1 cmscd1011 introduction to computer audio lecture 10: streaming audio for internet transmission dr...
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Introduction to Computer Introduction to Computer AudioAudio
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CMSCD1011 Introduction to CMSCD1011 Introduction to Computer AudioComputer Audio
Lecture 10: Streaming audio for Internet transmission
Dr David EnglandSchool of Computing and Mathematical Sciences
http://java.cms.livjm.ac.uk/homepage/staff/cmsdengl/Teaching/cmscd1011.html
Email: [email protected]
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In this session...In this session...
• How information is delivered via the Internet• Characteristics of audio transmission over
the Internet • Streaming technologies
– Real Networks’ RealAudio– Microsoft’s Advanced Streaming Format
• Streaming is a technology that lets you present multimedia data as it arrives at your computer
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How information is delivered via the How information is delivered via the InternetInternet
• The Internet is built on a protocol called TCP/IP– Transmission Control Protocol / Internet Protocol
• Information is sent in the form of small packets– Each packet has a sending and receiving address (IP)
• There are two forms of transport mechanism for packets: TCP and UDP– TCP guarantees that each packet will arrive and that
it will arrive in the right order– UDP does not guarantee that each packet will arrive
or get there in the correct order– TCP is slower but more reliable, UDP is much faster
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How information is delivered via the How information is delivered via the InternetInternet
Suppose we want to send some information over the Internet to a friend on the Greek island of Crete.
The information may hop across many countries en route (shown by the various paths on the diagram.)
There are no guarantees that the information will arrive in the order that we sent it due to the numerous routes that it can take.
TCP rearranges out of sequence information.UDP does not.
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Protocols for real-time informationProtocols for real-time information
• Because of the problems associated with TCP and UDP when dealing with multimedia data, other protocols have been developed which sit alongside or on top of the existing system:– RTP (Real time Transport Protocol)
• Usually sites on top of UDP adding extra features for synchronisation, ensuring the correct sequence and identifying different data types (e.g. audio or video)
– RTSP (Real Time Streaming Protocol)• Often described as an “Internet VCR remote control”, it is
used to co-ordinate how the actual media data transmission
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Unicasting and MulticastingUnicasting and Multicasting
Media Server
Multim edia PC
Multim edia PC
Multim edia PC
Multim edia PC
Multim edia PC
Unicasting
P acke ts o f da ta
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Unicasting and MulticastingUnicasting and Multicasting
Media Server
Multim edia PC
Multim edia PC
Multim edia PC
Multim edia PC
Multim edia PC
Multicasting
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Characteristics of audio transmissionCharacteristics of audio transmission
• Keywords: Delay, Jitter and Quality of Service
• Audio is one medium where we can tolerate a small amount of data loss– If we miss a word in a sentence, we can usually
infer what the speaker means– If a note is missing in some music, we don’t mind
• This describes our minimum Quality of Service (QoS)
• There will inevitably be delay in the transmission of audio data via networks and the variation in delay is known as jitter
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The effect of delay and jitter on The effect of delay and jitter on synchronisationsynchronisation
Sender
Video
Audio
Time
Receiver
Video
Audio
Time
Packets containing compressed data
If the jitter is pronounced, audio
may play back at the wrong time (earlier
or later than expected).
If the audio has to be synchronised to video, you will lose
the synchronisation.This usually has to be corrected at the
receiving end.
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Downloading audio dataDownloading audio data
Bytes to download
Time
Playback cannot begin until this point
Assume that you wish to download a 10 MB digital audio file in wav format.
The time it will take to download the data is a function of the size of the data and the speed
of the network connection.
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Streaming audio dataStreaming audio data
Bytes to download
Time
Start playing
Download
Playback
Buffer
The client software must download a certain amount of data first.
This is to compensate for the differences in required data rate and actual network transmission speed.
The size of the buffer will increase if the required data rate is higher than the
actual network speed.If they are equal the buffer is zero.
Note that the whole data isnot kept on the disk
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Points to note about streamingPoints to note about streaming
• There needs to be a data buffer– Unless the network speed is greater than or equal to the
required data rate (which is unlikely as more users request data at the same time)
• Playback commences when the buffer is initially full• During playback, the client software continues to
download the remaining data in the background– Filling the buffer (a FIFO data structure)
• The size of the buffer may need to be changed dynamically during playback if the client detects that the network speed has changed
• The entire audio is never stored on the local disk, only a small amount (equal to the size of the buffer) is ever stored
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Streaming data: The processStreaming data: The process
Encoder
Datafiles
The streaming media encoder converts the existing data into a format suitable for streaming.
ServerUNIX/LinuxWindows
A fast server is required to serve these files by sending small chunks of information over the network. Fast network connection and hard drive are important factors.
Player
Player
Player
NetworkTCP/IP
UDPHTTPRTP
RTSP
Various network protocols may be employed to deliver the actual data.
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Real Networks’ RealAudioReal Networks’ RealAudio
• RealAudio was the first major player in the field of streaming audio (and video and now images)
• The latest version uses a technology called SureStream™ to encode different quality settings in the same data– The higher your bandwidth, the better the quality
• RealPlayer is one of the most widely used multimedia applications on the Internet today
http://www.realnetworks.com/getstarted/index.htmlhttp://www.realnetworks.com/products/servers/bguide/index.html
http://www.realnetworks.com/getstarted/index.htmlhttp://www.realnetworks.com/products/servers/bguide/index.html
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The effects of bandwidth on data The effects of bandwidth on data transmissiontransmission
Server Client
28.8 Kbps modem
56 Kbps ISDN
56 Kbps of data over a 28.8 Kbps modem
Playback interrupted
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Microsoft’s Advanced Streaming Microsoft’s Advanced Streaming Format (ASF)Format (ASF)
• ASF is part of the Windows Media protocols
• The data is broken up into small packets with each numbered packet containing part of the media data
• The media server knows which packets to send to the user at any given time– If, for example, network congestion requires the server to send
less data, it will not send packets that the user won't receive
– Also, if the user starts skipping around in the file, by fast forwarding, rewinding, or skipping to markers within the file, the server sends the user the data that corresponds to what the user has requested
http://www.microsoft.com/windows/windowsmedia/http://msdn.microsoft.com/workshop/imedia/windowsmedia/tourwmtools.asp
http://www.microsoft.com/windows/windowsmedia/http://msdn.microsoft.com/workshop/imedia/windowsmedia/tourwmtools.asp
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ASF toolsASF tools Windows Media Author Windows Media Author
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SummarySummary
• Because in its current form, the Internet is so slow, we need different methods to deliver multimedia data– This is changing now with the introduction of
broadband communications, e.g. ADSL, Cable modems
• Streaming is one technique for delivering multimedia data in real time over the Internet
• Streaming can be used for live broadcasts or for transmitting pre-recorded information
• Streaming does not store the whole presentation on disk (which is good if you sell media!)