structure so that subcode, audio, and video for each frame are combined into a standardized structure of DIF blocks. A DV decoder relies on this structure to understand the specifications of the codec in order to interpret and playback the video and audio.

In this example, the second frame begins 33 DIF blocks that contain NULL data. The frame thus lacks the first header, first declaration of video specification and first few blocks of video data. This image shows the incoherent section of the file. Each line of data represents (in hexadecimal) a single DIF block of a DV frame. The first three lines of the image are the end of frame 1, while the rest of the image represents the beginning of the problematic second frame. The zero-filled DIF blocks occur where essential data is expected. Within the zero-filled DIF blocks the two lines that do contain meaningful data hold the first bits of audio data for the frame, which (somehow) appear in their expected location despite the missing header, subcode, and video data.

Implications

It may seem that such an error is minor in the capture of an hour long DV tape; however, since the frame fails to follow the expected structure of file-based DV video, the manner in which a DV decoder will interpret the data can not be easily predicted. In this case Quicktime and Final Cut Pro handled the error gracefully, the file plays continuously without stutter however the missing video data is noticeable.

The error first become apperant when the file was loaded into a broadcast server to play out during a live television news broadcast. The playout server did not handle this error as gracefully as Quicktime but stuttered when it reached this frame, resulting in the audio and video becoming significantly out of sync for the remaining of the file's duration. The live broadcast had to quickly rearranged since the error prevented a 6 minute clip from presenting correctly.

Conclusions

As mentioned, this file is our only known occurrance of this type of error (structural DIF incoherancy from invalid NULL DIF blocks); however the results were significantly problematic to the achievement of reliable presentation. Since the majority of the data for that particular head pass within the deck is valid, it is our assumption that the problem occurred within the DV deck's assembly of cached tape-based data into a DV stream output and does not represent a problem with the source DV tape.

For more information: http://www.avpreserve.com

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