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Best Practices in Digital Preservation of the Spoken Word

http://www.historicalvoices.org/oralhistory

EMELD Workshop of Digitizing Lexical InformationAugust 2002

Bartek Plichta, Michigan State University

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Why Best Practices?

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Why Best Practices?

• Highest possible audio “quality” of the speech signal.

• Platform and hardware-independent storage format.

• Platform and hardware-independent storage medium.

• Comprehensive metadata.

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Part I. Recording Speech

Common recording situations: – Field recording

– Studio and lab recording

– Telephone recording

Issues to be considered:– Recording techniques

– Hardware

– Software

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Analog or Digital? Acoustic properties of the speech signal:

– Frequency response: < 10000 Hz– Dynamic range: 30-40 dB

Analog tape:– Frequency response: < 10000-15000 Hz– Dynamic range: 45 dB

DAT (Digital Audio Tape, 16 bit, 48 kHz):– Frequency response: < 24000 Hz– Dynamic range: 96 dB

Digital telephone (ISDN)– Frequency response: < 4000 Hz– Dynamic range: 48 dB

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Microphones

• omnidirectional built-in

• omnidirectional lavalier

• hand-held dynamic

• hand-held condenser

• head-set dynamic or condenser

• highly directional shotgun

• digital telephoneRoom noise

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Recommended Microphones

1. Unidirectional (cardioid polar pattern) head-worn microphone.

– Shure SM 10A or AKG C 420

2. Unidirectional shotgun (highly directional polar pattern)

– Shure SM 89

3. Unidirectional dynamic or condenser– Shure SM 58 or AKG C 1000S

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Microphone PreamplifierA premium quality microphone preamp is CRUCIAL

to obtaining a reliable speech signal– 2 balanced XLR inputs – High gain (< 65 dB)– Phantom power (+48 V)

Built-in preamps (generally not recommended)– Marantz PMD222– TASCAM DA-P1

Stand-alone preamps– Symetrix 628– M-Audio DMP2– Shure FP24

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Recorders

Avoid using automatic levels settings, EQ, Dolby, etc.

• Field– Portable analog [Marantz PMD222]– Portable DAT (16 bit, 48 kHz) [TASCAM DA-P1]– Portable Hard Disk (24 bit, 48 kHz) [USB Pre]

• Studio/Lab– Reel-to-reel analog– Hard disk (24 bit, 48 kHz) [stand-alone ADC]

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Recording Technique

• Position the directional microphone close to the talker’s lips, 45 degrees off to the side, keeping the distance constant.

• Avoid low-frequency noise (refrigerator, traffic, fluorescent light buzz, computer hum)

• Use ONLY balanced XLR cables.• Use reliable microphone stands and clips.• Use manual gain control if possible.• Monitor ADC gain control to avoid clipping.• ALWAYS monitor your input with an earpiece.

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Part II. Processing

Analog to Digital Conversion

The main goal of A/D conversion is to obtain THE BEST POSSIBLE digital representation of the analog original for

the purposes of:

• Preservation

• Analysis

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Recommended Digitization Settings

• Sample rate: 48 kHz (96 KHz even better)

• Quantization: 24 bit

• Hardware: stand-alone, oversampling delta-sigma A/D converter with dither added prior to sampling.

• S/PDIF I/O interface.

• Store in an uncompressed (PCM) digital audio file format.

PC noise

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Processing

• Preservation– No further processing necessary.– Store as 48 kHz, 24 bit wav, aiff, or headerless

(e.g. raw)

• Analysis– Save as 16 bit, signed wav, aiff or raw– Downsample to 11025 Hz with anti-alias filter– Apply restoration processing if necessary

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Restoration processing

Restoration processing must be applied carefully to avoid removing information from the speech

signal itself.

• Hiss removal• Click and crackle removal• Clipped peak restoration• Volume normalization

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Example: preparing a DARE tape for analysis

• Original digitized at 48/24

• Converted to 16 bit

• Downsampled to 11025 Hz

• Band-passed for low and high frequency noise

• 2:1 compression starting at –15 dB

• Volume adjustment

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Before and After

• DM 0735 – S1 tape from DARE (Michigan)

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Recommended Software

Operating Systems:Windows 2000 Professional, Mac 9.1, Linux RedHat

Audio Editors:– Windows – GoldWave 2.24– Mac OS – Peak VST

Analysis Software:– Windows – Praat, WaveSurfer, MultiSpeech, MatLab,

SpeechStation2, Spectrogram 6.0– Mac OS – Praat– Linux – Praat, MatLab

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What to Avoid

• Using inexpensive, generic hardware (microphones, portable recorders, cables, sound cards, etc.)

• Outputting digital audio through analog D/A outputs (e.g., DAT to PC transfer)

• Capturing audio directly into analysis software• Ignoring metadata

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Metadata

• It is recommended to ALWAYS enter metadata in a common database or XML format.

• IDEALLY, metadata should be encoded in an OAI-compliant format (OLAC, METS)

• What about MPEG 7?

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METS

•Descriptive metadata•Administrative metadata•File groups•Structural Map•Behavior

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METS – Audio metadata

•File group•One master => many derivatives

•Structural map•Time alignment

•Technical metadata•Platform and hardware-independent storage

•Digital Provenance•How? Why? Who?

•Behavior •Executable code in metadata

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Bibliography

Harris, M.N. and R. Kelly, D.A.McLeod, M.J.Story. 1998. “Effects in High Sample Rate Audio Material”. DCS Ltd.

Johan, Lilkencrants. 1997 “Speech Signal Processing.” The Handbook of Phonetic Sciences. Eds. Hardcastle William and Laver John. Oxford: Blackwell.

Karl, J.H. 1989. An Introduction to Digital Signal Processing. Academic Press.Plichta, Bartek. 2001. “Digitizing Speech Recordings for Archival Purposes”. Working Paper. Matrix,

Michigan State University.Pohlmann, Ken. 2000. Principles of Digital Audio. New York: McGraw-Hill.Rabiner, Lawrence R., and Ronald W. Schafer. 1978. Digital Processing of Speech Signals. Englewood

Cliffs: Prentice-Hall.SONY Corporation. 1993. “Minidisk Specifications”. Digital Audio Disk Corporation.Stevens, Kenneth N. 1998. Acoustic Phonetics. Cambridge, Mass.: MIT Press. Story, Mike. 1997. “A Suggested Explanation for (Some of) the Audible Differences between High

Sample Rate and Conventional Sample Rate Audio Material”. DCS Ltd.Story, Mike and R. Kelly, D.A.McLeod. 1998. “Resolution, Bits, SNR and Linearity”. DCS Ltd.Titze, Ingo. 1994. Workshop of Acoustic Voice Analysis. National Center for Voice and Speech.Vanderkooy, J., and S.P. Lipshitz. 1984 “Dither in Digital Audio”. JAES, vol.32, no. 11.

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Built-in, omni directional microphone

‘Bob was positive that he heard his wife...’

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Omnidirectional, lavalier microphone

‘Bob was positive that he heard his wife...’

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Dynamic microphone, Shure SM58

‘Bob was positive that he heard his wife...’

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Unidirectional, shotgun microphone, Shure SM89

‘Bob was positive that he heard his wife...’

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Head-worn unidirectional microphone

‘Bob was positive that he heard his wife...’

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Digital telephone

‘Bob was positive that he heard his wife...’

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Room Noise

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LPC Comparison

f1 (formant + bandwidth Hz) f2 (formant + bandwidth Hz) built-in mic 871; 138 1670; 263 head-set mic 521; 146 1770; 45

Built-in

Head-set

Built-in

Head-set

frame length = 20ms, filter order = 12, pre-emphasis = 0.9

“Shannon”