Multimedia presentations consume vast amounts of storage space - a single colour television image takes up 105 Kb (Kilobytes) and a minute of live video would take up almost 200 Mb (Megabytes). Similarly, digitised sound consumes large amounts of storage space - a one-minute .aiff soundfile on the Macintosh sampled at 44.1kHz in 16-bit stereo consumes around 10.1Mb of disk space.

Sampling rate (kHz)	No. of bits	stereo/mono	Size
44.1	16	stereo	10.1Mb
22	16	stereo	5Mb
8	8	mono	938Kb

Table: Relative sizes of digitally-recorded 1-minute audio clips

Bandwidth

Bandwidth is a measurement of how much information can be transmitted between two points in a given period of time, and is a major concern when transmitting data over the Internet. Bandwidth is the maximum speed at which a modem operates - therefore various speeds of modem transmit and receive data at different bandwidths. A 2400 bps modem transmits and receives 2400 bits per second of information, which is approximately 240 characters ( a single letter, number or punctuation mark).

A 28.8kbps (thousand bits per second) modem transmits or receives 2880 characters per second, twelve times faster than a 2400 bps modem. Most domestic modems run at 14.4, 28.8 or 56 kbps, and though there are ways of transferring data at higher bandwidths, these are used primarily by corporate LANs or entertainment-based websites. A T-1 is a special data line which transfers data at around 15440 kbps, while a T-3 line runs 28 times faster than a T-1. An alternative which is fast becoming economically-viable and increasing in popularity is the ISDN system, which at its most basic transmits data at around 128kbps.

The more powerful the modem, the less time it takes to transmit the data from one terminal to another.

Modem speed	time taken
2400bps	9 hrs
28.8 Kbps	46 mins
128 Kbps (ISDN)	10 mins
T-1	50 secs
T-3	< 2 secs

 

table 2: Time taken by various modems to transmit 10 Mb of data.

A cheaper alternative to increasing the bandwidth to receive data is to use data compression, which shrinks data into even more manageable chunks. Several companies have developed software systems for the purpose of compression and transfer of audio down the Internet, such as AT&T, whose a2b music system is currently being used by BMG/RCA and online music distributor NK2's Music Boulevard. Other companies include Liquid Audio, Telos" Audioactive software, but one of the systems that seems to dominate the market at this time is Real Networks" Real Audio system.

A typical streaming media system consists of five elements - encoders and content creation tools (1) which are used to create or capture and then edit sound and/video data typesdata types (2), which are stored in centrally-located servers (3), to be transmitted over networks (4) to be received and decoded by client-side players (5) for live or on-demand playback.

fig3: A typical streaming media system, as used by Real Networks.

The Real System contains three main software applications

 
1. servers - purchased from Real Networks to store their coded media files. These files are connected to Web pages by metafiles - addresses which download the files from the media server rather than the web server. This allows users to continue navigating the site, moving from one document to another without interrupting the data stream from the media server.
   
2. encoders - after the video or audio has been digitised, the data is compressed into smaller files which can only be read by a Real Player at the user-end of the process. These files carry an .ra (Real audio) or .rm (Real media) extension, and vary in size depending on the method of encoding used. Real Networks offer a range of encoding technology designed for various computer platforms and modem speeds, the resulting file sizes from which are represented in the table below.
   
   

   unencoded	size	encoder version (bandwidth)	size
   .wav file, 44.1kHz, 16-bit stereo	10.1Mb	RA dual ISDN mono/stereo	586 Kb
   .wav file, 22kHz, 16-bit stereo	5Mb	RA single ISDN mono/stereo	296 Kb
   .wav file, 8kHz, 8-bit mono	938Kb	RA 3.0 (28.8) mono	RA 3.0 (28.8) mono
   		RA 2.0 (28.8)	113 Kb
   		RA 1.0 (14.4)	60 Kb
   		RA 4.0 (8.5)	64 Kb
   		RA 4.0 (6.5)	42 Kb

   table 4: Relative file sizes for Real Audio- encoded soundfiles.

   The different levels of encoding are known as codecs and offer varying quality of sound, e.g. the higher the codec (14.4, 28.8, 56, ISDN) the larger the resulting file and therefore the better the sound quality. The lower-bitrate codecs (6.5, 8.5 and 14.4) offer a sound quality equal to a clear, strong AM radio station, with slightly reduced frequency and dynamic range. The medium-range codecs (RA 2.0 and RA 3.0, both at 28.8) offer a sound quality equal to a clear FM mono radio station, with a slightly reduced frequency and dynamic range, while the higher codecs (ISDN and above) offer near-CD sound quality with a slight reduction in frequency and dynamic range.
   (All figures taken from Real Networks website)

3. Compression works by selecting and storing only the most relevant data and discarding the rest. Moving images are stored as frames relative to each other, so that the only information stored about two virtually identical frames is the actual variation in movement between the two. This cuts down considerably the amount of storage space needed to digitise film, and similarly music is compressed by recording the data in relative sound data.

4. players - These are decoders that are downloaded from Real Networks" website onto an individual's computer, enabling the user to hear/see the encoded data. Real Players will play both audio and video, and can be downloaded free. Once the player is installed, the user's web browser has to be adjusted to identify Real Media files and the application that will decode them. On Netscape 3 this is done by
   

   • selecting Preferences from the Options menu
   • clicking on Helpers
   • select the New button
   • enter the following information
     • Description: Real Player
     • MIME Type: audio/x-pn-realaudio
     • Suffixes: ra, rm, ram
   • click the Application button and find the directory in which the Player is stored.
   • select Real Player as the application to launch.
   • select PNRM or PNst in the File Type menu
   • click OK

   Steps 5-8 above may not be necessary if you can play the songs from the Welsh Pop Music Resource Pages directly from the Real Player provided on the CD. If this does not work, you may have to install a Player into your computer's hard drive.

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