1. 1 Best Practices in Digital Preservation of the Spoken Word http://www.historicalvoices.org/oralhistory http://www.historicalvoices.org/oralhistory EMELD Workshop of Digitizing Lexical Information August 2002 Bartek Plichta, Michigan State University

2. 2 Why Best Practices?

3. 3 Highest possible audio “quality” of the speech signal. Platform and hardware-independent storage format. Platform and hardware-independent storage medium. Comprehensive metadata.

4. 4 Part I. Recording Speech Common recording situations: –Field recording –Studio and lab recording –Telephone recording Issues to be considered: –Recording techniques –Hardware –Software

5. 5 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

6. 6 Microphones omnidirectional built-in omnidirectional lavalier hand-held dynamic hand-held condenser head-set dynamic or condenser highly directional shotgun digital telephone Room noise

7. 7 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

8. 8 Microphone Preamplifier A 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

9. 9 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]

10. 10 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.

11. 11 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

12. 12 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

13. 13 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

14. 14 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

15. 15 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

16. 16 Before and After DM 0735 – S1 tape from DARE (Michigan)

17. 17 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

18. 18 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

19. 19 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?

20. 20 METS Descriptive metadata Administrative metadata File groups Structural Map Behavior

21. 21 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

22. 22 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.

23. 23 Built-in, omni directional microphone ‘Bob was positive that he heard his wife...’

24. 24 Omnidirectional, lavalier microphone ‘Bob was positive that he heard his wife...’

25. 25 Dynamic microphone, Shure SM58 ‘Bob was positive that he heard his wife...’

26. 26 Unidirectional, shotgun microphone, Shure SM89 ‘Bob was positive that he heard his wife...’

27. 27 Head-worn unidirectional microphone ‘Bob was positive that he heard his wife...’

28. 28 Digital telephone ‘Bob was positive that he heard his wife...’

29. 29 Room Noise

30. 30 LPC Comparison Built-in Head-set Built-in Head-set frame length = 20ms, filter order = 12, pre-emphasis = 0.9 “Shannon”