Transcript Bit depth

Multimedia –
Digital Audio & Video
Outline
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Audio / Video on the Web
Basic Digital Audio Concepts
– Streaming Audio
– Web Audio Formats
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Basic Digital Video Concepts
– Video compression/decompression methods.
– Video File Formats
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Other Multi-media Formats
Summary
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AV on the Web?
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It is part of the Internet.
– Radio Stations
– Music, sound clips
– Streaming Audio / Video
– Video conferencing
– Digital Cameras
– Animation on the Web
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Shockwave, Flash
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Basic Terms
Term
Definition
Bit
Binary value 0 or 1
Kilobit (Kb)
1,000 bits (approx.)
Byte
8 bits
Kilobyte (KB)
1,000 bytes
Megabyte (MB)
1,000,000 bytes
Gigabyte (GB)
1,000,000,000 bytes
Kbps
Kilobits per second (1,000 bits in a second)
KB/sec
Kilobytes per second (1,000 bytes in a second)
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Analog vs. Digital
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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.
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Digital: A signal in which discrete steps
are used to represent information.
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Digital Signal
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Use Pulse Code
Modulation
(PCM) to
represent an
audio signal by
digital data.
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ADC & DAC
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
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Basic Digital Audio
Concepts
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Sampling rate
– Number of sample taken of a signal in a given
time (usually one second)
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Bit depth
– Describes the accuracy of the audio data
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Channels
– E.g. Stereo has two channels
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Bit rate
– Measured in kilobits per second (Kbps) is a
function of the bit depth and sampling rate
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Sampling rate
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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.
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Sampling Rate
demo
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Bit depth
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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
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Quantization
demo
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Channels
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Audio file can support one to six channels of
audio formats.
Mono – one channel
Stereo – two channels
Some others – three, four channels.
Six channels – 5.1-channel surround sound.
More multi-channel formats announce in the
coming years.
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Channel Examples
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Bit rate
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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.
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Quality at different compression rates
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Optimizing for the Web
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Length of the audio clip
– Keep the audio clip as short as possible.
– E.g. just keep the most sweetest part of
your greeting.
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Number of channels
– A mono audio file is halved the space of a
stereo file.
– Depends on your actual needs.
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Optimizing for the Web
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Bit depth
– Audio file on the Web are usually 8-bits.
– Half the size of a 16-bit file.
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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.
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Putting all things together: Mono, 8-bit,
22KHz, MP3 compression.
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Calculate audio size
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8-bit mono: seconds x KHz
16-bit mono: (seconds x KHz) x 2
8-bit stereo: (seconds x KHz) x 2
16-bit stereo: ([seconds x KHz] x 2)x2
E.g. the file size of 30 seconds of 16bit, 11KHz mono audio:
– (30 x 11) x 2 = 660K.
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Streaming Audio
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What is it?
– Play almost immediately after the request,
continues playing the transferring data.
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Advantages:
– Address the problem of long download time.
– Control distribution and protect copyright,
because the user cannot get a copy of the file.
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Disadvantages:
– Sound quality may be be affected by low speed
or unstable Internet connection.
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Reference
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What is Streaming?
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How does it work?
Streaming audio
Packet are sent to a buffer on the receiving
Computer, the RealPlayer will play the sound
File when buffer full
Web browser
Web browser request
a RealAudio from
the web server
RealAudio
Server
Web server
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Quality Comparison
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http://www.cit.cornell.edu/atc/itsupport/streamcompare.shtml
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Web Audio Formats
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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”.
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http://www.nuance.com/prodserv/demo_vocalizer.html
WAV/AIFF
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Try the Bell-lab synthesis link.
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http://www.bell-labs.com/project/tts/voices.html
Select the
Audio format
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MP3 (.mp3)
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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
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MP3 is actually MPEG-1 Layer-III
Good for distribution of HQ audio.
Demo: www.mp3.com
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What is MP3 digital
encoding?
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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.
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MP3 Players
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Apple QuickTime Audio
(.mov)
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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
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Netscape and IE have Plug-in now.
Quicktime : demo
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MIDI (.mid/.midi)
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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
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http://www.findmidis.com/
MIDI
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RealMedia/RealAudio
(.rm/.ra)
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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
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RealMedia/RealAudio
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Windows Media
(.wma/.asf)
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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.
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Windows Media
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Choosing an audio format
Audio Needs
Suggested formats
Short voice greetings
WAV, AIFF, MP3
News broadcasts
Streaming solutions (RealAudio,
Windows Media)
Background music
MIDI, WAV
Music samples for some
audience
MP3 or QuickTime
Radio-style or Live broadcasting
RealMedia System
Musical E-greeting card
MIDI, WAV
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Add Audio to your Web
Page
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A simple link
– <A HREF=“audio/song.wav”>Play the song </A>
– <A HREF=“music.mp3”><IMG SRC=“buttons/play.gif”></A>
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Background Sound
– <BGSOUND SRC=“audio/song.mid” LOOP=3>
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Link to RealMedia
– <A HREF=“song.ram”>Link to the song</A>
– pnm://domainname.com/song.rm
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Break Time – 15 minutes
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Basic Digital Video
Concepts
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Movie length
Frame size
Frame rate
Quality
Color bit depth
Data rate (bit rate)
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Movie length
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File size is proportional to the movie
length.
Videos longer than 1 or 2 minutes
cause long download times.
If it is a long video, consider to use
streaming video.
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Frame size
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“Full-screen” video is 640x480 pixels.
The most common frame size for web
video is 160x120 pixels.
Not recommend to use a frame size
larger than 320x240.
The size depends on the CPU power
and the Internet connection
bandwidth.
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Image and Video?
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Frame rate
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Frame rate is measured in number of
frames per second (fps).
Standard TV-quality video uses 30 fps.
For the web, 15 or even 10 fps is more
appropriate and produces fair smooth
quality for the user.
Commercial Internet Broadcasts are
using 0.5, 0.25 frames per second.
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Quality
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Many video-editors allow you to set
the overall quality of the video.
The degree of compression controls
the target quality.
The low or medium setting results a
fairly high compression which is
appropriate for web delivery.
Frame rate and quality are usually
tradeoff in different applications.
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Color bit depth
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The number of pixel colors in each
frame affects the size of the video.
The file size of the video will be
greatly reduced by changing the
number of colors from 24-bit to 8-bit.
It sacrifices the image quality of the
video.
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Data rate (bit rate)
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This is the rate that the data must be
transferred in order to ensure the video can
play smoothly without interruption.
It is measured in kilobytes per second
(K/sec or Kbps).
It can be calculated by dividing the size of
the file (in K) by the movie length (in
seconds).
– E.g. the video file size is 1.9MB  1900K
– Play 40 seconds long, Data rate = 47.5K/sec
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Consider the Internet bandwidth!
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Calculate space
requirements of Video
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NTSC video (640 x 480 and 29.97 fps)
– Frame size = ([Pixel width x pixel height
x bit depth]/8)/1024
– E.g. 200KB/Frame : 6.0 MB/sec
– 200KB x 30 fps = 6000KB/s, 6 MB/sec
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PAL video (768 x 576 and 25 fps)
– E.g. 200KB/Frame : 5.0 MB/sec
– 200KB x 25 fps = 5000KB/s, 5 MB/sec
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Video CODECs
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CODEC is
“Compression/Decompression”
algorithms.
The sound and frame images of a
digital video must be compressed.
– The vast amount of data
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Compressed in a number of ways
– Lossless and Lossy compression
– Spatial and Temporal compression
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Video Clip Demo
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reference
Lossless and Lossy
compression
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Lossless compression means no
information is lost and the final file is
the same as the original.
Most compression methods are lossy.
– Sacrifices some data from the file in order
to achieve higher compression rates.
– Use complicated algorithm to toss out
some data that is not discernible to the
human eye or ear.
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Spatial and Temporal
compression
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Spatial (Intraframe) compression takes
place on each individual frame of the video.
Temporal (Interframe) compression applies
on a series of frames, it takes advantages of
areas of the image remain unchanged from
frame to frame.
– Relies on the key frames and delta frames.
– A key frame is placed once every second.
– E.g. 15 fps, a key frame once 15 frames.
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Delta frame
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Video File Formats
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QuickTime Movie (.mov)
– Introduced by Apple Computer in 1991.
– First developed for Macintosh, now also
supports the PCs.
– Also supports streaming.
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How to create?
– Most video editor, QuickTime Pro.
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How to play?
– QuickTime plug-in or QuickTime player.
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http://www.apple.com/quicktime/gallery/cubicvr/times_square.html
Quicktime
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RealMedia (.rm)
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Industry standard streaming format.
RealPlayer for playback.
RealServer for serving streams.
RealProducer for creating .rm files.
Good for
– Long-playing video or broadcast to many people.
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How to create?
– RealSystem Producer
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How to play?
– RealPlayer (Free), RealPlayer Plus (Commerical)
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Windows Media
(.wmv/ .asf)
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Created by Microsoft, closely integrated with
Windows OS.
Support Windows Media Video (.wmv) and
Advanced Streaming Format (.asf) and
other formats (.avi, .mpeg, …)
Also support streaming.
How to create?
– Windows Media Encoder, Windows Media Author
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How to play?
– Media Player in Windows OS
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AVI (.avi)
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Stands for Audio/Video Interleaved.
Introduced by Microsoft in 1992.
In a AVI file, the audio and video
information are interleaved every frame.
Good for
– Short web clips, high-quality video
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How to create?
– Most video editing tools.
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How to play?
– Windows Media, QuickTime, etc.
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Video Clip Demo
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reference
MPEG (.mpg/ .mpeg)
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Created by Moving Picture Experts Group
(MPEG).
Supports 1) Video, 2) Audio, 3) Streaming.
Extremely high compression rates with small
quality degradation (lossy).
MPEG-1 : VHS quality
MPEG-2 : HQ standard for TV broadcast
MPEG-4 : Very HQ for AV compression
MPEG can be compressed by using three
schemes: Layer-I, Layer-II, Layer-III.
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Official MPEG page
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reference
MPEG Comparison
MPEG-1
MPEG-2
MPEG-4
Standard since
1992
1995
1999
Default Video resolution (NTSC)
352 x 288
640 x 480
640 x 480
Max. Audio Frequency range
48 KHz
96 KHz
96 KHz
Max. audio Channel
2
8
8
Regular data rate
1380 kbit/s
6500 kbit/s
880 kbit/s
Frames per sec (NTSC)
30
30
30
Video Quality
Satisfactory
Very good
Very Good
Hardware requirement for
encoding/decoding
Low
Medium
High
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Flash & Shockwave
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Advantages:
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File sizes are small
Image quality is high
It uses streaming technology
It uses high-quality streaming audio
It is scriptable
Disadvantages:
– A plug-in player is required
– Expensive authoring software
– Problems on printing their content
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Summary
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Discuss the basic digital audio/video
terminology.
Introduction to different formats: WAV,
MP3, QuickTime, RealMedia, Windows
Media, AVI, MPEG.
To deliver long-playing audio/video or live
broadcasts, you should choose one of the
streaming media.
Flash and shockwave are popular and
appropriate format for the Web.
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References
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Design Web Audio (J. Beggs, D. Thede), Oreilly.
E-Video (H. Peter Alesso), Addison-Wesley.
Audio Video Knowledge Center
Bible in MP3 format
MPEG
Musiq
Luke Video
The End.
Thank you for your patience!
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