Digital Audio G6DPMM
G6DPMM - Lecture 14 Digital Audio
Properties of Sound • Acoustics • Pressure waves in air • Pitch - frequency - time (kHz) • Amplitude - level - energy (dB) • Subjective Volume - frequency and energy
Analogue vs Digital • Analogue technologies • Physical • Magnetic • Digital technologies • Sample sound • Approximation of analogue • Must be captured (a2d conversion)
Sound in Multimedia • MIDI • Defines the synthesis of sound • Analogous to vector graphics • Digital Audio • Captured waveform • Analogous to bitmap graphics
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).
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.
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.
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
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”
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)
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).
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.
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
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).
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)
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