Chapter 6. Meeting 6, Recording: Digital Audio 6.1. Announcements • Next quiz will be next week 6.2. Quiz Review • ? 6.3. Listening: League of Automatic Composers • Listening: League of Automatic Music Composers, “Oakland One,” League of Automatic Music Composers 1978-1983 • The League of Automatic Music Composers: founded in the 1970s by Jim Horton and including John Bischoff, Tim Perkins, and Rich Gold (Holmes 2008, p. 276) • Made use of the KIM-1, created by MOS Technologies in 1975 (Holmes 2008, p. 275) 137Source: Wikipedia, by user en:Wtshymanski. Public domain image. 138Source: Wikipedia, by user Swtpc6800. Public domain image. 1396.4. Basics of Digital Encoding • Digital is discrete, analog is continuous • Take discrete time samples of a smooth analog signal • Each sample measures amplitude at a point in time • Time interval (spacing) is constant; often given as a rate in samples per second • Amplitude steps are positive or negative values within a fixed range of values • Encoding (analog to digital conversion) is always lossy • Decoding (digital to analog conversion) my repair some of the loss • PCM: Pulse Code Modulation 6.5. Two Parameters of Digital Encoding: Sampling Rate • Sampling rate • Bit depth 6.6. Two Parameters of Digital Encoding: Sampling Rate • Sampling rate • How quickly amplitudes are measured, or the time resolution 140Source: Ballora, Mark. Essentials of Music Technology. © Prentice Hall, 2002. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. 141• Determines what frequencies can be recorded: higher sampling rates can record higher frequencies • Doubling the sampling rate doubles the amount of data stored • Measured in Hertz (samples per second) • Examples: 44100 Hertz (CD Audio), 48000 Hertz, 88.2k, 96k 6.7. Two Parameters of Digital Encoding: Bit Depth • Bit depth • How accurate are amplitude measurements when sampled 142Source: Ballora, Mark. Essentials of Music Technology. © Prentice Hall, 2002. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. • Determines what range of amplitudes can be recorded, or dynamic range: higher bit depths can record more dynamic range • Doubling the bit depth doubles the amount of data stored • Measured in bits • Examples: 16 bit (CD Audio), 24 bit, 32 bit 1436.8. Encoding and Decoding • Encoding: smooth analog wave forms are measured in discrete samples Figure 2 in Tenney, James C. "Sound-Generation by Means of a Digital Computer." Journal of Music Theory 7, no. 1, 24-70.Copyright 1963, Yale University. Reprinted by permission of the present publisher, Duke University Press.• Decoding: digital information converted to pulses that are filtered into smooth analog wave forms 1446.9. Sampling Rates: The Nyquist Theorem • The highest frequency we can record is about half the sampling rate • To sample a sine wave, at least two points per cycle must be sampled 145 Figures 3 and 4 in Tenney, James C. "Sound-Generation by Means of a Digital Computer." Journal of Music Theory 7, no. 1, 24-70.Copyright 1963, Yale University. Reprinted by permission of the present publisher, Duke University Press.• Trying to sample a frequency higher than the sampling rate leads to confusion: new frequencies are generated below the sampling rate • Example: 44100 Hertz can record up to 22055 Hertz 6.10. The Limits of Sampling • Resampling an audio signal at the audio rate [samplingRate.pd] 6.11. Bit Depth: Dynamic Range • The bit depth determines dynamic range • 16 bit audio has 96 dB dynamic range (65536 discrete volume levels) • 24 bit audio has over 110 dB dynamic range • Each additional bit adds about 6 dB 1466.12. A Bit on Bits • Bits are a way of storing binary numbers • The number of bits tells us how many numbers (things, positions, values) available • One bit encodes two possible values: 0/1 • Two bits encode four possible values: 00/01/10/11 • Three bits encode eight possible values: 000/001/010/011/100/101/110/111 • 4 bits encode 16 possible values • 8 bits (or one byte) encode 256 possible values • 16 bits encode 65,536 possible values • 24 bits encode 16,777,216 possible values • In general: 2bits = possible values 6.13. The Sound of Degradation • Sample rate reduction and bit smashing • Both make curves more square • Making curves more square adds high frequencies [samplingReduction.pd] 1476.14. Listening: Alva Noto • Listening: Alva Noto, “Xerrox Meta Phaser,” Xerrox Vol. 2, 2008 6.15. Digital Storage • Audio files store digital sound information • Can store multiple channels of digital audio in a single file • Two components necessary 148• Header information: sampling rate, bit depth, number of channels • Data: a list of amplitude measurements • Playback system must properly interpret header and read sample data • Digital information can be stored on magnetic, optical, or other mediums 6.16. Digital Storage Size • MB / min == Sampling rate * bytes per sample * channels * 60 (s/min) • 44100 * 2 (16 bit) * 2 * 60 == 10 MB / min 6.17. Common PCM Digital Storage Formats • A long list of sample values, with header information describing channels, sampling rate, and bit depth • PCM Formats: AIFF, WAVE, others • May have bit depth from 8 to 32, may have sampling rates from 22050 to 96000 • Compressed, non-PCM formats: MP3, M4A, MWA, OGG 6.18. History of Digital Audio • 1928: Harold Nyquist at Bell Labs develops Nyquist Theorem • 1938: A. Reeves develops first patented pulse code modulation technique for message transmission • 1950s: Max Mathews at Bell Telephone Labs generates first synthetic sounds from a digital computer • First digital one-channel audio recorder demonstrated by NHK in Japan • 1973: Nippon Columbia (Denon) has digital audio recorder based on 1 inch video tape 149© Denon. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. • 1977: first commercial digital recording system: Sony PCM-1: encode 13 bit digital audio onto Sony Beta videocassette recorders © Sony. All rights reserved. This content is excluded from our Creative Commons license. For more