EET 450 Chapter 18 – Audio

Analog Audio Sound is analog Consists of air pressure that has a variety of characteristics  Frequencies  Amplitude (loudness)  Distortion Humans hear roughly in the range of 20 Hertz to 20,000 hertz

Analog Audio The sounds we hear, are actually combinations of many frequencies, added together This includes harmonic frequencies, which may be outside the audible range of a particular user.

Digital Audio Sound is ‘created’ by the computer from a digital source. In its simplest form, sound is created and/or stored on the computer in a digital form

Uses for Audio Adapters Games Sound effects for presentations Midi Music (hardware & software) Audio conferences and telephony OS Event sounds Document to audio

Disabled access Audio CD’s MP3 files AV clips DVD Films Voice control Speech to text

Quality of sound card Frequency response kHz bandwidth Total Harmonic Distortion Signal to Noise ratio Sampling rate 11kHz, 22kHz, 44kHz 8 bit versus 16 bit audio

PCM – Pulse Code Modulation A sound is ‘sampled’ periodically for amplitude. This amplitude is a digital representation of the VOLTAGE delivered to a transducer  Transducer = speaker If the sample rate is high enough, the output (integrated by the transducer) reproduces the original frequency or frequencies.

PCM Nyquist – sampling rate = 2 x base freq This can be reproduced back into the original sound Dynamic Range & resolution Adding the variety of frequencies back together may reconstruct a stored sound Resolution will allow accurate rebuilding

Computer sound Store either as a raw PCM data stream or as information needed to synthesize the original sound. Wave Table synthesis Waves are stored on sound card hardware Information is passed to card which calls up the particular wave

Other Issues MIDI FM synthesis (Sound Blaster) Wave Table (Ensonic) Compression

Moving Picture Experts Group MPEG1 digital storage media and bit rates at up to about 1.5 Mbits/s MPEG 2 – DVD audio quality data rates below 10 Mbit/s MPEG3 (MP3) – audio standard

MPEG4 – released 1998? MPEG-4 addresses the need for  Universal accessibility and robustness in error prone environments  High interactive functionality  Coding of natural and synthetic data  Compression efficiency MPEG7 Multimedia Content Description Interface

Computer Sound System Needs chips to generate sound D/A converters May include input A/D converter Amplifier to output sound to speaker system

Connections Stereo/Audio Line out Stereo/Audio Line in Speaker connection Amplified Microphone (mono) input Joystick MIDI connection Internal CD

Advanced features MIDI Input/output SPDIF – Sony/Phillips Digital Interface AKA Dolby Digital CD SPDIF – CD Rom drive with SPDIF output TAD in Telephone Answering Device Digital DIN output

Advanced features Aux in I2S in – Digital Audio input from external source

Speakers Specific transducers have frequency responses in a narrow range Tweeter Mid-Range Woofer Sub-Woofer

Microphones Low quality High quality microphones require interface electronics

Sound Blaster Resources DeviceInterruptI/O16bit DMA 8bit DMA AudioIRQ5220h-223hDMA5DMA1 MIDI Port-330h-331h-- FM Synth-388h-38Bh-- Game Port -200h-207h--

Ensoniq PCI Resources DeviceInterruptI/O16bit DMA 8bit DMA AudioIRQ5DC80h-DCBFh-- MIDI PortIncluded in above-- FM SynthIncluded in above-- Game Port -200h-207h--

Other Terms CODEC Coder / Decoder Streaming Audio Microphones