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Exploring Digital Audio: Properties, Creation, and Compression

Learn about digital audio properties, MIDI, sampling, distortion, editing, compression techniques, codecs, and file formats. Understand the advantages and disadvantages of MIDI and digital audio. Discover common codecs and file formats used to compress and store digital audio files.

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Exploring Digital Audio: Properties, Creation, and Compression

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  1. Digital Audio G6DPMM

  2. G6DPMM - Lecture 14 Digital Audio

  3. Properties of Sound • Acoustics • Pressure waves in air • Pitch - frequency - time (kHz) • Amplitude - level - energy (dB) • Subjective Volume - frequency and energy

  4. Analogue vs Digital • Analogue technologies • Physical • Magnetic • Digital technologies • Sample sound • Approximation of analogue • Must be captured (a2d conversion)

  5. Sound in Multimedia • MIDI • Defines the synthesis of sound • Analogous to vector graphics • Digital Audio • Captured waveform • Analogous to bitmap graphics

  6. MIDI • Musical Instrument Digital Interface. • Standard for synthesisers and electronic instruments and devices. • MIDI “score” describes time stamped sequence of notes. • Reproduction dependent upon hardware (MIDI device).

  7. Advantages of MIDI • Very compact (up to 1,000 times smaller than CD audio) • Sounds produced by hardware - therefore low system overhead. • Low bandwidth requirements. • High quality - dependent upon hardware. • Scaleable, Editable & cross platform.

  8. Disadvantages of MIDI • Hardware dependent - only a faithful reproduction if the equipment is constant. • Cannot record “real world” sounds. • Difficult to represent speech.Creation of MIDI • Via editing software (sequencer) • Via instruments / peripherals • Usually requires musicians or sound effects specialists.

  9. Digital Audio • Sampled Sound • At regular time intervals a sample is taken, and the information is stored digitally. • Applications • CD • Digital tape (DAT) • Digital broadcast (radio / TV) • Most multimedia sound

  10. Sampling (capturing) • Sampling hardware - from any analogue source (usually line in). • Sampling rate • frequency of samples • often called “frequency” • Sample size • amount of information stored. • often called “resolution”

  11. Sampling Rate

  12. Sample Size

  13. Sampling Parameters • Common Sample Rate (frequency) • 44.1 kHz (CD audio) • 22.05 kHz • 11.025 kHz • Common Sample Sizes (resolution) • 8 bit (256 amplitude states) • 16 bit (65,536 amplitude states)

  14. Examples

  15. Distortion • Distortion is caused when the reconstruction of the waveform is unacceptable. • Usually arises from incorrect settings of disparate equipment (amplitude / levels). • Also caused by insufficient sampling frequency (Nyquist theory).

  16. The Nyquist Rule

  17. Implications of Nyquist • Each half of the waveform must be recorded •  there must be 2 samples per period •  the sampling frequency must be at least twice the highest signal frequency. • For example:if the highest frequency is 14,080 Hz, then at least 2x14.08 = 28.16 kHz must be used. Thus 22.05 would not suffice - 44.1 would be needed.

  18. Editing Digital Sound • Wide range of software • Commercial (eg Sonic Foundry Sound Forge) • Shareware/Freeware • Bundled with hardware • Common Operations • Trimming • Splicing and assembly • Volume adjustments • Downsampling • Fades & other effects

  19. Sound Compression • In principle very similar to image or movies • Usually (not always) lossy • Requires codec for compression and playback • Streaming is commonplace and well developed (eg RealAudio, QuickTime, MS Media Player).

  20. Common Codecs • GSM - mostly for voice (3.5-28 Mb per hour) • ADPCM (Microsoft or IMA) - high quality, but low compression (14-152 Mb per hour) • Lernout & Hauspie - voice only (3.5-7 Mb per hour) • CCIT A-Law - European TAPI devices, very high quality (24-302 Mb per hour) • CCIT -Law - American TAPI devices, very high quality (24-302 Mb per hour)

  21. Common File Formats • Very large number • Audio IFF (AIFF) - developed by Apple for Macintosh, also used by SGI and various software. • AU Audio - uncompressed format developed by Sun, widely used on the Internet. • WAV - developed by Microsoft for Windows. • MP3 - highly compressed, high quality spin-off of the MPEG project • RealAudio RAM - developed for streaming by RealMedia. • Differ in terms of: • Compression algorithms • Metadata • Security & Encryption

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