File Background

This file is digitized video from an NTSC miniDV tape from The Archives of Traditional Music at Indiana University. This particular file is a severe example of audio errors in a DV ingest, rendering the digitized file mostly unusable. The source tape was recorded at LP (long-play) mode and in 4 track audio mode. By recording in LP the amount of data stored within a fixed surface area of the tape is greatly reduced, thus increasing the likelihood of errors. The tape also records in 4 track, 32kHz, 12 bit audio (as opposed to 2 track, 48kHz, 16 bit audio), thus each audio track is stored with half as much data, again increasing the likelihood for error. Given that the 3rd and 4th audio tracks of this scene were not utilized, there was no advantage to record in 4 track, 32kHz, 12 bit.

Reading Audio Datafrom a DV Tape and Associated Error Detection

When a DV tape plays in a deck, each pass of the head across the tape reads 9 blocks of audio data. The data transmitted from each head pass is referred to as a DIF sequence. In NTSC 10 tape passes (or DIF sequences) are needed to complete a full frame of DV (or 12 in PAL). Thus each NTSC frame contains a total of 90 audio blocks (or 108 in PAL). Each audio block contains parity data for error correction that can be used if the primary audio data is damaged or could not be read. In the case where the primary audio data and the parity data are both excessively damaged or not read properly, the deck may not have sufficient resources to perform error correction on that block of audio in the very short amount of time allowed to perform this work, thereby causing a drop out.

When the deck fails to correct the audio it uses a reserved audio sample value 0x8000 (or in 12 bit mode 0x808000) to state that the audio sample is erroneous. These audio error codes may be parsed from a dv file in order to automate the detection of audio errors (without non-stop listening).

Conclusions

Since this recording is configured to record 4 track audio but only contains 2 tracks, only DIF sequences 0, 1, 2, 3, and 4 contain audio data (DIF sequences 5-9 are empty although still often error-filled). This list reports on the total number of reported audio blocks are error-filled for the entire duration of the file (see this document for a more detail technical synopsis of the error detection performed):

• 648 Dseq=0
• 54 Dseq=1
• 702 Dseq=2
• 45 Dseq=3
• 684 Dseq=4

Interestingly, in this sample the great majority of the errors are on even-numbered tape passes, indicating that the problems are more likely generated by a bad recording or playback head than by tape damage.

This XML document reports on each frame that contains errors, providing the absolute time, timecode of the source tape, date and time of the recording, and information on the errors. For example frame number 6 has an error statement: “CH1: 27 audio errors ( 9 Dseq=0, 9 Dseq=2, 9 Dseq=4)”. The “Dseq” refers to the DIF sequence, or the number of the head pass along the tape to read a frame (in NTSC 10 head passes are used for a single frame). Since there are 9 audio blocks per pass (or DIF sequence), the statement “9 Dseq=0” means that all 9 audio blocks failed to be read during the first head pass of that frame, thus 10% of that frames audio is missing. This happens again during head pass number 2 and 4 for a total of 30% audio loss. However since this is a 4 track recording, DIF sequences (or head passes) 0-4 contain the audio for tracks 1 and 2 while sequences 5-9 contain the audio for tracks 3 and 4 (unused). Thus audio tracks 1 and 2 of frame 6 are missing 60% of the audio.

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

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