Podcasting and Serving Multimedia

Prepared by:

Kishan N. PatelIT Department,

SVMIT Bharuch

Podcasting

A podcast is a digital medium that consists of an episodic series of audio, video, digital radio, PDF, or ePub files subscribed to and downloaded through web syndication or streamed on-line to a computer or mobile device

Podcasting is the distribution of specially encoded multimedia content (normally MP3 files with ID3 tags) to subscribed personal computers via the RSS 2.0 protocol.

It is a form of narrowcasting, which sends content to people who have decided to listen, as opposed to broadcasting, which goes to everyone within a certain radius of the signal.

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Podcasters versus Podcatchers

Podcasters are the people who create the content.

Podcatchers are those who listen to the content.

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Potcatching Clients

The first podcatching client was iPodder, created by Adam Curry, the former MTV VJ.

iPodder is now called Juice Receiver and is freely downloadable. Juice Receiver is a so-called aggregator that accesses the podcasts to which

you have subscribed, retrieves any enclosed multimedia content, manages files for iTunes and iPhoto, and deletes old podcasts from your hard drive after they have expired.

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Rss Podcast Example

RSS templates for making podcasts are at www.udel.edu/fth/podcasting.

There you will find a simple example called simple.xml and a more complex example called complete.xml.

The simple example contains the minumum RSS for creating a podcast.

The complete example contains optional tags used by iTunes.

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Simple RSS Example

<?xml version="1.0"?><rss version="2.0">

<channel> <title>Introduction to Podcasting</title> <link>http://www.udel.edu/fth/podcasting/simple.xml</link> <description>How to create a podcast</description> <language>en-us</language> <managingEditor>fth@udel.edu (Fred Hofstetter)</managingEditor> <lastBuildDate>Fri, 03 Mar 2006 19:13:45 GMT</lastBuildDate>

<item> <title>Introduction</title> <description>What is podcasting? </description> <pubDate>Fri, 03 Mar 2006 01:15:00 GMT</pubDate> <author>fth@udel.edu (Fred Hofstetter)</author> <enclosure url="http://www.udel.edu/fth/podcasting/intro.mp3" type="audio/x-mp3" length="114688"/> <guid>http://www.udel.edu/fth/2006/03/03/podcasting/intro</guid> </item>

</channel></rss>

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Validating a Feed

After you create an RSS file, you should validate the feed to make sure it does not contain syntactical errors that may cause the podcast to fail.

You can validate the feed for free at http://feedvalidator.org.

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Advertising Your Feed

If you want your feed to be listed at podcast directories and listing sites, you can go to the site’s ping form, fill out the form, and click Submit.

Some ping forms are at: audio.weblogs.com/api.htmlwww.allpodcasts.com/Update.aspxwww.technorati.com/ping.html

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Where to Find Podcasts

The directory that started it all was iPodder, owned by and now called Podcast Alley at podcastalley.com.

You can register and create favorites at podcastpickle.com.

Yahoo’s audio search is at audio.search.yahoo.com

A directory of directories is at podcast411.com.

iTunes U; watch the movie about iTunes U at www.apple.com/education/itunesu_mobilelearning/itunesu.html

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Podcasting from a Blog

If you have a blog, you can use Feed Burner to turn your blog into a podcast. This is called “burning your feed.”

Feed Burner automatically creates and hosts the podcast’s RSS feed for you.

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Issues in Serving Multimedia

Even though you can serve multimedia documents on the Web as you can any other document, multimedia causes two kinds of issues:

1. Multimedia documents can be very big and serving them requires a lot of bandwidth.

2. Unlike text and image formats for which a small number of standardized and interoperable formats have emerged, there are a number of different and often proprietary multimedia formats for which interoperability is poor.

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Cont..

One of the motivations of these different formats is to provide compression techniques that reduce the size of the documents and the consumption of bandwidth.

On the other hand, optimizing document size and bandwidth means that you have to choose the best format depending on your constraints, and these two issues are tightly linked.

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Multimedia

Difference between multimedia applications and the applications we’ve seen so far is the notion of streaming.

A popular multimedia application is to simulate radio or television stations on the Net and synchronize what your users hear or see.

This is quite different from common Internet applications where each user reads each page at his or her own rhythm. Streaming applications require specific techniques.

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The Formats Labyrinth

It is categerised as follows:

1. MP3 is an audio

2. Ogg is a container format Descriptive formats that convey some kind of information about multimedia

streams or documents. Container formats that envelope multimedia streams. Documents and encoding formats that encode the actual audio or video.

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Descriptive Formats

Descriptive formats are used to describe a set of multimedia resources. They do not include any bit of multimedia as such, but use URLs to point to multimedia streams.

1. M3U: The simplest of all the descriptive formats is MPEG Version 3.0 URL (M3U). M3U playlist is a plain text file with a list of URLs. Each URL is placed on a single line and lines that start with a hash character (#) are considered to be comments.

2. PLS: PLS playlists are formatted like Windows .ini files and their media type is audio/x-scpls.

3. SMIL: Another approach to playlists is to use Synchronized Multimedia Integration Language (SMIL). SMIL (pronounced smile) is a W3C recommendation for a language that, like Atom, goes well beyond defining playlists. SMIL is about defining interactive multimedia presentations.

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Container and Encoding Formats

There is thus a need to define formats that will act as envelopes for all these types of information. These formats are known as container formats.

What further complicates the situation is that the different types of information carried in a container need to be synchronized and also that it must be streamable.

you can’t afford to send the audio for a full movie after the full video and need to interleave frames with the different types of information.

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Cont..

1. WAV and AIFF

The WAVeform (WAV) and Audio Interchange File Format (AIFF) container formats are remarkable in that they are only compatible with audio formats and that in practice they are almost exclusively used to enclose the Pulse Code Modulation (PCM) format also used to encode audio CDs.

2. AVI

Like WAV and AIFF, AVI (Audio Video Interleave) belongs to the ancient family of RIFF (Resource Interchange File Format).

3. ASF

Advanced Systems Format (ASF) is a Microsoft proprietary container format introduced to be the successor of AVI. Part of the Windows Media framework, ASF is commonly used with the corresponding proprietary formats Windows Media Audio (WMA) for audio and Windows Media Video (WMV) for video

4. MPEG

The MPEG standards that are pertinent to this section are MPEG-1, MPEG-2, and MPEG-4, three generations of a multipart standard

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Protocols

One of the first points to consider when choosing a protocol is to decide whether HTTP is good enough or if other protocols must be introduced.

To answer this question, you need to differentiate between two different usages of multimedia on the Web.

On the one hand, you have video or audio on demand, the domain of unicast; on the other hand, you have Web TV or Web radio, the domain of multicast with applications that stream the same multimedia content to a number of users.

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Cont..

1. Multicast Web TV and radio is a domain where HTTP would be very inefficient: the

same content would have to be sent separately to each subscriber. Multicast streaming protocols differentiate the stream itself from control

information needed to assert the quality of the link between the server and its clients.

The main protocols used to carry the stream are Real Time Protocol (RTP). RTP recommends that you split audio and video into two different RTP

sessions using different TCP ports so that users can subscribe independently to audio and video and container formats are in charge of synchronizing audio and video.

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Cont..

2. Unicast Unicast corresponds to audio and video on demand, a domain where the

choice between HTTP and a specific protocol is more open than for multicast applications.

Using HTTP to serve multimedia is often called HTTP streaming, pseudo-streaming, or progressive download.

HTTP streaming is not specific to multimedia but refers to lasting HTTP connections expecting that the client renders the results before the end of the exchange. This technique can be used by Ajax applications and has been formalized as the HTTP Streaming Ajax pattern.

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THANK YOU

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