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

GamesSound effects for presentationsMidi Music (hardware & software)Audio conferences and telephonyOS Event soundsDocument to audio

Disabled accessAudio CD’sMP3 filesAV clipsDVD FilmsVoice controlSpeech to text

Quality of sound card

Frequency response kHz bandwidth

Total Harmonic DistortionSignal to Noise ratioSampling 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 soundD/A convertersMay include input A/D converter

Amplifier to output sound to speaker system

Connections

Stereo/Audio Line outStereo/Audio Line inSpeaker connection Amplified

Microphone (mono) inputJoystickMIDI connectionInternal CD

Advanced features

MIDI Input/output SPDIF – Sony/Phillips Digital Interface AKA Dolby Digital

CD SPDIF – CD Rom drive with SPDIF outputTAD in Telephone Answering DeviceDigital DIN output

Advanced features

Aux inI2S 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 qualityHigh quality microphones require interface electronics

Sound Blaster Resources

Device Interrupt

I/O 16bit DMA

8bit DMA

Audio IRQ5 220h-223h

DMA5 DMA1

MIDI Port - 330h-331h

- -

FM Synth - 388h-38Bh

- -

Game Port

- 200h-207h

- -

Ensoniq PCI Resources

Device Interrupt

I/O 16bit DMA

8bit DMA

Audio IRQ5 DC80h-DCBFh

- -

MIDI Port Included in above - -

FM Synth Included in above - -

Game Port

- 200h-207h - -

Other Terms

CODEC Coder / Decoder

Streaming AudioMicrophones