LECTURE # 6

RAZIA NISAR NOORANI

Multimedia Digital Audio

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Outline2

AudioBasic Digital Audio Concepts

Streaming Audio Web Audio Formats

Sound

Sound comprises the spoken word, voices, music and even noise.

It is a complex relationship involving a vibrating object (sound source), a transmission medium (usually air), a receiver (ear) and a perceptor (brain). Example banging drum.

As the sound vibrates it bumps into molecules of the surrounding medium causing pressure waves to travel away from the source in all directions

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Sound

So, Sound are rapid vibrations that are transmitted as variations in air pressure.

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Waveforms

Sound waves are manifest as waveforms A waveform that repeats itself at regular intervals is

called a periodic waveform Waveforms that do not exhibit regularity are called

noise

The unit of regularity is called a cycle This is known as Hertz (or Hz) after Heinrich Hertz

One cycle = 1 Hz Sometimes written as kHz or kiloHertz (1 kHz

= 1000 Hz)

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Waveforms

distancealong wave

Cycle

Time for one cycle

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The characteristics of sound waves

Sound is described in terms of two characteristics: Frequency Amplitude (or loudness)

Frequency the rate at which sound is measured Number of cycles per second or Hertz (Hz) Determines the pitch of the sound as heard by our

ears The higher frequency, the clearer and sharper the

soundthe higher pitch of sound

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The characteristics of sound waves

Amplitude Sound’s intensity or loudness The louder the sound, the larger amplitude.

In addition, all sounds have a duration and successive musical sounds is called rhythm

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The characteristics of sound waves

distancealong wave

Cycle

Time for one cycleAmplitude

pitch

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Example waveforms

PianoPiano

Pan flutePan flute

Snare drumSnare drum

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Capture and playbackof digital audio

Air pressurevariations

Captured viamicrophone

Air pressurevariations

ADC

Signal isconverted intobinary(discrete form)01010011010110101111

Analogueto DigitalConverter

DAC

Converts back into

voltage

Digital toAnalogueConverter

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The Analogue to Digital Converter (ADC)

An ADC is a device that converts analogue signals into digital signals

An analogue signal is a continuous value It can have any single value on an infinite scale

A digital signal is a discrete value It has a finite value (usually an integer)

An ADC is synchronised to some clock

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The Analogue to Digital Converter (ADC)

It will monitor the continuous analogue signal at a set rate and convert what it sees into a discrete value at that specific moment in time

The process to convert the analogue to digital sound is called Sampling. Use PCM (Pulse Code Modulation)

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Digital samplingSampling frequency

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Digital samplingSampling frequency

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Sampling

Two parameters:

Sampling Rate Frequency of sampling Measure in Hertz The higher sampling rate, higher quality sound but size

storage is big. Standard Sampling rate: - 44.1 KHz for CD Audio

- 22.05 KHz - 11.025 KHz for spoken- 5.1025 KHz for audio effect

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Sampling

Size sampleThe resolution of a sample is the number of bits it uses to store a given amplitude value, e.g. 8 bits (256 different values) 16 bits (65536 different values) A higher resolution will give higher quality but will

require more memory (or disk storage)

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Quantisation

Samples are usually represented the audio sample as a integers(discrete number) or digital

S am ple po in ts

0

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Basic Terms19

TermTerm DefinitionDefinition

BitBit Binary value 0 or 1Binary value 0 or 1

Kilobit (Kb)Kilobit (Kb) 1,000 bits (approx.)1,000 bits (approx.)

ByteByte 8 bits8 bits

Kilobyte (KB)Kilobyte (KB) 1,000 bytes1,000 bytes

Megabyte Megabyte (MB)(MB)

1,000,000 bytes1,000,000 bytes

Gigabyte Gigabyte (GB)(GB)

1,000,000,000 bytes1,000,000,000 bytes

KbpsKbps Kilobits per second (1,000 bits in a Kilobits per second (1,000 bits in a second)second)

KB/secKB/sec Kilobytes per second (1,000 bytes in a Kilobytes per second (1,000 bytes in a second)second)

Analog vs. Digital20

Analog: A signal that has a continuous nature rather than a pulsed or discrete nature. Note: Electrical or physical analogies, such as

continuously varying voltages, frequencies, or phases, may be used as analog signals.

Digital: A signal in which discrete steps are used to represent information.

Digital Signal21

Use Pulse Code Modulation (PCM) to represent an audio signal by digital data.

ADC & DAC22

Figure 4.3 Conversion from Analog to Digital requires an Analog-to-Digital Converter

Figure 4.4 Conversion from Digital to Analog requires a Digital-to-Analog Converter

Basic Digital Audio Concepts23

Sampling rate Number of sample taken of a signal in a given

time (usually one second)Bit depth

Describes the accuracy of the audio dataChannels

E.g. Stereo has two channelsBit rate

Measured in kilobits per second (Kbps) is a function of the bit depth and sampling rate

Sampling rate24

The more sample taken per second, the higher the accuracy.

Typically measured in kilohertz (KHz).CD audio has 44,100 samples per second

(44.1KHz).8 KHz produces lower quality radio sound.Standard sampling rates include “8KHz”,

“11.025KHz”, …The high-end 96K is used in DVD, but is not

applicable to the Web.

Sampling Rate25

demo

Bit depth26

Also called “sampling resolution” or “word length”.

The more bits, the better is the quality of the audio (and a larger file of course).

Common bit depths are 8-bit (telephone like), 16-bit (CD quality), and 20, 24, 32, 48-bit depths.

How many signal can a 8-bit and a 16-bit data represent? 0000 0000 1111 1111 0000 0000 0000 0000 1111 1111 1111 1111

Quantization27

demo

Channels28

Audio file can support one to six channels of audio formats.

Mono – one channelStereo – two channelsSome others – three, four channels.Six channels – 5.1-channel surround sound.More multi-channel formats announce in

the coming years.

Channel Examples29

Bit rate30

Audio files are measured in terms of bit rate which is measured in kilobits per second (Kbps).

It can be calculated by dividing the file size by the time (in second) to play the audio clip. E.g. 3Mb file play in 30 seconds 3000k / 30 = 100kbps.

Quality at different compression rates

Optimizing for the Web31

Length of the audio clip Keep the audio clip as short as possible. E.g. just keep the most sweetest part of your greeting.

Number of channels A mono audio file is halved the space of a stereo file. Depends on your actual needs.

Optimizing for the Web32

Bit depth Audio file on the Web are usually 8-bits. Half the size of a 16-bit file.

Sampling rate Half the sampling rate will also halve the space

needed. Voice only audio file can be reduced to 8KHz. 22 KHz music clips are acceptable.

Putting all things together: Mono, 8-bit, 22KHz, MP3 compression.

Calculate audio size33

8-bit mono: seconds x KHz16-bit mono: (seconds x KHz) x 28-bit stereo: (seconds x KHz) x 216-bit stereo: ([seconds x KHz] x 2)x2E.g. the file size of 30 seconds of 16-bit,

11KHz mono audio: (30 x 11) x 2 = 660K.

Streaming Audio34

What is it? Play almost immediately after the request,

continues playing the transferring data.Advantages:

Address the problem of long download time. Control distribution and protect copyright,

because the user cannot get a copy of the file.Disadvantages:

Sound quality may be be affected by low speed or unstable Internet connection.

Reference

What is Streaming?35

How does it work?Streaming audio

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Web browser

Web server

RealAudioServer

Web browser requesta RealAudio fromthe web server

Packet are sent to a buffer on the receivingComputer, the RealPlayer will play the soundFile when buffer full

Quality Comparison37

http://www.cit.cornell.edu/atc/itsupport/streamcompare.shtml

Web Audio Formats38

WAV/AIFF (.wav/.aif/.aiff) The Waveform Audio File format (.wav) was developed

by Microsoft, supports arbitrary sampling rates and bit depths.

The Audio Interchange File format (.aif, .aiff) was developed for Macintosh platform.

They are less likely used on the Web, because people use “mp3” or “streaming”.

WAV/AIFF39

Try the Bell-lab synthesis link.http://www.bell-labs.com/project/tts/

voices.html

Select theAudio format

http://www.nuance.com/prodserv/demo_vocalizer.html

MP3 (.mp3)40

Able to maintain excellent sound quality at very small file sizes.

The compression reduces an audio file to one-tenth of its original size. E.g. 40MB file 3.5MB

MP3 is actually MPEG-1 Layer-IIIGood for distribution of HQ audio.Demo: www.mp3.com

What is MP3 digital encoding?41

MP3 is actually the file extension for MPEG, audio layer 3. Layer 3 is one of three coding schemes (layer 1, layer 2, and layer 3) for the compression of audio signals. Layer 3 uses perceptual audio coding and psychoacoustic compression to remove all superfluous information. (More specifically, it removes the redundant and irrelevant parts of a sound signal--the stuff the human ear doesn't hear anyway). It also adds a MDCT (Modified Discrete Cosine Transform) that implements a filter bank, increasing the frequency resolution 18 times higher than that of layer 2.

MP3 Players42

Apple QuickTime Audio (.mov)43

QuickTime is a well-known video format, but it can create audio-only movies.

QuickTime is a container format, which is able to store still images, movie formats, … Excellent compression, true streaming

Netscape and IE have Plug-in now.Quicktime : demo

MIDI (.mid/.midi)44

MIDI stands for “Musical Instrument Digital Interface” which is developed for electronic musical instruments.

MIDI files are very compact and very good for low-bandwidth delivery.

Instruments are “piano, drums, bass, orchestral strings, …”

It is very attractive for adding MIDI file to your website with very little download time.

Demo: www.findmidis.com

MIDI45

http://www.findmidis.com/

RealMedia/RealAudio (.rm/.ra)46

RealAudio is a server-based streaming audio format.

The RealServer responses to the requests and delivers the streaming packets, including the bandwidth negotiation.

A robust RealServer can support thousands of simultaneous listeners.

Good for continuous-playing audio and live broadcasts to a large group of people.

Example: RTHK Radio

RealMedia/RealAudio47

Windows Media (.wma/.asf)48

Windows Media is a streaming system.It wraps all media elements into a Active

Streaming File (.asf).Audio may be saved as non-streaming

Windows Media Audio format (.wma).Good for continuous-play audio.The encoder and player is Free, shipped with

Windows OS.

Windows Media49

Choosing an audio format50

Audio NeedsAudio Needs Suggested formatsSuggested formats

Short voice greetingsShort voice greetings WAV, AIFF, MP3WAV, AIFF, MP3

News broadcastsNews broadcasts Streaming solutions Streaming solutions (RealAudio, Windows Media)(RealAudio, Windows Media)

Background musicBackground music MIDI, WAVMIDI, WAV

Music samples for some Music samples for some audienceaudience

MP3 or QuickTimeMP3 or QuickTime

Radio-style or Live Radio-style or Live broadcastingbroadcasting

RealMedia SystemRealMedia System

Musical E-greeting cardMusical E-greeting card MIDI, WAVMIDI, WAV

Add Audio to your Web Page51

A simple link <A HREF=“audio/song.wav”>Play the song </A> <A HREF=“music.mp3”><IMG SRC=“buttons/play.gif”></A>

Background Sound <BGSOUND SRC=“audio/song.mid” LOOP=3>

Link to RealMedia <A HREF=“song.ram”>Link to the song</A> pnm://domainname.com/song.rm

Digital audio editing software

One of the most powerful and professional PC-based packages is a tool called Sound Forge

http://www.sonicfoundry.com/

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Editing Digital Audio

TrimmingSplicing and assemblyVolume adjustmentsFormat conversionResampling or downsamplingFade ins and fade outsEqualizationTime StretchingDigital Signal processingReversing Sounds

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Editing Digital Audio

Simple audio editing software allows: Recording of digital audio segments Trimming Splicing and assembly Volume adjustments of the entire segment Reversing Sounds Copy, cut, paste and delete segments of digital audio

Others audio editing software: COOL Edit Pro Gold Wave PROSONIQ SonicWORX Samplitude Studio

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How audio can be used effectively

Examples of uses of audio:

Cautions and warningsIt is a good medium for alerting users to critical information. Some uses include: Sounding an alarm when a limit is reached Alerting users when data is entered incorrectly

Music and Sound EffectsThese make multimedia interaction more real. Some uses include:

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How audio can be used effectively

Musical background for a video segment Birds Songs accompanying photographs in

biological field training.

Sound-related data. Some uses include: Helping mechanics diagnose engine trouble Training medical students to recognize different

breathing sounds

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How audio can be used effectively

Direct voice communication. Some uses include: Leaving a voice message for other users of an

application Consulting with an expert during a troubleshooting

procedure.

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Advantages and Disadvantages of using audio

Sound adds life to any multimedia application and plays important role in effective marketing presentations.

Advantages

Ensure important information is noticed

Add interest

Can communicate more directly than other media

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Advantages and Disadvantages of using audio

Disadvantages

Easily overused

Requires special equipment for quality production

Not as memorable as visual media

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Summary

There are two main types of digital audio Sampled audio

Captured by sampling an analogue waveform at a set rate

MIDI data Instructions on how to perform some musical

compositionSampled audio requires more storage space

than MIDI information

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