Audio Technology

Audio Technology

introduction

Iwan Sonjaya,MT

What is sound?Sound is a physical phenomenon caused by vibration of material (ex.: violin). As the matter vibrates, pressure variations are created in the air around it. The pressure waves propagate in the air. When a wave reaches the human eardrums, a sound is heard.

Ear: receive 1-D waves.Cochlea: convert to frequency dependent nerve firings, sent to the brain.Suara (Sound)Fenomena fisik yang dihasilkan oleh getaran bendaGetaran suatu benda yang berupa sinyal analog dengan amplitudo yang berubah secara kontinyu terhadap waktu

The wave form occurs repeatedly at regular interval or periods. A sound with a recognizable periodicity is called music. Non-periodic sounds called noises.periodamplitudetimeAirpressureA sound frequency is the reciprocal value of its period. The frequency represents the number of periods per seconds and is measured in hertz (Hz). A kHz describes 1000 oscillations per second or 1000 Hz.Frequency RangesThe frequency range is divided into: Infrasonic: 0 to 20 Hz Audio-sonic: 20Hz to 20 kHz (Human hearing frequency) Ultrasonic: 20kHz to 1 GHz Hypersonic: 1GHz to 10 THzIn multimedia we are concerned with sounds in the audiosonic range.Frequency-Change of pitch

AmplitudeThe amplitude of the sound is the displacement of the air pressure from its quiescent state, which humans perceive subjectively as loudness or volume. Sound pressure levels are measured in (db).OR The amplitude of a sound is a measuring unit used to deviate the pressure wave from its main value.

0 db - no sound20 db - rustling of paper35 db - quiet home70 db - noisy street130 db pain threshold

Amplitude-determines loudness of soundAmplitude-change volume

Audio Representation on ComputersA computer measures the amplitude of the waveform at regular time intervals It then generates a series of sampling values. The mechanism that converts an audio signal into digital samples is the analog-to-digital converter (ADC). A digital-to-analog converter (DAC) is used to achieve the opposite conversion.Audio SamplingDetermine number of samples per second;At each time interval determine the amplitude;Stored the sample rate and the individual amplitudes.Sampling Rate: The number of samples per second. The CD standard rate of 44100 Hz means that the wave form is sampled 44100 times per second.Quantization: is a value of the sample. The resolution of a sample value depends on the number of bits used in measuring the height of the waveform. The sampled waveform with a 3 bits quantization results in only eight possible values: 0.75, 0.50, 0.25, 0.00, - 0.25, - 0.50, -0.75 and -1. An 8 bit quantization yields 256 possible values, 16 bit result in over 65536 values.

Quantization introduces noiseThe lower quantization, the lower quality of the sound.File size versus quality 1: Sampling at higher rates more accurately captures the high frequency content. 2: Audio resolution determines the accuracy with which a sound can be digitized. 3: Using more bits yields a recording that sounds more like its original. 4: High sample rate with high resolution = large files.

Here are the formulas for determining the size (in bytes) of a digital recording:For monophonic recording: Sampling rate * duration of recording in second * bit resolution / 8For stereo recording: Sampling rate * duration of recording in second * bit resolution / 8 * 2Thus the formula for a 10 second recording at 22.05 KHz, 8 bit resolution would be: 22050 * 10 * 8/8 * 1 = 220, 500 bytesA ten second stereo recording at 44.1 KHz, 16 bit resolution would be : 44100 * 10 * 16 / 8 * 2 = 1,764, 000 bytes

3D Sound Projection: The shortest path between sound source and the auditor is called the direct sound path. All other sound paths are reflected which means they are temporarily delayed before they reach the auditor's ear.MIDI ( Musical Instrument Digital Interface ) versus digital audioThe MIDI is a small piece that plugs directly into the computers serial port and allows the transmission of music signals. Note that: MIDI does not produce sound, only produce the parameters that are needed to be sent to the device that translates those numbers into sound.Physical Specification: 1: 5 pin DIN a - pin 2: ground b - pins 4 and 5: data c - pins 1 and 3: unconnected 2: Shielded twisted pair of 50 feet max length

Electrical Specification: 1: Asynchronous serial interface. 2: 8 data bits, 1 start bit, and 1 stop bit 3: Logic 0 is current ON 4: Rise and fall time