Sound and Digital Sound v © Allan C. Milne Abertay University

Agenda Visualising Sound Human Perception Digital Sound Sampling Issues Notes available in WhatIsSound.doc WhatIsDigitalSound.doc

Sound is A form of energy which propagates through a host medium in the form of longitudinal waves –form of energy –propagates through a medium –longitudinal waves

Visualising Sound Longitudinal waves are difficult to draw and hence to visualise. Visualise as a transverse wave plotting density against time at a fixed point in the medium.

Human Perceptions Loudness is the human perception of amplitude –Measured in decibels (dB): a logarithmic scale –Note 0dB is not absolute silence but rather a reference point –Loudness perception depends on frequency Pitch is the human perception of frequency –We perceive from about 20Hz to 20,000Hz (20kHz) –Pitch is closely related to frequency but not synonymous, other factors intrude

Digital Sound Sound is an analogue property We must have a digital representation for use within a computer –Saving –Processing –Recording –Playback The conversion process from analogue sound to digital representation is known as sampling

Sampling Overview Microphone picks up sound waves and outputs an analogue electrical signal The analogue signal is sampled at fixed clock intervals (usually many kHz) At each clock tick the analogue signal’s amplitude is measured The measurement amplitude is output as a binary integer

Some Terminology Analogue to digital converter (ADC): the device that samples the analogue signal & outputs the stream of binary integers Sampling frequency: the rate at which the clock ticks to sample the signal Resolution or bit depth: the number of bits used for one binary sample Example: a 16-bit samples at 11kHz

General Audio Pipeline. Original sound  microphone  ADC  wav file  pre-production  audio program code  soundcard  DAC  amp  speakers  sound

Aliasing High frequency sounds are transformed to low-frequencies if too small a sampling rate is used If the sound frequency is greater than ½ the sampling rate then new freq = sample freq – original freq Thus to recreate the original sound the sampling frequency must be at least twice the source sound frequency

Quantisation & Distortion Quantisation noise: sampling may not recreate the exact level of the original signal due to the limited resolution (bit- depth) used Digital distortion: more unpleasant than analogue distortion since it overflows / underflows the binary sample values

Digital Playback Samples are streams of binary values Converted by a digital to analogue converter (DAC) to an analogue signal The signal then passes through a low- pass smoothing filter to remove quantisation noise The signal can now be presented to an amplifier & loudspeakers

Development Domains. Audio design. –Pre-production. Game programming. –Framework & engine programming.

Tools & Environments. Sound composers. –Sound editors. XACT. –XAudio2 –OpenAl –DirectSound. »Core APIs.