Sound and Digital Audio.ppt

7/27/2019 Sound and Digital Audio.ppt

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From the air to the iPod


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Minute disturbances in the air, caused bya vibrating object

Air molecules bunch together, thenspread out

Changes in density (air pressure, orsound pressure)

Causes a chain reaction; sound pressurewave propagates

What is Sound?


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Sound Wave Propagation


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Compression / Rarefaction

High

Low

Normal

Sou

ndPressure

Time

Time domain plot of a waveform


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Periodic Waveforms

1 Cycle

Period:

Frequency:

How long does one cycle last?

How many cycles per second?

Expressed in Hertz (Hz)Ex: 440 Hz (the A above middle C)

Period = 1 / Frequency

(for A440: 0.0023 sec.)

Amplitude


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Frequency and Pitch

Frequency is an acoustic fact.Pitch is a human perception.

Our sense of pitch has a logarithmic relation tofrequency its based on ratios.

Our ear is like a microphone. It changes thephysical wave (acoustic energy) into an electricalsignal.

Range of human hearing

20Hz to 20,000 Hz (20 kHz), approximately(for young folks; old folks cant hear as high)


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The Harmonic Series

Arithmetic series of frequencies givesever-shrinking intervals.

Frequencies in Hz:

Double frequency: octave higher

(flat)


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Analog Representations of Sound

Magnified phonograph grooves, viewed from above:

When viewed from the side, channel 1 goes upand down, and channel 2 goes side to side.


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Analog versus Digital

Analog

Continuous signal that mimics shapeof acoustic sound pressure wave

DigitalStream of discrete numbers thatrepresent instantaneous amplitudesof analog signal, measured at equally

spaced points in time.


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Analog to Digital Recording Chain

ADC

Continuously varying electrical energy isan analog of the sound pressure wave.

Microphone converts acoustic toelectrical energy. Its a transducer.

ADC (Analog to Digital Converter)converts analog to digital electrical signal.

Digital signal transmits binary numbers.

DAC (Digital to Analog Converter) converts digitalsignal in computer to analog for your headphones.


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Analog to Digital Conversion

Instantaneous amplitudes ofcontinuous analog signal, measuredat equally spaced points in time.

A series of snapshots


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[a.k.a. sample word length, bit depth]Precision of numbers used formeasurement: the more bits, the higherthe resolution.

Example: 16 bit

Analog to Digital Overview

Sampling Rate

How often analog signal is measured

Sampling Resolution

[samples per second, Hz]

Example: 44,100 Hz


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Sampling Rate

Nyquist Theorem:

Sampling rate must be at least twice as high as

the highest frequency you want to represent.

Determines the highest frequency that youcan represent with a digital signal.

Capturing just the crest and trough of a sinewave will represent the wave exactly.


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Aliasing

What happens if sampling rate not high enough?

A high frequency signal

sampled at too low a rate

looks like

a lower frequency signal.

Thats called aliasing.


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Common Sampling Rates

Sampling Rate Uses

44.1 kHz (44100) CD, DAT

48 kHz (48000) DAT, DV, DVD-Video

96 kHz (96000) DVD-Audio

22.05 kHz (22050) Old samplers

Most software can handle all these rates.

Which rates can represent the range offrequencies audible by (fresh) ears?


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3-bit Quantization

A 3-bit binary (base 2) number has 23 = 8 values.

0

1

2

3

4

5

6

7

A rough approximation

Amplitud

e

Time measure amp. at each tick of sample clock


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4-bit Quantization

A 4-bit binary number has 24 = 16 values.

0

2

4

6

8

10

12

14

Amplitud

e

A better approximation

Time measure amp. at each tick of sample clock


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16-bit Sample Word Length

A 16-bit integer can represent 216, or

65,536, values (amplitude points).

We typically use signed 16-bit integers,and center the 65,536 values around 0.

32,767

-32,768

0


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Quantization Noise

Round-off error: difference between actual

signal and quantization to integer values

Random errors: soundslike low-amplitude noise


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Common Sampling Resolutions

Word length Uses8-bit integer Low-res web audio

16-bit integer CD, DAT, DV, sound files

24-bit integer DVD-Video, DVD-Audio

32-bit floating point Software (usually only forinternal representation)


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The Digital Audio Stream

Its just a series of sample numbers, to be

interpreted as instantaneous amplitudes:one for every tick of the sample clock.

Previous example:

11 13 15 13 10 9 6 1 4 9 15 11 13 9

This is what appears in a sound file, along

with a header that indicates the samplingrate, bit depth and other things.


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Audio File Size

CD characteristics

- Sampling rate:

44,100 samples per second (44.1 kHz)

How big is a 5-minute CD-quality sound file?

- Sample word length:

16 bits (i.e., 2 bytes) per sample

- Number of channels:

2 (stereo)


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Audio File Size

5 minutes * 60 seconds per minute= 300 seconds

How big is a 5-minute CD-quality sound file?

44,100 samples * 2 bytes per sample * 2 channels

= 176,400 bytes per second

300 seconds * 176,400 bytes per second

= 52,920,000 bytes = c. 50.5 megabytes (MB)


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MPEG Compression

MPEG 1-Layer 3 (.mp3) Motion Picture Experts Group

Different levels for different purposes

E.g. MPEG 2 used for DVDs

Takes out parts of the sound signal thathumans cant hear

How does the size change? Lossy compression


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MPEG Compression


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What is an iPod?

Its a computer! Look at the von Neumann architecture

I/O

2-4GB microdrive, click wheel, LCD panel,USB connection, FireWire connection, audiooutput

Memory 32MB RAM, 32MB ROM

CPU Special digital to analog converter chips Well-designed UI Stores music in various digital formats

AAC (16 to 320 Kbps), MP3 (32 to 320Kbps), MP3 VBR, Audible, AIFF, Apple

iP d S ifi i


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iPod Specifications

CPU: ARM7TDMI Playtime: up to 8 hours Scroll wheel: solid state touch Buttons: mechanical CPU Speed: dual 80 Mhz

embedded Data Path: 32 bit ROM: 32 MB Onboard RAM: 32 MB Screen: 1.67" LCD w/ LED

Maximum Resolution: 1-bit 138x110 Hard Disk: 1" 4 GB 4200 RPM holds 1000 songs. USB: via Dock Connector FireWire: via Dock Connector Audio Out: stereo 16 bit mini Weight: 0.225 lbs. Dimensions: 3.6" H x 2.0" W x 0.5" D

OS: iPod OS 1.1 I t d d J 2004

Date post:	14-Apr-2018
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Sound and Digital Audio.ppt

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