Transcript 20042009115355_W12L1..

Week 12 Lecture 1
Audio and Video File Formats
Audio on the web
• Can be linked to and downloaded like any
other file
• Traditional audio files are quite large
• Long download time
• Advances
– Steaming audio
– MP3 format – lossy compression- shrinks files
to 1/10 their original size
Digital Audio
• An analog signal must be converted to digital
information
• To convert an analog sound wave into a digital
description of that wave, samples of the wave
are taken at timed intervals
• The number of samples taken per second is
called the sampling rate
• The more samples taken per second, the more
accurately the digital description can recreate
the original shape of the sound wave, and
therefore the better the quality of the digital
audio
Digital Audio
• Sample rates are typically measured in kilohertz (KHz).
• CD-quality audio has a sampling rate of 44.1 KHz (or
44,100 samples per second)
• The higher the sampling rate, the more information is
contained in the file, and therefore the larger the file size
• Like images, audio files are also measured in terms of their
bit depth (also called sampling resolution or word length).
• The bit depth corresponds to the resolution of the amplitude
(or volume) of the sound file
• Some common bit depths are 8-bit and 16-bit, which is
required to describe music of CD quality
Commonly used Audio qualities
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8 KHz, telephone
32 KHz miniDV camcorders
44.1 KHz CD, MP3
88.2 & 176.4 KHz Professional recording
equipment
• 96-192 KHz DVD-Audio, HD DVD,Blu-ray
Audio File Formats
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AIFF - Audio Interchange File Format
WAV - WAVE File Format
MIDI - Musical Instrument Digital Interface
AU - audio format originally by SUN
MP3 - MPEG Audio Layer 3 format
RA - RealAudio
WAV and AIFF file formats
• The WAV and AIFF audio formats are very similar
in peformance
• The Waveform Audio File Format (.wav) was
originally developed as the standard audio format
for the Microsoft Windows operating system, but it
is now supported on the Macintosh as well
• The Audio Interchange File Format (.aif or .aiff)
was developed as the standard audio format for
the Macintosh platform, but it is now supported by
Windows and other platforms
• WAV and AIFF: 8 KHz and 11.025 KHz at 8- or 16bit are/were most common for Web use
WAV and AIFF
• Now less commonly used on the Web than they
once were, now that we have MP3 or formats
designed specifically for the Web (streaming
formats).
• WAV and AIFF files are typically used as the
source format for audio that then gets
compressed into more web-friendly formats
• They sound good when uncompressed, but they
suffer drastic loss of quality when compressed to
small file sizes
• They are useful for very short, downloadable
audio clips, such as short greetings
WAV and AIFF
• Good for: Storing high-quality source
audio before converting to web formats,
delivering short audio clips.
• Creation tools: The majority of sound
editing tools can save files in WAV and
AIFF format.
• Player: WAVs and AIFFs generally play
using the browser's default function for
sound handling
MIDI – Musical Instrument Digital
Interface
• MIDI (.mid) was originally developed as a standard way for
electronic musical instruments to communicate with each
other
• A MIDI file contains no actual audio information (the digital
representation of analog sound), but rather numeric
commands that trigger a series of notes (with instructions
on each note's length and volume).
• These notes are played by a MIDI player using the
available "instrument" sounds on a computer's sound card.
• As a result, MIDI files are incredibly compact and ideal for
low-bandwidth delivery. They are capable of packing a
minute of music into just 10K, which is 1,000 times smaller
than a one-minute WAV file (approximately 10 MB).
Au file Format
• “AU” was developed by Sun Microsystems and is
a standard for Unix computers.
• Most Web browsers include the capability to play
“AU” files directly, so it makes the format a good
choice for Internet work that will be received by a
larger Net audience.
• This type of file can be larger than other types of
audio files so it is best used for short sound clips
for effective download times.
MP3
• MP3 is the most popular file format on the Web
for distributing music
• The key to its success is MP3's ability to maintain
excellent sound fidelity at very small file sizes.
• Its compression scheme can reduce an audio
source to just one-tenth of its original size
• Four minutes of high-quality music:
– in WAV format - 40 MB of disk space
– in MP3 format - 3.5 MB
MP3
• MPEG is a family of multimedia standards created
by the Moving Picture Experts Group. It supports
three types of information: video, audio, and
streaming (synchronized video and audio).
• MPEG uses a lossy compression scheme that is
based on human auditory perception.
• Sounds that are not discernible to the human ear
are thrown out in the compression process
• The resulting file sounds nearly the same, but
contains much less data than the original
RealAudio - RA
• RA is a proprietary streaming audio format
developed by RealNetworks (once Progressive
Networks)
• RealAudio is a server-based streaming audio
solution
• It has grown to be the standard for streaming
audio, including live broadcasts
• To listen to RealAudio files, users must have
RealPlayer, which is available for Windows,
Mac, and Unix systems.
• For more info see http://www.real.com.
Audio and Video
• Delivering video via the Web is especially problematic
because video files require huge amounts of data to describe
the video and audio components, making for extremely large
files.
• Many of the same technologies that have improved the
experience of receiving audio over the Web have been
applied to video as well.
• As with audio, you have the option of simply linking a video to
your web page for download and playback, or you can choose
from a number of streaming solutions (begins playing almost
immediately continues playing as the data is transferred;
however, the file is never downloaded to the user's machine)
• Many of the principles for developing and delivering video
content for the Web are the same as those for audio.
Digital Video
• Movie length : Limiting the length of your video
clip limits its file size. Videos longer than a
minute or two may cause prohibitively long
download times. If you must serve longer videos,
consider one of the streaming video solutions.
• Frame size: Obviously, the size of the frame has
an impact on the size of the file. "Full-screen"
video is 640 X 480 pixels. The most common
frame size for web video is 160 X 120 pixels.
Digital Video
• Frame rate: The frame rate is measured in
number of frames per second (fps). Standard
TV-quality video uses a frame rate of 30 frames
per second to create the effect of smooth
movement. For the Web, a frame rate of 15 or
even 10 fps is more appropriate and still capable
of producing fairly smooth video playback.
• Quality: Many video-editing applications allow
you to set the overall quality of the video image.
The degree to which the compression algorithms
crunch and discard data is determined by the
target quality setting.
Digital Video
• Color bit depth : The size of the video is affected
by the number of pixel colors in each frame.
Reducing the number of colors from 24- to 8-bit
color will drastically reduce the file size of your
video, just as it does for still images. Of course,
you also sacrifice image quality.
• Data rate (bit rate): This is the rate at which data
must be transferred in order for the video to play
smoothly without interruption. The data rate
(also called "bit rate") for a movie is measured in
kilobytes per second (K/sec or Kbps).
Digital Video Compression
• Digital video wouldn't be possible without methods for
compressing
• Compression can be "lossless," which means no
information is lost and the final file is identical to the original
• Most compression schemes use forms of lossy
compression. Lossy compression sacrifices some data
from the file to achieve much higher compression rates.
• Lossy compression schemes, such as MPEG, use
complicated algorithms that remove sound and image data
details that are not discernible to the human ear or eye.
Compression Algorithms
• The decompressed file is extremely similar in character to
the original, yet is not identical. This is similar to the way
JPEG handles still images.
• Spatial (or intraframe) compression takes place on each
individual frame of the video, compressing the pixel
information as though it were a still image.
• Temporal (or interframe) compression happens over a
series of frames and takes advantage of areas of the image
that remain unchanged from frame to frame, throwing out
data for repeated pixels.
• There are a number of compression/decompression
algorithms (codecs) that can be used to compress video
files for the Web. Many of these codecs can be applied to
several different file formats
Digital Video File Formats
• MPEG - Moving Picture Experts Group
Video File
• MOV - QuickTime (Apple Computer) file
format
• AVI - Audio Video Interleave
• RM - Real Media
• ASF - Advanced Systems Format
QuickTime Movie (.mov)
• QuickTime, a system extension that makes it
possible to view audio/video information on a
computer, was introduced by Apple Computer in
1991
• Although developed for the Macintosh, it is also
supported on PCs via QuickTime for Windows
• Most hardware and software offer QuickTime
support.
• While originally developed as a video format, it
has evolved into a container format capable of
storing all sorts of media (still images, audio,
video)
RealMedia (.rm)
• RealMedia is the industry standard streaming
media format.
• RealNetworks (Progressive Networks) first
launched its streaming video capabilities in
Version 3.0 of its RealMedia line of products (of
which RealAudio is the star component).
• RealMedia files (.rm) are viewed using
RealPlayer 3 and higher.
• The wide distribution of RealPlayer and a proven
track record of effective playback have made
RealNetworks' products the de facto standard
for adding streaming media to a web site.
Windows Media (.wmv or .asf)
• Windows Media is the new standard for audio
and video, created by Microsoft and therefore
very closely integrated with the Windows OS.
• The Windows Media Player is capable of playing
Microsoft's proprietary Windows Media Video
(.wmv) and Advanced Streaming Format (.asf ),
as well as a number of other formats such as
AVI, MPEG, MP3, and QuickTime.
• Windows Media movies are encoded using the
proprietary Windows Media Video algorithms
AVI format
• AVI (Audio/Video Interleaved) was introduced by
Microsoft in 1992 as the standard movie format
to work with its Video for Windows (VFW)
multimedia architecture for Windows 95.
• The AVI format has been replaced by the more
robust Windows Media as the standard media
format for Windows.
• AVI movies are not as common as they once
were for web distribution. More often, they serve
as the high-quality source file for the video which
is then converted into a more web-friendly
format.
MPEG-4
• MPEG-4, introduced in late 1998, is the designation for a
group of audio and video coding standards and related
technology agreed upon by the ISO/IEC Moving Picture
Experts Group (MPEG).
• The primary uses for the MPEG-4 standard are web
(streaming media) and CD distribution, conversational
(videophone), and broadcast television.
• MPEG-4 absorbs many of the features of MPEG-1 and
MPEG-2 and other related standards
• Most of the features included in MPEG-4 are left to
individual developers to decide whether to implement
them