EET 450 Chapter 18 – Audio. Analog Audio Sound is analog Consists of air pressure that has a...
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Transcript of EET 450 Chapter 18 – Audio. Analog Audio Sound is analog Consists of air pressure that has a...
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EET 450
Chapter 18 – Audio
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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
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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.
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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
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Uses for Audio Adapters
GamesSound effects for presentationsMidi Music (hardware & software)Audio conferences and telephonyOS Event soundsDocument to audio
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Disabled accessAudio CD’sMP3 filesAV clipsDVD FilmsVoice controlSpeech to text
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Quality of sound card
Frequency response kHz bandwidth
Total Harmonic DistortionSignal to Noise ratioSampling rate 11kHz, 22kHz, 44kHz
8 bit versus 16 bit audio
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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.
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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
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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
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Other Issues
MIDI FM synthesis (Sound Blaster) Wave Table (Ensonic)
Compression
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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
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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
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Computer Sound System
Needs chips to generate soundD/A convertersMay include input A/D converter
Amplifier to output sound to speaker system
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Connections
Stereo/Audio Line outStereo/Audio Line inSpeaker connection Amplified
Microphone (mono) inputJoystickMIDI connectionInternal CD
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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
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Advanced features
Aux inI2S in – Digital Audio input from external
source
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Speakers
Specific transducers have frequency responses in a narrow range Tweeter Mid-Range Woofer Sub-Woofer
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Microphones
Low qualityHigh quality microphones require interface electronics
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Sound Blaster Resources
Device Interrupt
I/O 16bit DMA
8bit DMA
Audio IRQ5 220h-223h
DMA5 DMA1
MIDI Port - 330h-331h
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FM Synth - 388h-38Bh
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Game Port
- 200h-207h
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Ensoniq PCI Resources
Device Interrupt
I/O 16bit DMA
8bit DMA
Audio IRQ5 DC80h-DCBFh
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MIDI Port Included in above - -
FM Synth Included in above - -
Game Port
- 200h-207h - -
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Other Terms
CODEC Coder / Decoder
Streaming AudioMicrophones