SWE 423: Multimedia Systems Chapter 3: Audio Technology (2)

Audio Signal Representation Waveform representation –Focuses on the exact representation of the produced audio signal. Parametric form representation –Focuses on the modeling of the signal generation process. –Two major forms Music synthesis (MIDI Standard) Speech synthesis

Waveform Representation Audio Capture Sampling & Digitization Storage or Transmission Receiver Digital to Analog Playback (speaker) Audio Source Human Ear Audio Generation and Playback

Digitization To get audio (or video for that matter) into a computer, we must digitize it (convert it into a stream of numbers). This is achieved through sampling, quantization, and coding.

Example Signal Amplitude

Sampling Sampling: The process of converting continuous time into discrete values.

Sampling Process 1.Time axis divided into fixed intervals 2.Reading of the instantaneous value of the analog signal is taken at the beginning of each time interval (interval determined by a clock pulse) 3.Frequency of clock is called sampling rate or sampling frequency The sampled value is held constant for the next time interval (sampling and hold circuit)

Sampling Example Amplitude

Quantization The process of converting continuous sample values into discrete values. –Size of quantization interval is called quantization step. –How many values can a 4-bit quantization represent? 8-bit? 16-bit? The higher the quantization, the resulting sound quality

Quantization Example Amplitude

Coding The process of representing quantized values digitally

Analog to Digital Conversion Amplitude

MIDI Interface Musical sound can be generated, unlike other types of sounds. Therefore, the Musical Instrument Digital Interface standard has been developed –The standard emerged in its final form in August 1982 –A music description language in binary form A given piece of music is represented by a sequence of numbers that specify how the musical instruments are to be played at different time instances.

MIDI Components An MIDI studio consists of –Controller: Musical performance device that generates MIDI signal when played. MIDI Signal: A sequence of numbers representing a certain note. –Synthesizer: A piano-style keyboard musical instrument that simulates the sound of real musical instruments –Sequencer: A device or a computer program that records a MIDI signal. –Sound Module: A device that produces pre-recorded samples when triggered by a MIDI controller or sequencer

MIDI Components

MIDI Data Describes –Start/end of a score –Intensity –Instrument –Basis frequency –.... Header Chunk Track Header Track Chunk Track Header Track Chunk Track 1Track 2 Status Byte Data Bytes Status Byte Data Bytes MIDI File Organization Actual Music Data

MIDI Data MIDI standard specifies 16 channels –A MIDI device is mapped onto one channel E.g. MIDI Guitar controller, MIDI wind machine, Drum machine. –128 instruments are identified by the MIDI standard Electric grand piano (2) Telephone ring (124) Helicopter (125) Applause (126) Gunshot (127)

MIDI Instruments Can play 1 single score (e.g. flute) vs. multiple scores (e.g. organ) Maximum number of scores that can be played concurrently is an important property of a synthesizer –3..16 scores per channel.

3D Sound Projection After experimentation, it was shown that the use of a two-channel sound (stereo) produces the best hearing effects for most people.

Spatial Sound Direct sound path: The shortest path between the sound source and the auditor. –Carries the first sound waves to the auditors head. All other sound paths are reflected All sound paths leading to the human ear are influenced by the auditor’s individual head-related transfer function (HRTF) –HRTF is a function of the path’s direction (horizontal and vertical angles) to the auditor. Pulse response in a closed room