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Audio and video
Digital representations of sound (audio, music, voice,...)
As distinct from analog representations. Similar to representation of images, i.e.
Images: real world -> technology -> representation -> eyes -> brain
Sound: real world -> technology -> representation -> ears -> brain
Many different applications with diferent requirements, e.g.
Music - classical, rock, jazz....
Online radio (talk, music) - e.g.
BBC World Service
Cellphones, phones, and other duplex voice
Two-way and emergency communications
Many different environments, e.g.
Streaming on the web
Real time transmission over wireless or wired networks
The Science of Sound Waves Explained in Great ...
Sound is essentially a waves disturbance of the air. A pure note will be a sine wave with a frequency and amplitude: the higher the frequency, the higher the note that is perceived; the higher the amplitude the louder the sound. You can look at the
figures on this page
to see a pure sine wave and how it corresponds to air compressions.
Different instruments (including voices, etc) produce different waveforms which define the sound of the instrument. When multiple instruments are used, complex waveforms arise. See
has a few examples of waveforms for different instruments.
You can also try out the spectrum analyzer at
Note that these waves are continuous, or
. To use them on a computer, we we have to digitize them. To explore this, we will go through the
them. A few things to note:
Analog-Digital conversion can be done in hardware or software
Sampling occurs on the x (time) axis; the higher the sampling rate, the better the recreation of the wave forms (e.g. CD has 44.1KHz); the sampling rate limits the bandwidth (i.e. the range of frequencies that can be represented).
The highest frequency that can be represented is half the sampling rate (the
). You can play around with
to see how the sampling rate affects the reproduction of the original frequency (you can vary both the original frequency and the sampling frequency and study the effects).
The amount of bits used to discretize the y (amplitude) axis, or quantization, e.g. "16 bit audio" in CD's using 16 bits (65536 values), affects the "dynamic range" of the amplitude
occurs on the x-axis, and the sampling rate determines the
and the maximum frequency sound that can be represented (
occurs on the y-axis, and determines the amount of different values of amplitude (or volume) that can be represented (known as the
Human audible range:
The human audible range of frequencies is roughly 20 Hz to 20 KHz. One of the reasons why "CD quality" is good, is because sampling at 44 KHz has a Nyquist frequency of 22 KHz, which is adequate to capture the sounds the human ear can capture. Well, as we get older, we lose our ability to hear higher frequencies first.
Check your own audible range here!
(just for fun, not a scientific test).
All of this is a necessary precursor to the particular format we use to store (or stream!) sound. This problem is similar to images: i.e. a quality vs space tradeoff, with options for lossy or lossless compressions. Note there is a difference between a codec (the system used for encoding the samples) and the file format (the implementaiton of a codec in a file). The standard way of representing the digitized audio (which lays behind the codec) is called
Pulse Code Modulation
For details of audio formats, see the
Compact Discs have the following parameters for audio stored on them:
Sample rate: 44.1 kHz
Channels: 2 (stereo)
Bits per sample, per channel: 16
Levels per sample: 65,536
Total data rate (Mb/s): 1.4112
Compact Discs sample the audio 44,100 times per second.
The total information needed for 1 second of audio for 2 channel is therefore 44,100 x 2 x 16 = 1,411,200 bits.
for music player
Digital representation of video
Video cameras (digital and analog)
Analog / digital TV
ins and outs of video compression
specifies how a video is compressed and decompressed
specifies how a video is stored or transmitted (one format might support several codecs)
Common computer / internet video codecs
- Original DVD
MPEG-4 PT 2
- Blu-Ray, Vimeo, YouTube, iTunes, Adobe Flash
Adobe Flash home
Flash 10 Demo
Flash on the Web
help on how to format text
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