Audio Digitisation Workflow
This paper discusses the considerations needed when planning a workflow for digitisation. It outlines some of the preparation needed when creating a workflow and precepts that need to be incorporated. The paper then offers an example generic workflow for digitising open-reel audiotapes onto a computer hard drive system.
For some time best practice guidelines for still image digitisation projects have been widely available but only until recently has work begun on producing such standards for audio (and in fact other digital media) digitisation projects.
Two of the main proponents of this work have been the IASA (International Association of Sound and Audiovisual Archives) and the American Library of Congress who have produced both TC-04 Guidelines on the Production and Preservation of Digital Audio Objects and Capturing Analog Sound for Digital Preservation, respectively. This work has set a good grounding upon which we can begin to identify a best practice for digitising analogue audio materials.
Establishing a workflow
Design of a workflow begins with reference to well-defined project aims. Defining those aims is the joint responsibility of project managers and key stakeholders. Of course, hands-on digitisation staff also have a vital role to play in defining project aims by offering advice and undertaking ‘feasibility' studies. Results from these trial runs (for instance, the number of audiotapes which can be digitised in a day) allow project managers to realistically shape the expectations of stakeholders into achievable project deliverables.
It is important though, to delineate between the feasibility study phase and initiation of the digitisation phase proper. Digitisation is a resource intensive process which cannot begin without first clearly established required project outcomes.
Your day-to-day workflow should be robust enough to allow most materials to be handled in a standardised way while allowing time to be spent on exceptions. It should be flexible enough to allow future refinement as experience grows. Crucially, your working processes should be recorded in detail in a ‘workflow manual' which acts as a clear guide to all members of the team.
With several hundred different formats of analogue audio and an infinite amount of possible project aims it is not possible to offer a highly detailed one-size-fits-all workflow for the digitisation of analogue audio. Each project will have its own unique aspects. It is possible however, to offer a generic workflow (see below) for the digitisation of analogue video to computer hard drive. It is also possible to identify a number of precepts which can be used to inform the design of each step in your own project.
Collect required technical metadata as work progresses
Metadata or ‘structured data about data' needs should be identified in the initial phase of the project (for more information see Metadata for Digital Audio) technical metadata handling can then be embedded into the digitisation workflow. This might mean simply filling out a small number of Dublin Core fields as the work progresses. Some metadata, such as original tape format or original label information may be vital to future use of the collection and risks being lost if not recorded at the time of digitisation. It is recommended that at least a basic list of analogue tapes to be digitised is compiled into a skeleton database before digitisation begins. This list then becomes the basis for metadata collected throughout the project. In-depth collection of metadata, such as the construction of complex descriptive records, is best carried out parallel to but outside of the digitisation process by a subject specialist.
- Archive original capture files
If no re-mastering is required then these original files are write-protected and used in order to generate lower-resolution delivery copies.
In an ideal situation, two identical and geographically separate generations of these mater files would be created. While this tactic does offer protection against disaster for unique or valuable materials, the related costs may prove prohibitive.
- Re-master only copies
Audiotapes may have deteriorated and audible defects may be digitised along with the original signal itself. Many effective and exciting digital restoration techniques are possible once audio is in the digital realm. A simple application such as ‘de-noiser' for example, can filter out any unwanted frequencies. But these techniques are not always successful. For this reason restoration techniques are best thought of as potentially destructive. Therefore, such techniques should only ever be carried out on a copy rather than a unique file.
- Archive re-mastered generation
Once footage has been re-mastered or digital restored these files too should be archived. Restorative techniques represent a significant investment of time and resources. Once successfully completed these files should be of similar status to preservation masters. Re-mastered files (if created) will generally be used as the source of delivery or ‘access' copies.
- Create access generation versions for delivery
Copies of audio intended for delivery will typically be of lower-resolution than master copies, perhaps in a propriety format. Although the creation of these can be largely automated, access copies also represent an investment of time and resources and so should also be archived safely.
- Plan for sustainability
Even in the initial stages your project's output should be viewed as valuable and so deserving of a long lifespan. Simple steps such as generating a checksum value (see FAQ) for each captured file will assist in future error checking.
- Use open technologies, avoid propriety archival formats
Some software, hardware and file formats are the property of a single company or institution; these are described as ‘propriety' technologies. Others are developed and supported by a group of several companies or institutions; these are known as ‘open' technologies. An advantage of using an open technology is the support of a user community which often accompanies it. There is also a perceived reduction in the risk of obsolescence; support for a propriety format can disappear along with the single company who developed it but this is not so for open technologies. Proprietary file formats can safely be used for delivery but never for long-term preservation purposes.
This section presents a generic example workflow for digitising analogue audio open-reel tapes to a computer hard drive. As the format of open reel tapes and the specifics of the playback and capture equipment varies so much, the following example presents an overview of considerations and points of good practice in a useful and practical generic order of events.
It is recommended that a workflow of this type is trialled before being directly put to task. A trial day would shed light on any problems that may arise, the time taken to complete processes and would familiarise digitisation staff with an effective routine.
Before the act of digitization is considered tapes should be inspected and analysed for their suitability for digitisation. Tapes which show signs of significant deterioration may be further damaged by the digitisation process and these should be dealt with accordingly with your preservation outlines previously set in place. Tapes should be individually checked again before playback and only those that appear to be in good condition should be used for capture.
It is important to optimise your working space in the most effective way possible. The most important factor in this is the acoustic environment in which you are monitoring the audio signals. Your workspace should be carefully prepared and designed. Its productivity and also output quality can be affected by a multitude of factors and for more information about this refer to the advice document Preparing your Audio Digitisation Workspace.
Check playback equipment
Analogue open-reel tape machines are old pieces of mechanical and electrical components. As such they can be prone to faults and require regular maintenance by a qualified technician. They should be visually inspected daily and it is recommended that they are calibrated before each session of use, and that the replay heads are cleaned at least once every four hours.
Check capture equipment
The computer, interface and hard drive are the key components in the digitisation chain and although they are likely to be of newer technology that your analogue playback device they are also prone to faults and unpredictable changes and should be checked before every session. A good time saving method is to program in settings to create presets that can be stored digitally in software and hardware systems. This also prevents any important settings being missed out during daily configuration
Once your analogue and digital systems have been checked and calibrated, a ‘dummy run' is worth undertaking to ensure proper connectivity and that both systems are synchronised correctly. Tapes should be aligned manually and monitored throughout the test both at the playback and ingest stages alternately. Once a sample has been captured, the digital file should be monitored to check for any imperfections which can be linked to the system. Noise, intermittent signal, loss of quality, loss of volume, jittering or incorrect playback speed will all indicate that a configuration or component in the chain is incorrect and needs to be addressed.
Once tested the further capture of tapes should be constantly monitored, again, regularly monitoring at the playback and ingest stages. Attention should be paid to the volume levels of different tapes and gain adjustments should be made accordingly. Files should be captured in an open standard format and saved to a hard disk which is separate from the disk which contains the computer's operating system.
Digital and/or analogue errors may have occurred during capture. Depending on your resources available you may wish to carry out some form of error testing before files are passed to be archived. Note that the most effective error testing is done by audibly listening to the files.
Top and Tail/Trim clips
If necessary separate clips of audio from the tape and top and tails clips to remove unwanted silence within your editing software. For preserving the capture quality the output format of the capture and editing software should be identical to the input format to prevent any changes being made before archiving.
Prepare files for archiving
If any cuts or basic editing has taken place then a new file will need to be rendered from within the editing software. Files will generally need renaming into a pre-decided format, preferably with a unique identifier. If files were captured in a proprietary format then they should be transcoded into an open standard format for long-term storage.
Archive master generation copy
At this stage your open standard file, with no digital errors may be labelled as your ‘preservation' master and entered into your digital asset management system. This can be an external device, a removal storage device or networked storage.
It is maybe worthwhile considering duplicate storage options in separate locations. Furthermore online storage management systems are becoming increasingly popular due to their lowering cost and automated processes including error checking, automatic metadata extraction, and format obsolescence checking.
Capture master generation metadata
Once your master generation copy has been stored, it is a good time to collect and save the new metadata relating to the new archived master file into your chosen schema. This should include file name, file size, file duration, and any inaccurate audible artefacts. This last point is useful for later detecting possible problems with the capture process rather than the original file.
Improving the audible quality may be desirable before the delivery of audio files to your audience. Repairing noise created from aged media or enhancing the quality of the original recording can be achieved in a variety of cutting edge software packages. Making adjustments to the master generation file should only ever be carried out on a saved COPY of the master generation file.
Archive re-mastered files
A second-generation of archived files can now be created with the addition of the re-mastered files. Once optimisation has been carried out to a desired level, these files should be treated in a similar way to the master archived files, that is; given a new file name and stored correctly.
Capture second-generation metadata
After file optimisation it is important to document the changes made in the metadata records. Information regarding the mastering software used and the location and information surrounding the project file in which re-mastering occurred should also be included.
Transcode for delivery
Now that your files are audibly ready for delivery the only thing which remains to be done is to convert the files into an accessible format for your audience. Formats for delivery are often proprietary and change quickly to keep up with current trends. The most common presents audio formats for delivery are mpeg-3, real media audio, and windows media audio files.
Archive third-generation access files
If it is financially feasible it is recommended that you archive the files prepared for access. Although perhaps not as important as your master generation files which have been prepared for long-term storage, you may also find you need access to these at a later date, and they too represent time spent by staff and should therefore be saved.
Capture third-generation access metadata
As previously mentioned, most delivery formats are likely to be proprietary and as a result their shelf life is unpredictable. Therefore metadata will be required for access versions and any other information regarding the format and codecs used.
The purpose of ‘signing-off' is the final check that all of the previous stages have been completed to a satisfactory standard and that the resultant files are deemed fit for purpose. A signing-off facility such as a log book or a simple spreadsheet should be utilised to ensure a strict log is kept.
Assembling an effective workflow isn't an easy task. The type, age and condition of tapes can vary widely even within a single collection. As noted above, audiotapes are infinitely variable and complex working objects. Through research, planning and well documented implementation the task can be made a lot easier. Feasibility studies, though crucial, are only an indication of what is possible; your workflow can only be truly refined through practice. For this reason, every workflow should be an iterative process.
You can obtain advice relating specifically to your collection via JISC Digital Media's helpdesk service.