3. Digital Audio
SOUND
◦ Sound is simply waves of compressed
air that move out from its source, similar to dropping a stone in
water.
◦ Your ears are sensitive to these compressed waves. As the sound
waves interact with your ear drum, the signal is sent to your
brain, converting them to the sounds that you recognize.
Two types of Sounds
Analog sound(recorded &reproduce the signals)
Digital sound(stored as numerical data &
easily reproduced & manipulated)
4. Characteristics of Sound & Digital
Audio
Sound is based on two things: Energy & Transport medium.
Energy from a banging drum is converted into pressure that is
transmitted by the air.
There are two characteristic of sound waves
◦ Amplitude
◦ Amplitude is the power or intensity of the sound.
◦ The louder the sound, the larger the amplitude.
◦ Frequency
◦ Frequency is the rate at which sound is measured.
◦ It is measured in cycles per second or Hertz(Hz).
5. Voice and Music
The Three classes of sounds:
Voices,
Music and
Sound effects.
Voice is defined as talking, not singing by human. It is does
not have a pitch and tone.
Music can have rapid changes in tone and pitch. It is
usually recorded and played in stereo.
Sound effects can be voice or music. It is vary in tone, pitch
and time.
6. Digitizing Sound
Digitizing Sound means capturing the sound
amplitude at a specific frequency.
The three characteristics for recording sound:
Frequency rate
Amplitude measurement
Sound channels (Monaural or stereo)
For multimedia computers, standard
frequency rates includes
11.025 Kilohertz(KHz)
22.5 KHz
44.1 KHz
◦ Amplitude measurement is based on 8 bits(1byte) for 256 levels
and 16 bits for 65,536 levels
7. Calculating Audio Data Size
There are three characteristic is used to calculating audio file sizes,
amplitude, sound channels and frequency rates
c=number of channels(mono-1channel, stereo- 2channel)
s=sampling rate in Hertz(cycle per second)
t=a time(second)
b=bytes(1for 8 bits, 2 for 16 bits)
File size=c*s*t*b
8. Digital Audio System
Digital audios created when the characteristic of sound wave is represent
using number – a process referred to as Digitizing.
We can digitize any sound(Microphone, Synthesizer…)
Digitized sound is sampled sound
Every nth fraction of a second, a sample of a sound is taken & stored as
digital information in bits and bytes.
The quality of digital recording depends on the how often the samples
are taken
i.e. Higher the sampling rate better is the quality of sound.
9. sampling rates are normally used in multimedia 44.1KHz(CD-
Quality),22.05 KHz and 11.025KHz
Larger the sample size, more accurate data will describe the recorded
sound
The value of each sample is rounded off to the nearest integer called
Quantization.
Digital Audio software support
Playback
Record
Stop
Fast forward
Reverse
Rewind
15. Sound Controls
Multimedia computes usually have software that acts as a control for
the computer’s sound playback and recording. This software includes:
◦ Selection of components(Microphones, Speakers, Line-in, Line-out)
◦ Volume level, both monaural and stereo
◦ Recording input level
◦ Bass and treble controls
These controls are usually accessible
for a user from the operating system of
the computers as well as from within multimedia products
themselves
16. MIDI(Musical Instrument Digital
Interface)
It is a communications standard developed in the 1980’s for
electronic instruments and computers
It allows instruments from different manufactures to communicate
But MIDI data is not digitized, its only music data stored in
numeric form.
Digital Audio is recorded where as MIDI is a score
Device Independent
This file is a list of time stamped commands
that are recordings of musical actions
17. Using MIDI, we can easily and quickly compose our own original
score
The process of creating MIDI is quite different from digitizing a
recorded audio
Digitized Audio – Bitmap Image the MIDI –Vector graphics
For digitized audio we simple need to play the recording through a
computer device that can digitally record the sound
◦ To make MIDI, we need
Notation software
Sequencer software
Synthesizer
MIDI Keyboard
18. MIDI vs. Digital Audio
Advantages
MIDI files are more compact that audio files
MIDI is small, can be easily embedded into the web pages
MIDI sound source is high quality, this sounds better than audio
files
MIDI data is completely editable
MIDI data can be easily converted to musical notations and vise
versa.
19. Audio File Format
A sound files format is a recognized
methodology for compressing data bits
of digitized sound into a data file
The structure of the file must be known even before the data
can be saved or later loaded into a computer to be edited or
played as sound
The file name extension identifies the method of storage used
On the Macintosh, digitized sounds may be stored as data
files, resources, or applications such as AIFF or AIFC
In Windows, digitized sounds are usually stored as WAV files
Both can use MIDI files(.mid)
20. Audio File Format
The method used for consumer grade music CD’s is
Linear Pulse Code modulation(LPCM shortened to
PCM)
CD-ROM/XA (Extended Architecture) format for
reading and writing CD’s was developed later so one
can put several recording sessions of music to be
placed on a single CD-R
LPCM tracks from an Audio CD are usually
converted and stored on a computer in
uncompressed AIFF or WAV format
◦ AIFF-Audio Interchange File Format Both contain
◦ WAV-Wave Format Uncompressed data
21. Audio File Format
There are huge no of file formats &
“Multimedia Containers” that store sound data
There are converter to read & write sound
files from one format to another
MP3 was developed by MPEG (Moving Picture Expert Group)
Common method for storing consumer audio
Contains lossy compression algorithm to save space
Evolved in early 1990’s
WMA(Windows Media Audio) is a proprietary format
OGG was developed as an open-source and royalty free
”Container” for sound compressed using the “ Vorbis
algorithm”
Vorbis sound data resides within an OGG container – OGG
Vorbis
22. MP$ format is based on Apple’s Quick Time
Movies (.mov) “container” model
It is similar to the MOV format that stores
different types of media
MP4 extension is used when file streams audio & video together
M4a extension is used only when file contains only audio data
M4p extension contains only audio, but encrypted for Digital
Rights Management(DRM)
M4r files are used for ringtones on Apple’s iPhone
◦ Other GSM phones use 3gp file format for ring tones
23. Advanced Audio Coding(AAC) format is adopted by
Apple’s iTunes store
Default format for iPod, iPhone, PlayStation, Wii, Dsi,
Motorala, Nokia, Philips, Samsung, Siemens and Sony
Ericssion
Codec is a software that compresses a stream of audio or
video for data storage or transmission and then
decompresses it for play back
Some are “lossy” – trade quality for reduced file size
transmission speed
Some are “lossless” – original data is not at all altered.
24. Using Audio In Multimedia
Application Audio
Adding sound to Multimedia project & the Following Step:
Determine the file formats that are compatible with MM authoring
S/W
Determine the sound playback capabilities that the end user’s system
offers
Decide the type of sound needed
Decide where and when you want to use the digital audio or MIDI
data
Acquire source material by creating it from scratch or buying it
Edit the sounds to fit your project
Test the sounds to be sure they are timed properly with the project’s
images.(This may involve steps 1-4 until everything is in sync)
25. Space Considerations
Large amount of digital information is required for high
quality sound, which in turn requires large amount of
space.
If monaural sound is adequate then one can cut the
storage space by half or get double the playing time in
the same memory space.
With compression codec's we can reduce the size to 1/8th
the space but there is loss of quality & we can also
downsample.
Formula
◦ Sampling rate* bits per sample/8=bytes/sec
◦ Sample rate* sample size/8 *#sec*2 (if stereo)= file size in KB
26. Audio Recording
If we require CD quality recording, then digitize sound at 44.1KHz
and 16 bit to store, at a sound studio.
If we decide to do it our self then we need to invest in:
◦ Acoustically treated room
◦ High end amplifiers
◦ Recording equipments
◦ Powerful microphones
Digital Audio Tape(DAT)systems provide a tape based 44.1KHz
and 16 bit record an record playback capability
DAT recordings are too accurate, precisely recording glitches,
background noises, microphone pops, including coughs in the
next room.
27. Audio Recording
A good editor can reduce the impact of
these noises but at the cost of time & money
Mobile phones can record audio. USB, Flash memory
recorders can be used to record and the recordings can be
downloaded directly as digital files using USB cables or Card
readers
Audio CD’s – the method for digitally encoding the high
quality stereo of the consumer CD market is an international
standard, ISO 10149.
Also known as “Red Book Audio” Standard
Converter & burning s/w’s such as Toast & CD creator from
Roxio can translate digital files of RBA found on CD directly
to digital sound file formats like MP3 & WAV.
28. Sound For Mobile
Ringtones are normally associated with
phones
Telephone system – Pulsating at 90 volt – signal is
sent to copper wire to energize the hammer like
klangs a bell
But in the present systems, the mobile’s s/w takes the
charge when come one calls, depending on the
programmed options, plays the users choice of
ringtone
Ringtones play on very small speakers & often
compete in a noisy environment
The other tones include answer tones, sing tones,
true tones, real tones, video tones & ringels
