What is an archival version of an audio file?
An archival version of an audio file is a file which represents the original sound faithfully. In archiving we want to keep a version of the audio which can be used to make other products and also be used directly itself if needed. This is usually done through PCM. There are several file types which are associated with PCM or RAW uncompressed faithful (to the original signal) digital audio. These are:
- Standard Wave
- Wave 64
- Broadcast Wave Format (BWF)
[One way to understand the difference between audio file formats is understanding how different format are used. One place which has been helpful to me has been the DOBBIN website as they explain their software and how it can change audio from one PCM based format to another.]
Each one of these file types has the flexibility to have various kinds of components. i.e. several channels of audio can be in the same file. Or one can have .wav files with different bit depths or sampling rates. But they are each a archive friendly format. Before one says that a file is suitable for archiving simply based on its file format one must also consider things like sample rates, bit depth, embedded metadata, channels in the file, etc. I was introduced to DOBBIN as an application resource for audio archivists by a presentation by Rob Poretti. One additional thing that is worth noting in terms of archival versions of digital audio pertains to born digital materials. Sometimes audio is recored directly to a lossy compressed audio format. It would be entirely appropriate to archive a born-digital filetype based on the content. However it should be noted that in this case the recordings should have been done in a PCM file format.
What is a presentation version? (of an audio file)
A presentation version is a file created with a content use in mind. There are several general characteristics of this kind of file:
- It is one that does not retain the whole PCM content.
- It is usually designed for a specific application. (Use on a portable device, or personal audio player)
- It can be thought of as a derivative product from an original audio or video stream.
In terms of file formats, there is not just one file format which is a presentation format. There are many formats. This is because there are many ways to use audio. For instance there are special audio file types optimized for various kinds of applications like:
- 3G and WiFi Audio and A/V services
- Internet audio for streaming and download
- Digital Radio
- Digital Satellite and Cable
- Portable players
[A brief look a an explanation by Cube-Tec might help to get the gears moving. It is part of the inspiration for this post.]
This means there is a long list of potential audio formats for the presentation form.
- AAC (aac)
- AC3 (ac3)
- Amiga IFF/SVX8/SV16 (iff)
- Apple/SGI (aiff/aifc)
- Audio Visual Research (avr)
- Berkeley/IRCAM/CARL (irca)
- CDXA, like Video-CD (dat)
- DTS (dts)
- DVD-Video (ifo)
- Ensoniq PARIS (paf)
- FastTracker2 Extended (xi)
- Flac (flac)
- Matlab (mat)
- Matroska (mkv/mka/mks)
- Midi Sample dump Format (sds)
- Monkey’s Audio (ape/mac)
- Mpeg 1&2 container (mpeg/mpg/vob)
- Mpeg 4 container (mp4)
- Mpeg audio specific (mp2/mp3)
- Mpeg video specific (mpgv/mpv/m1v/m2v)
- Ogg (ogg/ogm)
- Portable Voice format (pvf)
- Quicktime (qt/mov)
- Real (rm/rmvb/ra)
- Riff (avi/wav)
- Sound Designer 2 (sd2)
- Sun/NeXT (au)
- Windows Media (asf/wma/wmv)
Aside from just the file format difference in media files (.wav vs. .mp3) there are three other differences to be aware of:
- Media stream quality variations
- Media container formats
- Possibilities with embedded metadata
Media stream quality variations
Within the same file type there might be a variation of quality of audio. For instance Mp3 files can have a variable rate encoding or they can have a steady rate of encoding. When they have a steady rate of encoding they can have a High or a low rate of encoding. WAV files can also have a high or a low bit depth and a high or a low sample rate. Some file types can have more channels than others. For instance AAC files can have up to 48 channels where as Mp3 files can only have up to 5.1 channels.
One argument I have heard in favor of saving disk space is to use lossless compression rather than WAV files for archive quality (and as archive version) recordings. As far as archiving is concerned, these lossless compression formats are still product oriented file formats. One thing to realize is that not every file format can hold the same kind of audio. Some formats have limits on the bit depth of the samples they can contain, or they have a limit on the number of audio channels they can have in a file. This is demonstrated in the table below, taken from wikipedia. This is where understanding the relationship between a file format, a file extension and a media container format is really important.
[table id=9 /]
Media container formats
Media container formats can look like file types but they really are containers of file types (think like a folder with an extension). Often they allow for the bundling of audio and video files with metadata and then enable this set of data to act like a single file.
[On wikipedia there is a really nicecomparison of container formats.]
MP4 is one such container format. Apple Lossless data is stored within an MP4 container with the filename extension .m4a – this extension is also used by Apple for AAC audio data in an MP4 container (same container, different audio encoding). However, Apple Lossless is not a variant of AAC (which is a lossy format), but rather a distinct lossless format that uses linear prediction similar to other lossless codecs such as FLAC and Shorten. Files with a .m4a generally do not have a video stream even though MP4 containers can also have a video stream.
MP4 can contain:
- Video: MPEG-4 Part 10 (H.264) and MPEG-4 Part 2
Other compression formats are less used: MPEG-2 and MPEG-1
- Audio: Advanced Audio Coding (AAC)
Also MPEG-4 Part 3 audio objects, such as Audio Lossless Coding (ALS), Scalable Lossless Coding (SLS), MP3, MPEG-1 Audio Layer II (MP2), MPEG-1 Audio Layer I (MP1), CELP, HVXC (speech), TwinVQ, Text To Speech Interface (TTSI) and Structured Audio Orchestra Language (SAOL)
Other compression formats are less used: Apple Lossless
- Subtitles: MPEG-4 Timed Text (also known as 3GPP Timed Text).
Nero Digital uses DVD Video subtitles in MP4 files
This means that an .mp3 file can be contained inside of an .mp4 file. This also means that audio files are not always what they seem to be on the surface. This is why I advocate for an archive of digital files which archives for a digital publishing house to also use technical metadata as discovery metadata. Filetype is not enough to know about a file.
Possibilities with embedded metadata
Audio files also very greatly on what kinds of embedded metadata and metadata formats they support. MPEG-7, BWF and MP4 all support embedded metadata. But this does not mean that audio players in the consumer market or prosumer market respect this embedded metadata.
[ARSC has in interesting report on the support for embedded metadata in audio recording software. ] Aside from this disregard for embedded metadata there are various metadata formats which are embedded in different file types, one common type ID3, is popular with .mp3 files. But even ID3 comes in different versions.
In archiving Language and Culture Materials our complete package often includes audio but rarely is just audio. However, understanding the audio components of the complete package help us understand what it needs to look like in the archive. In my experience in working with the Language and Culture Archive most contributors are not aware of the difference between Archival and Presentation versions of audio formats and those who think they do, generally are not aware of the differences in codecs used (sometimes with the same file extension). From the archive’s perspective this is a continual point of user/submitter education.
[This past week have taken the time to listen to a few presentations by Audio Archivist from the 2011 ARSC convention. These in general show that the kinds of issues that I have been dealing with in the Language and Culture Archive are not unique to our context. ]
The Complete Audio Package