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Multimedia – Digital Audio & Video CSC1720 – Introduction to Internet Essential Materials Outline Audio / Video on the Web Basic Digital Audio Concepts Streaming Audio Web Audio Formats Basic Digital Video Concepts Video compression/decompression methods. Video File Formats

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Multimedia digital audio video l.jpg

Multimedia –Digital Audio & Video

CSC1720 – Introduction to Internet

Essential Materials


Outline l.jpg
Outline

  • Audio / Video on the Web

  • Basic Digital Audio Concepts

    • Streaming Audio

    • Web Audio Formats

  • Basic Digital Video Concepts

    • Video compression/decompression methods.

    • Video File Formats

  • Other Multi-media Formats

  • Summary

All copyrights reserved by C.C. Cheung 2003.


Av on the web l.jpg
AV on the Web?

  • It is part of the Internet.

    • Radio Stations

    • Music, sound clips

    • Streaming Audio / Video

    • Video conferencing

    • Digital Cameras

    • Animation on the Web

      • Shockwave, Flash

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Basic Terms

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Analog vs. Digital

  • 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.

  • Digital: A signal in which discrete steps are used to represent information.

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Digital Signal

  • 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

  • Sampling rate

    • Number of sample taken of a signal in a given time (usually one second)

  • Bit depth

    • Describes the accuracy of the audio data

  • Channels

    • E.g. Stereo has two channels

  • Bit rate

    • Measured in kilobits per second (Kbps) is a function of the bit depth and sampling rate

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Sampling rate l.jpg
Sampling rate

  • 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

  • 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

  • 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 l.jpg
Bit rate

  • 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.

  • Quality at different compression rates

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Optimizing for the Web

  • Length of the audio clip

    • Keep the audio clip as short as possible.

    • E.g. just keep the most sweetest part of your greeting.

  • 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 web17 l.jpg
Optimizing for the Web

  • Bit depth

    • Audio file on the Web are usually 8-bits.

    • Half the size of a 16-bit file.

  • 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.

  • Putting all things together: Mono, 8-bit, 22KHz, MP3 compression.

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Calculate audio size

  • 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 16-bit, 11KHz mono audio:

    • (30 x 11) x 2 = 660K.

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Streaming audio l.jpg
Streaming Audio

  • What is it?

    • Play almost immediately after the request, continues playing the transferring data.

  • Advantages:

    • Address the problem of long download time.

    • Control distribution and protect copyright, because the user cannot get a copy of the file.

  • Disadvantages:

    • Sound quality may be be affected by low speed or unstable Internet connection.

  • Reference

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What is streaming l.jpg
What is Streaming?

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How does it work streaming audio l.jpg

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

How does it work?Streaming audio

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Quality comparison l.jpg
Quality Comparison

  • http://www.cit.cornell.edu/atc/itsupport/streamcompare.shtml

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Web Audio Formats

  • 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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Wav aiff l.jpg

http://www.nuance.com/prodserv/demo_vocalizer.html

WAV/AIFF

  • Try the Bell-lab synthesis link.

  • http://www.bell-labs.com/project/tts/voices.html

Select the

Audio format

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MP3 (.mp3)

  • 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

  • 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?

  • 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)

  • 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

  • Netscape and IE have Plug-in now.

  • Quicktime :demo

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MIDI (.mid/.midi)

  • 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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MIDI

http://www.findmidis.com/

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RealMedia/RealAudio (.rm/.ra)

  • 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 l.jpg
RealMedia/RealAudio

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Windows Media (.wma/.asf)

  • 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 l.jpg
Windows Media

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Choosing an audio format

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Add Audio to your Web Page

  • A simple link

    • <A HREF=“audio/song.wav”>Play the song </A>

    • <A HREF=“music.mp3”><IMG SRC=“buttons/play.gif”></A>

  • Background Sound

    • <BGSOUND SRC=“audio/song.mid” LOOP=3>

  • 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 l.jpg
Break Time – 15 minutes

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Basic Digital Video Concepts

  • Movie length

  • Frame size

  • Frame rate

  • Quality

  • Color bit depth

  • Data rate (bit rate)

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Movie length l.jpg
Movie length

  • 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

  • “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 l.jpg
Image and Video?

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Frame rate l.jpg
Frame rate

  • 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

  • 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

  • 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 l.jpg
Data rate (bit rate)

  • 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

  • Consider the Internet bandwidth!

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Calculate space requirements of Video

  • 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

  • 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

  • CODEC is “Compression/Decompression” algorithms.

  • The sound and frame images of a digital video must be compressed.

    • The vast amount of data

  • Compressed in a number of ways

    • Lossless and Lossy compression

    • Spatial and Temporal compression

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Video Clip Demo

  • reference

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Lossless and Lossy compression

  • 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

  • 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

  • QuickTime Movie (.mov)

    • Introduced by Apple Computer in 1991.

    • First developed for Macintosh, now also supports the PCs.

    • Also supports streaming.

  • How to create?

    • Most video editor, QuickTime Pro.

  • How to play?

    • QuickTime plug-in or QuickTime player.

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Quicktime l.jpg

http://www.apple.com/quicktime/gallery/cubicvr/times_square.htmlhttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

Quicktime

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Realmedia rm l.jpg
RealMedia (.rm)http://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • 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.

  • How to create?

    • RealSystem Producer

  • How to play?

    • RealPlayer (Free), RealPlayer Plus (Commerical)

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Windows media wmv asf l.jpg
Windows Media http://www.apple.com/quicktime/gallery/cubicvr/times_square.html(.wmv/ .asf)

  • 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

  • How to play?

    • Media Player in Windows OS

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Avi avi l.jpg
AVI (.avi)http://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • 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

  • How to create?

    • Most video editing tools.

  • How to play?

    • Windows Media, QuickTime, etc.

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Video clip demo57 l.jpg
Video Clip Demohttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • reference

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Mpeg mpg mpeg l.jpg
MPEG (.mpg/ .mpeg)http://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • 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 l.jpg
Official MPEG pagehttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • reference

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Mpeg comparison l.jpg
MPEG Comparisonhttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

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Flash shockwave l.jpg
Flash & Shockwavehttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • Advantages:

    • 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 l.jpg
Summaryhttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • 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 l.jpg
Referenceshttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html

  • 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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