1. IT 2002 – Graphics & Multimedia
IT 2002 – Graphics & Multimedia
Week 7- Audio
ATI Kurunegala

2. What is Audio ? Audio is a key component of the communication.
What is Audio ?
Audio is a key component of the communication.
Presence of sound greatly enhances the effect of mostly graphics presentation.
Especially in a video or with animation
Sound is the brain’s interpretation of electrical impulses being sent by inner ear through the nervous system
In multimedia projects-you can use sound in different ways.

3. Audio Sound is a continuous wave that travels through the air.
Audio
Sound is a continuous wave that travels through the air.
The wave is made up of pressure differences.
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4. How do we hear sound? www.hndit.com
Sound is detected by measuring the pressure level at a point
When an acoustic signal reaches the otter- ear (Pinna), the generated wave will be transformed into energy and filtered through the middle-ear. The inner-ear (Cochlea) transforms the energy into nerve activity.
In similar way, when an acoustic wave strikes a microphone, the microphone generates an electrical signal, representing the sound amplitude as a function of time.
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5. How does the ear work? www.hndit.com
As the sound waves enter the ear, the ear canal increases the loudness of those pitches that make it easier to understand speech and protects the eardrum - a flexible, circular membrane which vibrates when touched by sound waves.
The sound vibrations continue their journey into the middle ear, which contains three tiny bones called the ossicles, which are also known as the hammer, anvil and stirrup. These bones form the bridge from the eardrum into the inner ear. They increase and amplify the sound vibrations even more, before safely transmitting them on to the inner ear via the oval window.
The Inner Ear (cochlea), houses a system of tubes filled with a watery fluid. As the sound waves pass through the oval window the fluid begins to move, setting tiny hair cells in motion. In turn, these hairs transform the vibrations into electrical impulses that travel along the auditory nerve to the brain itself.

6. Properties of sound wave
Properties of sound wave

7.
Properties of sound
Period- the difference between the formation of two crests(peak) is termed as period
Frequency –the number of peaks that occur in one seconds(1/period)
Human hearing frequency range: 20 Hz kHz (audio), voice is about 500 Hz to 2 kHz.
Wavelength – is the distance from the midpoint of one crest to the midpoint of the next crest

8.
Properties of sound
Amplitude of a sound is the measure of displacement of the air pressure wave from its mean
Bandwidth –is defined as difference between the highest and lowest frequency contained in a signal

9. Analog to Digital Recording Chain
Analog to Digital Recording Chain
ADC
Microphone converts acoustic to electrical energy. It’s a transducer.
Continuously varying electrical energy is an analog of the sound pressure wave.
ADC (Analog to Digital Converter) converts analog to digital electrical signal.
Digital signal transmits binary numbers.
DAC (Digital to Analog Converter) converts digital signal in computer to analog for your headphones.

10. Computer Representation of Audio
Computer Representation of Audio
Speech is analog in nature and it is converted to digital form by an analog-to-digital converter (ADC).
A transducer converts pressure to voltage levels.
Convert analog signal into a digital stream by discrete sampling
Discretization both in time and amplitude (quantization)
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11. Audio Encoding www.hndit.com electrical voltage input
Audio Waves Converted to Digital
electrical voltage input
sample voltage levels at intervals to get a vector of values: (0, 0.2, 0.5, 1.1, 1.5, 2.3, 2.5, 3.1, 3.0, 2.4,...)
A computer measures the amplitude of the waveform at regular time intervals to produce a series of numbers (samples).
The ADC process is governed by various factors such as sample rate and quantization: binary number as output
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12.
Audio Encoding
Sampling Rate: rate at which a continuous wave is sampled (measured in Hertz)
Examples: CD standard Hz, Telephone quality Hz
The audio industry uses kHz, kHz, kHz, and 44.1 kHz as the standard sampling frequencies. These frequencies are supported by most sound cards.
How often do you need to sample a signal to avoid losing information?
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13. Audio Encoding Nyquist Theorem:
Audio Encoding
Nyquist Theorem:
Sampling rate must be at least twice as high as the highest frequency you want to represent.
Capturing just the crest and trough of a sine wave will represent the wave exactly.

14. Audio Encoding Nyquist Sampling Theorem:
Audio Encoding
Nyquist Sampling Theorem:
If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal.
Example:
CD's actual sampling frequency Hz,
Due to Nyquist's Theorem - sampling frequency is Hz.
It depends on how fast the signal is changing,. Real Answer: twice per cycle (this follows from Nyquist sampling theorem)
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15. Nyquist’s sampling Theorem
A signal s(t) can be reconstructed from its samples, provided that it is sampled at least at a rate of twice of its maximum frequency.
Maximum frequency of the Analog signal = fm Hz
Sampling rate = 2fm samples /second
Analog samples are called PAM (Pulse Amplitude Modulated) samples
Amplitude
Time
Sampling time = 1/2fm seconds
Voice band width – 4000 Hz (with a guard band 900 Hz) Sample rate required to reconstruct the signal form its samples is 8000 samples / sec Color TV - BW = 4.6 MHz

16. Aliasing What happens if sampling rate not high enough?
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.
That’s called aliasing or foldover. An ADC has a low-pass anti-aliasing filter to prevent this.
Synthesis software can cause aliasing.

17. Common Sampling Rates Sampling Rate Uses 44.1 kHz (44100) CD, DAT
Common Sampling Rates
Which rates can represent the range of frequencies audible by (fresh) ears?
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.

18.
Digitization
Process of converting analog data into digital data is known as digitization. Ea
Analog to Digital Conversion Ed
Representing with a minimum loss of information
Contains an infinite number of amplitudes
Sampling - (PAM) Measuring the amplitude of the analog signal at regular intervals

19.
Audio Encoding
The best-known technique for voice digitization is Pulse-Code Modulation (PCM).
Voice 4000 Hz
What is the PCM sampling rate?
PCM provides analog samples which must be converted to digital representation. Each of these analog samples must be assigned a binary code. Each sample is approximated by being quantized as explained next.
If voice data are limited to 4000 Hz, then PCM samples 8000 samples/second which is sufficient for the input voice signal.
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20. Pulse-Code Modulation (PCM).
Pulse-Code Modulation (PCM).

21. Three different sampling methods for PCM
Three different sampling methods for PCM

22. Pulse-Code Modulation
Encoding
Quantizing
Done in three steps
Sampling (PAM)
Quantizing and Linear Encoding
Binary Stream

23.
Audio Encoding
Quantization (sample precision): the resolution of a sample value.
Samples are typically stored as raw numbers (linear PCM format) or as logarithms (u-law or A-law)
Quantization depends on the number of bits used measuring the height of the waveform
Example: 16-bit CD quality quantization results in over values
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24. 3-bit Quantization 7 6 5 4 Amplitude 3 2 1
3-bit Quantization
A 3-bit binary (base 2) number has 23 = 8 values. 1 2 3 4 5 6 7
Amplitude
Time — measure amp. at each tick of sample clock
A rough approximation

25. 4-bit Quantization 14 12 10 8 Amplitude 6 4 2
4-bit Quantization
A 4-bit binary number has 24 = 16 values. 2 4 6 8 10 12 14
Amplitude
Time — measure amp. at each tick of sample clock
A better approximation

26. The Digital Audio Stream
The Digital Audio Stream
It’s just a series of sample numbers, to be interpreted as instantaneous amplitudes: one for every tick of the sample clock.
Previous example:
This is what appears in a sound file, along with a header that indicates the sampling rate, bit depth and other things.

27. Common Sampling Resolutions
Common Sampling Resolutions
Word length
Uses
8-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 for internal representation)

28. 16-bit Sample Word Length
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

29. Audio File Size CD characteristics… - Sampling rate:
Audio File Size
CD characteristics…
- Sampling rate:
44,100 samples per second (44.1 kHz)
- Sample word length:
16 bits (i.e., 2 bytes) per sample
- Number of channels:
2 (stereo)
How big is a 5-minute CD-quality sound file?

30. Audio File Size How big is a 5-minute CD-quality sound file?
Audio File Size
How big is a 5-minute CD-quality sound file?
44,100 samples * 2 bytes per sample * 2 channels
= 176,400 bytes per second
5 minutes * 60 seconds per minute
= 300 seconds
300 seconds * 176,400 bytes per second
= 52,920,000 bytes = 50.5 megabytes (MB)

31. DAC(Digital to Audio converter) Sample and Hold
DAC(Digital to Audio converter) Sample and Hold
To reconstruct analog signal, hold each sample value for one clock tick; convert it to steady voltage. 1 2 3 4 5 6 7
Amplitude
Time

32.
DAC: Smoothing Filter
Apply an analog low-pass filter to the output of the sample-and-hold unit: averages “stair steps” into a smooth curve. 1 2 3 4 5 6 7
Amplitude
Time

33.
Audio Formats
Audio Formats are described by the sample rate and quantization
Voice quality: 8-bit quantization, 8000 Hz u-law mono (8kBytes/s)
22 kHz 8-bit linear mono (22 kBytes/second) and stereo (44 kBytes/s)
CD quality 16-bit quantization, Hz linear stereo (176.4 kBytes/s = samples x 16 bits/sample x 2 (two channels)/8000)
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34. Audio Formats Available formats on SUN au - Sun File Format
Audio Formats
Available formats on SUN
au - Sun File Format
wav - Microsoft RIFF/waveform Format
al - Raw A-law Data Format
u - Raw u-law Data Format
snd - NeXT File Format
Available formats on Microsoft-Windows-based systems ( RIFF formats):
Waveform audio file format for digital audio hardware
MIDI file format for standard MIDI files
Audio Video Interleaved (AVI) Indeo file format
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35.
Audio Formats
RIFF (Resource Interchange File Format) forms the basis of a number of file formats. RIFF (similarly to TIFF - Tagged Image File Format) is a tagged file format. Tags allow applications capable of reading RIFF files to read RIFF files by another application, hence the word interchange in RIFF.
Other Formats/Players - RealPlayer 7 (Windows NT) with RealAudio, MP3 (MPEG Audio Layer 3) audio, Midi players; MP3 players (MP3.com)
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36. Use of Audio in Multimedia
Use of Audio in Multimedia
Can use sound in multimedia projects in two ways
Content Sound
provides information to audiences.(dialogs in movies or theater)
Ambient sound
consists of array of background & sound effects

37. Content Sound used in multimedia are :
Content Sound used in multimedia are :
Narration : provides information about an animation that is playing on the screen
Testimonials : these could be auditory or video sound tracks used in presentations or movies
Voice-overs : these are sound for short instructions ,for example, to navigate the multimedia application
Music –music may be used to communicate(as a song)

38. Ambient Sound used in multimedia
Ambient Sound used in multimedia
Message reinforcement : the background sound you hear in real life, such as crowds at a ball game, can be used to reinforce the message that you wish to communicate.
Back ground music : set the mood for audience to receive and process information by starting and ending a presentation with music.
Sound effect : sound effects are used in presentation to liven up the mood and add effects to your presentations, such as sound attached to bulleted lists.

39.
Audio File formats

40.
Audio File Formats

41.
Audio File Formats

42.
Project
Find all the sound files on your computer: WAV files (.wav), MIDI (.mid or .rmi), and any others (like .mp3 files) that you are able to download from the Web. Listen to several files of each format carefully, check their properties (right- clicks properties > advanced), and compare their quality with the properties like the size of file, time of the clip, compression used if any, sampling rate, etc. Make a table and record your results in the table. Mention whether these sounds are used (or can be used) as content audio or ambient audio, and explain in one sentence the message they give (if any).