Technical specifications for digitising audiovisual records

Audio digitisation

The National Archives recommend the standards, practices and strategies detailed in the publication: International Association of Sound and Audiovisual Archives (IASA) – Guidelines on the production and preservation of digital audio objects (IASA TC-04 second edition, 2009).

Audio formats

This standard applies to all physical formats containing audio recordings. Examples include:

  • compact cassette
  • micro cassette
  • mini cassette
  • ¼" open reel tape
  • gramophone discs
  • digital audio tape (DAT)
  • CD audio
  • 16 mm, 17.5 mm and 35 mm magnetic or optical film sound

Preferred standard for audio digitisation

The number of audio channels should always be based on the format of the original item, rather than the recorded signal. For example, a single microphone recorded on a stereo cassette recorder will have a mono signal but the format is stereo for playback and so the copy should be stereo. When in doubt, copy to stereo.

Films intended for playback on a projector in a theatre will have been recorded at 24 frames per second, whereas those intended for television will be 25 frames per second. It is important that magnetic or optical film sound always be played back at the original recorded speed.

TypeFile formatAudio stream
  • gramophone disc
  • music on ¼ inch tape or compact cassette
  • speech on ¼ inch tape 7.5ips or higher
  • magnetic or optical film sound

wav

  • 96 kHz sampling
  • 24 bit
  • speech on ¼ inch 3.75ips or less
  • speech on compact cassette
  • micro cassette

wav

  • 48 kHz sampling
  • 24 bit
  • digital formats (e.g. DAT)

wav

  • original sample rate
  • original bit rate

Minimum standard for audio digitisation

All copies made from analogue sources should be stored in uncompressed file based formats at a sample rate no lower than 44.1kHz and a bit rate of 16 bits.

Video digitisation

Video formats

This standard applies to all physical formats containing video recordings, either as a cassette or an open reel. Examples include:

  • VHS
  • Betamax
  • U-Matic (low band, BVU and BVU-SP)
  • Hi-8
  • DV
  • DVCPro/DVCPro50
  • DVCam
  • 1"
  • Betacam SP
  • Digital Betacam

Preferred standard for video digitisation

Digitised video content should be produced to the following specifications or higher:

 

TypeFile wrapper(s)Video streamAudio stream

Standard definition

mov, avi, mxf

50 Mb/s 10-bit uncompressed or lossless compression encoding

  • PAL - 4:3 720 x 576 pixels 25fps
  • NTSC - 4:3 720 x 486 pixels 29.97 fps

BWF 48 kHz, 16 bit PCM encoding

High definition

mov, avi, mxf

100Mb/s 10bit uncompressed or lossless compression encoding

  • 1280 x 720 or 1920 x 1080 as per the recorded format
  • 23.98 PsF, 24P, 25P, 50i, 29.97P, 59.94i as per the recorded format

BWF 48kHz, 24 bit PCM encoding

The National Archives uses the following specifications for its video preservation copy:

  • file wrapper - mxf
  • video stream - 50 Mb/s 10bit lossless motion JPG 2000
  • audio stream – BWF 48kHz 16bit PCM

Minimum standard for video digitisation

If the preservation copy is made from a digital videotape source the minimum acceptable specification would be that of the original recording format. Excessive compression should always be avoided. For example:

  • Mini-DV could be captured as 8 bit 25Mb/s DV codec
  • DVCPro could be captured as 8 bit 50Mb/s DVCPro codec
  • Digital Betacam should be captured as 10 bit 50Mb/s uncompressedv

Motion picture film digitisation

Film scanning is the preferred process for digitising motion picture film. It produces an individual image for each frame of footage.

Motion picture film formats

This standard applies to all motion picture film formats. Examples include:

  • 8 mm
  • 9.5 mm
  • 16 mm
  • 17.5 mm
  • 35 mm
  • 70 mm

Minimum standard for motion picture film digitisation

Digitised preservation files should be produced to the following specifications or higher:

TypeResolutionVideo streamAudio stream

16 mm positive material

SD or HD telecine transfer

uncompressed AVI; or uncompressed Quicktime; or lossless MJPEG2000

BWF 48 kHz, 24 bit PCM encoding

16 mm and Super 16 mm negative

scanned at 2K resolution
(2048x1080)

minimum 10bit log or linear DPX ( SMPTE-268 2003)

BWF 48 kHz, 24 bit PCM encoding

3 5mm positive and negative

scanned at 4K resolution
(4096 x 2160)

minimum 10bit log or linear DPX ( SMPTE-268 2003)

BWF 48 kHz, 24 bit PCM encoding

Acceptable quality for access or reference purposes

For internal access or reference purposes, the following formats could be considered:

Original contentFormat

Video or motion picture film

  • mov (Quicktime) – H.264 or DV codec
  • wmv – Playback compatibility with QuicktimePlayer depends on the availability of the wmv player component.
  • DVD

Audio – easily intelligible speech

  • CD
  • MP3 at 128 kbps

Audio – music or poorly recorded speech

  • CD
  • MP3 at 256 kbps

File-based born digital media

In this instance, the term born-digital refers to materials that originate in a digital data format recorded on an internal hard drive or removable flash memory, as opposed to digital signals recorded on videotape.

Born-digital material should always be submitted to the Archives in its original file format. If transcoding is necessary additional compression should be avoided.

Process considerations when digitising

Migrating audiovisual formats to digital files is considered the best practice for preservation digitisation.

The preservation copy is the digital file that is saved for long-term storage. It represents the most accurate reproduction of the original item without enhancement of the signal.

The most original source item is usually selected for preservation digitisation. However, if this is not possible due to physical or chemical deterioration, the most complete and appropriate duplicate must be selected.

Playback and signal path equipment (cables, switchers, analogue-to-digital converters, etc.) should be of a high quality and be well maintained. For any conversion off a digital audio or video tape source a digital signal path must be maintained with no further compression (e.g. Firewire is a compressed digital signal). At no stage should the signal be decoded to analogue then re-digitised.

Digitisation should be done in a single pass wherever possible. Where it is not possible due to the physical condition of the original item, each pass should be maintained in its entirety without any editing.

All tape based formats should be spooled through from end to end at least twice prior to playback.  This will help even out the tension and improve playback stability.

When producing digitised content, you should conform to internationally agreed standards. Non-standard and proprietary formats and files risk being unsupported in future preservation pathways.

File naming conventions that will be meaningful to future users in identifying items by file name should also be developed. Unique file names are essential to avoid potentially over-writing existing content. File names should never include blank spaces or special characters (e.g. &, * % # ; * ( ) ! @$ ^ ~ ' { } [ ] ? < > -).

The National Archives recommends that digital files be stored on hard drives rather than on recordable media (CD, DVD). The best practice option for file storage is Linear Tape Open (LTO tape). CD, DVD and Blu-ray are not considered acceptable as preservation storage formats due to long-term instability of the physical item.

Quality assurance checks

Visual and/or audio monitoring should be conducted for all conversions in their entirety and any issues documented. When the conversion is complete a quality control on all digital files is needed to ensure the files created match the visual and audio characteristics of the original film.

Operators should verify that the digital content is complete and matches the original source item. Notes should also be made about any issues with the conversion, such as poor framing.

The operator must also confirm that:

  • All digital files play properly
  • Artefacts that were not in the original source material do not appear in the transfer
  • The entire program should be digitised without clipping peak levels, or distorted audio
  • The files have been created in the specified file format, adhering to agreed naming conventions

For video or motion picture film digitisation the operator must also confirm that the aspect ratio is the same as the original, and that there is accurate sync between audio and video. The preservation copy must also have the same interlacing as the original source tape.

Copyright National Archives of Australia 2017