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Matt Ottewill
Don Kallenbach
Ben Henderson
Paul Hazel

(See also our article on Choosing appropriate settings for optimising audio and video files.)

Introduction

New media developers are familiar with the constraints of data size and rate when authoring for fixed capacity storage media (such as CD-ROM) and network bandwidth. Even small images must be optimised to their smallest possible size. Because video and audio files contain a great deal of data, extra care needs to be taken to ensure fast delivery and smooth playback on desktop computer systems.

Playback of multiple audio tracks is no longer problematic for desktop systems (although speedy transfer over networks is). For a variety of reasons, playing back full frame PAL or NTCS video (with its associated audio tracks) is still a major challenge. Transferring files over networks is even more problematic. Therefore video files usually need optimising or compressing before being published for delivery.

You may need to ...

• Have video running from a CD-ROM or DVD-ROM

• Have video running from an internal HD

• Have video running off a floppy disc

• Have video running off a Zip disc

• Have video files download from a web site

• Have video streaming over the internet

• Have video running over an intranet

You may therefore need to consider ...

• End-user system

• Network performance (bandwidth, traffic etc)

• Frame size

• Frame rates

• Compression settings (audio and video)

• File format

• Storage devices available on an end-user system

• Capacity of delivery media (CD-ROMs etc)

Data rate (or Bit rate)

"Data rate"(or bit rate) is used to measure ...

• The average amount of data per second that a file (video, audio etc) contains.

• The speed with which data can be transferred from a storage device to RAM (normally referred to as "sustained data transfer rate")

Clearly data will arrive in RAM faster from an internal HD than a 56K internet connection.

Data size

"Data size" is the total size of a given video or audio file (data rate multiplied by the video or audio file's duration in seconds).

You will be concerned with data size for the following reasons ...

• Because all storage devices have finite capacity you will need to ensure that your data will fit on your chosen storage media.

• If an end-user must download a complete file before viewing and time is an issue. How long will your end-user be willing to wait?

• If your data files are so large that they place unreasonable demands on the free application RAM of your end-user's system.

Bandwidth

You may wish to read the articles Computer counting systems (binary / bit depth explained) and Units of memory and storage (bits, bytes megabytes etc) before reading this section.

"Bandwidth" is the amount of data per second that a particular network connection, storage device or piece of hardware is capable of delivering.

Be aware that bandwidth can be expressed in ...

• Bits per second (Bits/sec, b/sec, bps). Be warned, sometimes the expression "bps" may mean bytes per second.

• Kilobits per second (kb/sec, kbps). Network bandwidths are often expressed in kilobits per second.

• Bytes per second (Bytes/sec, B/sec, bps). Be warned, sometimes the expression "bps" may mean bits per second.

• Kilobytes per second (Kbytes/sec, KB/Sec, Kb/sec, KBps, Kbps)

• Megabytes per second (Mbytes/sec, MB/sec, MBps, Mbps)

Network connection technology bandwidths

56K (V90) MODEM: Because 1 byte is 8 bits, a 56K (56 Kilobits/sec) modem connection has a maximum bandwidth of 7 Kilobytes/sec (56 divided by 8). in practice such modems rarely connect at more than 42Kbps (see Contention and other problems below).

ISDN (single channel): A single channel ISDN connects at 64Kbps everywhere in the world.

ISDN (dual channel): Given that most residential ISDN lines are 2 channel capable you can expect a minimum bandwidth for ISDN of 128Kbps. Business lines with multiple channels (up to 30) may offer significant speed increases.

T1: This is a US standard high speed link and offers speeds of 30 x
56Kbps

E1: This is the UK equivalent to T1 and offers 30 x 64Kbps.

BROADBAND: Broadband has no fixed definition, so can mean anything better than dual-channel ISDN up to gigabits/sec.

Contention and other problems

Modems and other network technologies suffer from several problems, preventing them reaching their theoretical maximum quoted performance ...

• NOISE is analogue interference which enters the telephone line between home and digital exchange.

• BANDWIDTH CONTENTION is the number of people who share the bandwidth at the receiving end.

• MODEM CONTENTION ratios refer to the number of subscribers in any given time period divided by the number of modems available in that time period.

(If you've got this far (well done!) you'll probably want to read our article on Choosing appropriate settings for optimising audio and video files.)

For a more in-depth discussion of Bandwidth click here.