The Challenges of Audio Digitization: An Analysis of Best Practices and How They Apply to Oral History Digitization Projects.

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1 Running Head: THE CHALLENGES OF AUDIO DIGITIZATION The Challenges of Audio Digitization: An Analysis of Best Practices and How They Apply to Oral History Digitization Projects Grant Outerbridge Professor Krystyna Matusiak University of Denver February 28, 2012

2 Abstract It is important to preserve audio collections and make them accessible because they contain valuable information not communicated by books and images. When a library or other institution embarks upon an audio digitization project there are many challenges to be faced and many questions to be answered. Because it is outside the scope of this paper to study the challenges of both music and oral history digitization projects, this paper will explore the challenges in digitizing oral history collections faced by a large-scale project, the Louie B. Nunn Center for Oral History at the University of Kentucky Libraries; and a small-scale project, the Manuscripts, Archives, and Special Collections (MASC) at the Washington State University Libraries. Best practices for sampling rates and bit depth for archival master and web derivative files, as well as storage issues and workflow management suggestions and examples, are discussed. This paper is only an introductory investigation of the challenges facing libraries and archives that embark on audio digitization projects. There is, as this paper argues, a growing nationwide understanding of the importance of preserving audio recordings through digitization, and the recommendation is made that further research be conducted in the area of audio digitization, to more fully understand and broadly articulate to libraries, museums, and archives the importance of preserving audio recordings at the highest possible fidelity for present and future generations. Keywords: audio digitization, file conversion, sampling rate, bit depth, oral history, music, hard drives, RAID, storage, preservation 2

3 Introduction It is important to preserve audio collections and make them accessible because they contain valuable information not communicated by books and images. As concerns oral histories, Stevens & Latham (2009) write: The human voice, with all its intonation, inflection, and emotion, can convey so much more than the written page. Books teach about the past; oral histories give it voice (p. 212). But many valuable sound recordings either from negligence, ignorance, or deterioration due to over-use or the passage of time are in danger of disappearing forever (National Recording Preservation Board, 2010). When a library or other institution embarks upon an audio digitization project there are many challenges to be faced and questions to be answered. What sample rate and bit depth will be employed? What type of storage solution is needed: external hard drives, a redundant array of independent disks (RAID), or remote servers? In what format will the audio files be, and how will they be delivered? Because it is outside the scope of this paper to study the challenges of both music and oral history digitization projects, this paper will explore the challenges in digitizing oral history collections faced by a large-scale project, the Louie B. Nunn Center for Oral History at the University of Kentucky Libraries, which houses over 8,000 interviews that concentrate on Kentucky history, Appalachia, Kentucky writers, and agriculture (University of Kentucky Libraries, 2011); and a small-scale project, the Manuscripts, Archives, and Special Collections (MASC) at the Washington State University Libraries, which is home to the 51 interviews with African-American pioneers that comprise the Black Oral History collection. Analog to Digital Conversion 3

4 Sound, as described by the CDP (2006), is a series of air-pressure waves (p. 6). The grooves on a vinyl record and the transmission of electrical impulses through a microphone onto magnetic tape are physical analogs of sound waves, thus the term analog recording. When digitizing analog materials, two main factors govern quality and resolution: the sampling rate, and the bit-depth. Sampling rate is measured in kilohertz (khz), which is how many thousands of samples per second are taken of a sound wave s amplitude (the highest and lowest point). Bit depth refers to the range of numbers used to represent each amplitude measurement Eight-bit numbers range from 0-255; 16-bit numbers range from 0-65,535; and 24-bit numbers range from 0-16,777,215 (CDP, 2006, p. 7). The higher the bit-depth, the more information is captured. Traditional audio science contends that humans are unable to hear frequencies above khz (22.05 khz in each of two channels = 44.1 khz) (CDP, 2006; Weig, Terry, & Lybarger, 2007; CARLI, 2009), and many audio scientists agree, that the human ear can only resolve between 15 and 17 bits per sample. Thus, some people may not be able to differentiate between a 16-bit recording and a 24-bit recording (CDP, 2006, p. 8). This led the music industry to use the 44.1 khz sample rate with a 16-bit depth as the standard for audio CDs (CDP, 2006; Weig, et al., 2007; CARLI, 2009), since that would capture all sound audible to humans. However, many audio engineers report a distinct difference in sound between 16-bit and 24-bit recordings (CDP, 2006). The reported differences could be because many recordings, especially analog oral histories, do not take full advantage of the available bit depth. According to CARLI (2009), the current standard for archival digitization of analog recordings requires the sample rate to be 96 khz with a 24-bit depth (p. 1). The CDP (2006) contends that the two acceptable choices for sampling rates are 96 khz and 44.1 khz [but] the 4

5 96 khz sample rate is rapidly being accepted for archival preservation standards (CDP, 2006, p. 16). Additionally, the CDP (2006) recommend[s] adhering to the current professional standard 24 bit depth (p. 16). Many consumer devices and out-of-the-box audio software packages, though, are not compatible with a 96 khz sampling rate with 24-bit dept (referred to as 24/96), and those files require a huge amount of storage space; so, for some institutions, the 24/96 archival standard may not be attainable. CARLI (2009) cites many reasons for the more dynamic files afforded by 24/96: The accurate capture of noise like clicks, pop, and other inaudible information that resides in frequencies higher than 44.1 khz; [the] desire to communicate inaudible harmonic information that impact perception of sound; [the] ability to record and provide content that, although not necessarily heard, helps listeners understand and hear better space, depth, and instrument location in stereo and surround sound recordings; and to accommodate future user applications. A higher bit-depth allows more audio information to be stored within the digital file, including support for a greater dynamic range (p.1). That said, a 44.1 khz sampling rate with a 16-bit depth (referred to as 16/44.1) may be sufficient for oral history digitization projects. While most audio digitization practitioners believe that anything less than 16/44.1 a disservice to the process (Weig, Terry, & Lybarger, 2007), and agree that, for preservation purposes, 24/96 is considered ideal, with many magnetic tape recordings, sampling at a rate higher than 44.1 khz often adds little more than noise or tape hiss (Weig, et al., 2007). As the CDP (2006) asserts: Recording at a sampling rate higher than 44.1 khz may not be effective in preserving more information for particular source materials, such as vinyl records or analog audio cassette tapes, because those sources are not capable of recording frequencies above 5

6 22.05 khz. Indeed they may only be 10kHz and could, theoretically, be sampled at 20kHz. Therefore 44.1 may be enough (p. 8-9). Eric Weig, Kopana Terry, and Kathryn Lybarger, who headed up the two-year pilot project to establish a digital preservation methodology and workflow at the Louie B. Nunn Center for Oral History at the University of Kentucky Libraries, decided to capture their audio at 16/44.1 rather than 24/96 (Weig, et al., 2007). Because the oral histories being digitized are spoken word only and their highest frequency is quite low, there would need to be a very clear indication that a significant amount of information would be lost capturing the audio recordings at 16/44.1 in order to justify doubling the amount of storage space required to capture the audio recordings at 24/96. The project decided that for analog magnetic cassette tape holding recorded speech, we would not capture at 24/96, and therefore opted to follow [16/44.1] as our analog-to-digital conversion standard (Weig, et al., 2007). Trevor James Bond, Special Collections Librarian for MASC at Washington State University Libraries also opted for the 16/44.1 sampling rate when digitizing the Black Oral History Collection. Although Bond (2004) does not discuss the preservation standard 24/96 sampling rate, he does explain that the 16/44.1 sampling rate was chosen for its CD quality (p. 16). Master Files and Web Derivatives In addition to the sampling rate and bit depth, the format of master audio files for preservation and web derivatives for online presentation and delivery must be considered. For audio master files, it is essential to use an uncompressed file format. There are two best options: WAV PCM (.wav), and AIFF (.aif), both of which support uncompressed audio. WAV PCM, developed by Microsoft, is the most widely used uncompressed audio format and is compatible 6

7 with almost any device; AIFF, developed by Apple, is an acceptable alternative to WAV PCM (CARLI, 2009). For web derivative files, a compressed file format is necessary, to reduce the file size and increase the speed of online delivery. There are two options: MP3 or M4A. For audio master files, a 16/44.1 WAV or AIFF file is considered a minimum standard; for oral histories with no music or background sounds, it is an acceptable choice. A 24/96 WAV or AIFF file is considered ideal, especially for recorded music or oral histories with music or background sounds (CARLI, 2009). For web derivative files, an MP3 or M4A file with a bit rate of 128 Kbps (kilobytes per second) is considered a minimum standard for oral histories without music; an MP3 or M4A file with a bit rate of 192 Kbps is the recommended standard for most audio; and an MP3 or M4A file with a bit rate of 320 Kbps is ideal when very high fidelity is required (CARLI, 2009). Weig, Terry, and Lybarger captured the Louie B. Nunn audio cassettes using a 16/44.1 sampling rate and saved them as uncompressed WAV files. For each interview, two uncompressed WAV files were created, one was the archival master file and the other was the service master file, which received whatever edits were necessary to clean up any signal distortion from the analog to digital conversion process (Weig, et al., 2007). When editing the service master WAV files, each was treated on its own merits rather than applying blanket settings to a host of files as a batch process (Weig, et al., 2007). Once the WAV masters had been processed, the derivative MP3 files for web delivery were created through an automated production process (Weig, et al., 2007). Weig, Terry, and Lybarger do not specify what bit rate was used for the Louie B. Nunn project s MP3s. The file size and format of web derivatives is often dictated by the content management system and type of server in use at the parent institution. For example, the Black Oral History 7

8 Collection captured their analog cassettes using a 16/44.1 sampling rate, saving them as uncompressed WAV files, and then encoded the WAV masters as RealAudio (rm) files so that they could be streamed through the WSU Libraries RealSystem Server 8 (RealServer) (Bond, 2004, p. 16). Bond (2007) does not specify if web derivatives were ever created, but does note that one of the advantages of the RealServer is that it allows for various rates of streaming audio, can accommodate extremely large files, users can fast forward or rewind easily as need be, and it accepts a wide range of formats, including mp3 and video (p. 17). Storage Issues There are no agreed upon best practices for audio storage, but there are several logical solutions depending upon the scope of a digitization project. The main choices are external hard drives, a RAID hard drive system, magnetic data tapes, optical discs (CDs and DVDs), and remote servers (CDP, 2006). For smaller digitization projects, an external hard drive setup is sufficient. For larger digitization projects, a RAID hard drive system or remote servers are preferable. Whether the scope of a project is extensive or modest, it is imperative that the master audio files be stored in a minimum of two locations, one on site and one off-site; this protects against data corruption or data loss and facilitates migration to future media (CDP, 2006). Weig, Terry, and Lybarger utilized a secure FTP to send one copy of each master file to an automated mass storage, robotic tape drive system, and two more copies of each master file were stored on 1.2TB external hard drives and a fourth copy [was] archived to DVD (Weig, et al., 2007). The Louie B. Nunn project also employs a 5-year refreshment process, as new media emerge, which helps to avoid possible corruption of storage media and future incompatibility issues. 8

9 As far as the Black Oral History Collection is concerned, Bond provides no specific information about what methods were employed for storing the audio master files. Because the audio masters, as well as any MP3 web derivates there may be, were uploaded to the Washington State University Library s RealServer, it logically follows that that is where the project s audio files are stored. Workflow Management Regardless of the scope of an audio digitization project, a basic workflow must be established. The process begins with the analog audio playback device, such as a reel-to-reel tape deck, cassette tape deck, or turntable. As the audio is played back, it can be run through an audio mixer if necessary, and then through an analog to digital (A/D) converter. Once the analog audio has been digitized, it is processed on a computer with audio software such as ProTools or Sony Sound Forge and stored on external hard drives, optical discs, or magnetic data tape before being sent to a RAID hard drive system or remote server (CDP, 2006). When choosing hardware and software to build a digitization workstation, it is important to recognize that all components of the analog-to-digital conversion process come with some amount of signal distortion (Weig, et al., 2007). Additionally, Weig et al. (2007) contend that documentation of the preservation process is of principal importance; for their project, a text file containing the technical specs of the conversion process accompanies each master file, and metadata is inscribed on the cassette sleeve as well as entered into a database; [and] an MD5 checksum is also created for each master file, allowing for later retrieval, should the original file ever be corrupted. Also, when deciding how web derivate files will be presented, it is essential to understand who will primarily be accessing the collection. Bond and the MASC project team kept their 9

10 target audience in mind, and selected the lowest Internet connection rates to the RealSystem Server 8 as 28K and 56K modems, to ensure that even those with the slowest connections could still access the recordings (Bond, 2004). Conclusion In response to an increasing amount of disturbing anecdotal evidence affecting the survival and preservation of sound recordings collected by the staff of the Library of Congress, the U.S. Congress passed The National Recording Preservation Act of 2000 (Public Law ), which affirmed the nation s collective interest in preserving sound recordings for posterity [and] established the Library of Congress National Recording Preservation Board (National Recording Preservation Board, 2010, p. vi). In the foreword to The State of Recorded Sound Preservation in the United States: A National Legacy at Risk in the Digital Age, a report by the National Recording Preservation Board (2010), James Billington, Librarian of Congress, writes: sound recordings have existed as one of the most salient features of America s cultural landscape for more than 130 years However, our collective energy in creating and consuming sound recordings in all genres has not been matched by an equal level of interest, over the same period of time, in preserving them for posterity (p. vi). This paper is only an introductory investigation of the challenges facing libraries, museums, and archives that embark on audio digitization projects. But, as demonstrated by the projects undertaken at the Louie B. Nunn Center for Oral History at the University of Kentucky Library and the MASC division of the Washington State University Libraries, there is a growing nationwide understanding of the importance of preserving audio recordings through digitization. However, such an understanding should be considered in its infancy. There is general agreement as to standards and best practices, but such standards are not always adopted, due to 10

11 philosophical differences over the necessity of 24/96 files versus 16/44.1 files and the high cost of adequate storage space for digital audio masters. It is the recommendation of this paper that further research be conducted in the area of audio digitization, not only for oral histories but also for music recordings, to more fully understand and broadly articulate to libraries, museums, and archives the importance of preserving audio recordings at the highest possible fidelity for present and future generations. 11

12 References Bond, T. J. (2004). Streaming audio from African-American oral history collections. OCLC Systems & Services: International Digital Library Perspectives, 20(1), doi: / CDP Digital Audio Working Group. (2006). Digital audio best practices: Version Consortium of Academic and Research Libraries in Illinois. (2009, Nov. 1). Guidelines for the creation of digital collections: Digitization best practices for audio. CARLI Digital Collections Users Group, 1-13 National Recording Preservation Board. (2010). The state of recorded sound preservation in the United States: A national legacy at risk in the digital age. Washington, DC: Council on Library and Information Resources, Library of Congress. Stevens, K. W., & Latham, B. (2009). Giving voice to the past: digitizing oral history. OCLC Systems & Services: International Digital Library Perspectives, 25(3), University of Kentucky Libraries. (2011). Louie B. Nunn Center for Oral History. Accessed February 25, 2012: Weig, E., Terry, K., & Lybarger, K. (2007, May/June). Large scale digitization of oral history: A case study. D-Lib Magazine, 13(5/6). doi: /may2007-weig 12

GUIDELINES FOR THE CREATION OF DIGITAL COLLECTIONS

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