Presentation – Victoria Sloyan – 07/07/2010
Archiving Digital Audio Files
Hello and welcome to my presentation “Archiving digital audio files”.
I am the trainee for the futureArch Project, which aims to expand the Bodleian’s
capability to archive and preserve born-digital material. As part of this we have been
developing a procedure for extracting digital files from deposited media such as
floppy disks and moving them into our secure repository. This procedure involves the
use of forensic imaging equipment to create bit-by-bit duplicates of files, but it is only
suitable for data files. Therefore my project brief was to research an effective way of
extracting and preserving audio files.
Audio material requires its own method for three reasons:
1. Unlike data disks, audio CDs do not hold the audio data within a file system,
and so forensic imaging kits have difficulty creating an image file.
2. Audio files have different metadata. Some of the metadata fields are the same,
such as ‘title’ and ‘creation date’, but other metadata is specific to audio files,
such as ‘duration’ and ‘file format’.
3. The final difference involves delivery to users: the way you access a word
document is different from the way you access a sound recording.
Therefore, the overall aim of this project can be divided into three objectives:
1. First is selecting the most appropriate format to store the audio in. There are
many formats available such as MP3, FLAC and WAVE files and many
others. So, the first decision to be made was which to use.
2. Secondly, since imaging audio disks does not work I had to find a way to
extract the files from their original media and move them onto the secure
3. Finally, I had to devise an effective way of delivering digital audio to users.
Selecting a format
The most important consideration when choosing a format is finding one which is
uncompressed. Basically, formats fall into three categories: uncompressed formats
are, as the name suggests, uncompressed, which means sound and silence are encoded
at the same bit/time rate. Lossless compression is where the file is compressed to
shrink the file size, but done without reducing the sound quality. This is typically
achieved by compressing any silence. Lossy compression is where the whole file is
compressed. This can significantly reduce the file size, though often the reduction in
quality is unnoticeable by ear. For archiving purposes you want an exact replica of the
original data, therefore an uncompressed format must be used.
The second consideration is to find an open source format in order to aid accessibility,
because open source means you do not have to worry about licensing and legal issues.
Also, a standardised format is preferred, as this tends to mean it has been thoroughly
reviewed and is more likely to be longer lasting.
After evaluating all the formats I concluded the best one to use is WAVE. Crucially it
is an uncompressed format, also it is open source and standardised and it is
recommended by the International Association of Sound and Audiovisual Archives,
the Library of Congress and the British Library Sound Archive.
Capture audio and extract metadata
So, once the format was decided I needed a way of extracting files from a CD and the
best way is one you may well be familiar with: ripping, although the technical term
for it is ‘digital audio extraction’. However, normal ripping, like that done by
Windows Media Player is what is known as ‘fast ripping’, whereas for archiving we
wanted a ‘secure ripper’. The main difference between the two is that secure rippers
perform various validation tests to ensure maximum accuracy.
There are quite a few secure ripping programmes available but the one I decided to
use is Exact Audio Copy. It is well regarded by audio professionals, it works with
Windows OS, and it is free to download and rips to WAVE.
Here is a screen shot of EAC.
• Down the left-hand side you can see the available ripping options including
ripping as a MP3 and burning to a CD, but for my project I was only interested
in the first icon – ripping as a WAVE file.
• Along the toolbar there are various options to further specify the format. For
instance you can state whether the recording is ripped in mono or stereo.
• You can also specify the sample rate to use, although the maximum sample
rate available for WAVE files is 44.1 kHz. This is because Red Book Audio
states that audio on CDs should be recorded at a sample rate of 44.1 kHz with
a 16 bit-depth, thus there is little benefit to ripping audio at a higher rate.
Moreover, from an archiving perspective, the British Library stipulates that
audio transferred from one medium to another should retain the same sample
Once EAC has ripped the files it will produce a report recording the rip result. Under
each track it will say either OK or Finished. If it says Finished you know the rip was
achieved but the resulting file is not identical to the original. This could occur for
several reasons, the most common being if the disk is dirty or scratched.
Once the disk has been ripped, metadata needs recording in a spreadsheet. This is
based on the futureArch project’s metadata spreadsheet for digital files, but has been
modified to suit audio material. Here you can see all the fields that need to be
completed. The ones in bold are ones that have been added for audio files.
Archives exist not only to preserve material, but to also make it available to
researchers. Therefore, audio files need to be delivered in an efficient and effective
way. There are two potential problems with audio files. Firstly, they can be extremely
large, particularly if they are in an uncompressed format and secondly, the quality of
the recording can be quite poor. So, in order to combat this two issues I created two
versions of each file: I already had the master file so I processed this to create a
processed WAVE file and an optimised MP3 file. The table illustrates the intended
use for each derivative. MP3 files are lossy; therefore the file size is significantly
Processing was done using Audacity, which looks like this. The processing done to a
file will depend on its content and quality, but the tools I most often used were:
• Silencing and cutting to trim the beginnings and ends of recordings and
remove long pauses.
• Noise removal tool to either remove or reduce the volume of background
noise, like high pitched hissing on poor quality recordings.
It is very important to record every change that is made to the master file, so I created
a Process History Spreadsheet to record these changes. This includes the name of the
original file (22cd), all actions done to it in detail (such as two second noise cut at
22:54) and the name of the resulting file (22cd_mp3).
After processing was finished each file was exported from Audacity, first as in the
WAVE format and then the MP3 format.
Once the files are processed the MP3 versions and possibly the processed WAVE
files can be made available to listen to in the reading room. The master WAVE files
will be stored in the repository and will not be touched be users. All digital material,
both data and audio, will be accessed via a specific laptop in the reading room. This
laptop will have a specially designed interface similar in feel to an internet browser
and audio will be streamed, so accessing audio will be a similar experience to using
something like MySpace.
So, to sum up very briefly, if we go back to the three aims you can see I’ve pretty
much answered them:
1. The best format to use is WAVE
2. The way to capture audio is by securely ripping it
3. Audio will be processed and compressed and will be accessed by streaming
the files through a self-contained interface within the reading room.