Acquiring Digital Audio
- Physical Format Standards
- Digital Format Standards
- MP4 and QuickTime Benefits
- Distributing Digital Video
- Archival Recommendations
- File Extension Standards
Digital audio files are obtained most commonly by one of three methods: conversion, use of a digital recording device or, by obtaining an existing audio file. In all cases copyright law must be considered. Permission from owners and subjects may require a signed release form. Prior to recording campus events, a signed media release form must be obtained from presenters.
Creating your own digital audio files involves numerous crucial technical decisions. Poor decisions can result in digital files that are inadequate for their intended uses. By anticipating the most demanding end-use before the audio file is created or digitized, it is possible to choose specifications that will result in satisfactory audio quality without squandering resources such as time or network storage.
Conversion involves the use of both hardware interfaces and software to read an analog (or digital) file and translate it into the desired digital audio file. Conversion is quite complicated both because there are many different analog formats, each requiring different hardware, and because of the various options and settings involved in the software "translation". Moreover, each piece of hardware and software will have its own settings and user interface. If your digital audio collection will involve significant transfer from existing analog collections, it is highly recommended that you consult with IT.
Physical Format Standards
The DVD format, because it both produces a high quality sound and has been widely adopted by manufacturers and users of consumer-level audio equipment, is the recommended audio format for Bowdoin College students, faculty, and staff. The DVD uses high density media to record audio tracks, and is easily transferable to either a PC or to a Mac platform.
Bowdoin College Information Technology and the Library’s Language Media Center support DVD playback and editing, and the Library’s Special Collections and Archives is able both to play DVD's and to convert them to other formats (network/web file, iTunes U) for distribution or for classroom use.
Recommended Audio Setting for Video Recording
To ensure better compatibility among miniDV playback devices, it is recommended that miniDV cameras have their audio set to 16 bits. Selection of higher bit depths can jeopardize audio playback on some machines.
Digital File Format Standards
These recommendations are based on established format protocols currently used to deploy media content to the Bowdoin Web site and other Web-based applications. I.e., Blackboard, e-Reserves, iTunes U, Podcasts, Bowdoin web site.
Recommended Media Containers
Bowdoin's baseline standard for both open and commercial software audio formats is based on MPEG-4 Part 14. MPEG-4 Part 14 is a multimedia container format standard specified as a part of MPEG-4. It is most commonly used to store digital audio and digital video data, especially those defined by MPEG, but can also be used to store other data such as subtitles and still images. Additionally, MPEG-4 Part 14 also allows streaming over the Internet and is compatible with current server hardware installed at Bowdoin. The official filename extension for MPEG-4 Part 14 files is .mp4, thus the container format is typically referred to as MP4. The only exception to Bowdoin's baseline standard for an open audio format is MPEG-1 Audio Layer 3, more commonly referred to as MP3. MP3 audio is used to guarantee successful delivery of digital audio to the widest variety of computer platforms and mobile devices, such as MP3 players, iPods and cellular phones. To determine which media container to use, please consult with IT.
Recommended Software Platform
Quicktime is the recommended software platform for encoding and decoding of media content, but other encoders are freely and openly available to encode media into the recommended containers. Additionally, Quicktime is freely available to the public for both PC and Macintosh computer platforms and is currently the most used encoding engine for mobile video and audio devices (such asiPods). Quicktime also allows Bowdoin to leverage server side solutions, such as Apple's Podcast Producer, to automate the encoding, and deployment of media content in support of web related applications.
MP4 and QuickTime Benefits
The primary reason behind the selection of Quicktime as the recommended software platform is because both the MOV and MP4 containers can use the same MPEG-4 codecs, and are mostly interchangeable in a QuickTime-only environment. However, MP4, being an international standard, has more open support. This is especially true on most hardware devices.
Additionally, QuickTime's MPEG-4 Export dialog contains an option called "Passthrough" which allows a clean export to MP4 without affecting the audio or video streams. The MOV file format supports multichannel audio with more than two channels while QuickTime's support for audio in the MP4 container is limited to stereo. Therefore multichannel audio must be re-encoded during MP4 export.
Distributing Digital Audio
Digital audio can be distributed via physical media or the internet. Tape is not a suitable physical medium to be used for distribution. When distributing audio recorded on tape, alternative formats, such as DVD or a computer file (see recommendations below), are recommended. For more information about audio captured during video recording, consult the video guidelines.
Tape and personal hard drives work well for short term capturing of digital audio, but this technology is not ideally suited to archival preservation. Magnetic media are inherently unstable and will, over time, begin to decay, thereby compromising the integrity of the source material. In addition, given the “all or nothing” nature of digital recording – data is either readable or it is not – relatively minor tape decay or deformation caused by repeated use or file corruption can render digital audio unusable. This is unlike analog tape recordings, which may exhibit sound distortions when the tape has begun to decay or has been deformed, but, depending on the severity of the distortions, still allow the possibility of listening to damaged source material.
In order to minimize the risk that an audio recording is lost due to decay or damage of the master, members of the Bowdoin community are recommended to preserve digital media in more than one format. A DVD can be used as a short term back-up for a master, but because DVDs are created using a compression algorithm, which produces audio of diminished quality from the original, and because DVDs are prone to “bronzing” as the inner foil is exposed to air and playback is compromised, DVDs should not be used as the primary archival storage media. It is recommended that in addition to the master, a computer file of the audio also be maintained on the Bowdoin network (see recommendations below).
File Extension Standards
MPEG-4 (.mp4, .m4a, .m4b)
MPEG-4 audio and video files generally use the standard .mp4 extension. Audio only MPEG-4 files typically use the standard .m4a extension. Digital audiobooks are typically use the .m4b extensions
MPEG-1 Audio Layer 3 (.mp3)
MPEG-1 Audio Layer 3 audio files use the .mp3 extension.
Downloadable Audio (.mov)
Audio purposed for download on the Bowdoin web site and related web applications should make use of Quicktime and the .mov extension to offer the best possibility of successful playback.
Streaming Audio (.mp4, .mov)
Audio purposed for streaming playback on the Bowdoin web site and related web applications should make use of Quicktime and the .mov extension to offer the best possibility of successful playback.
Mobile Audio (.mp3, .m4a)
Audio purposed for download and listening on mobile devices (such as iPods and iPhones) should use the .mp3 extension to offer the best possibility of successful playback.
Mobile Video with Sound (.m4v)
Video purposed for download and viewing on mobile devices may also include audio content, and should make use of Quicktime and the .m4v extension to offer the best possibility of successful playback.