Key Concepts in Streaming Media Technologies:
Summary: This article explains some basic concepts of digital media
delivery over a network, and contains information about capturing, distributing,
playing, downloading, streaming, compressing, encoding Media content.
Introduction
To put together a solution for delivering digital media on a network, you
first need to understand some basic concepts. It is easy to deal with one box
that does everything with the push of a button, like a TV. However, to
understand digital media delivery on a network, you need to have an
understanding of a system that is bigger than one box.
A network digital media system, like the type you can put together with
StreamCanada Media Technologies, involves several boxes and a lot of cable. It
also involves integration with other components, like network hardware and video
capture cards. To understand all there is to know about network multimedia, you
would need to be an expert in producing audio and video content, computer
software development, networking, Web scripting and design, and business plans,
among other things.
Fortunately, you don't have to know everything to start using Streaming Media
Technologies to deliver digital media over a network. If you know how to produce
audio and video content, know how to use a computer, and have general knowledge
of network multimedia concepts, you can get started. From there, you can expand
your knowledge and explore the possibilities.
This part of the Introduction to Streaming Media Technologies explains some
basic concepts of digital media delivery over a network, and contains the
following sections:
- Getting from Here to
There. The process of delivering digital media over a network.
- Downloading. A method of
delivering digital media over a network in which users copy files.
- Streaming. A method of digital
media delivery in which audio and video content is delivered to users, but
files are not copied.
- Compressing. Making digital
media small enough to fit through the network.
- Encoding. Putting digital media in
the correct format to be streamed.
From a high-level viewpoint, the basic process of getting content from here
to there follows this sequence:
- Capture
- Convert
- Distribute
- Play
To help you understand the process, let's say you want to make a movie on VHS
tape available for streaming on the Internet.
The first thing you do is capture the movie to a file on your computer. This
process is called digitizing or capturing. You convert your analog audio and
video to a digital form that can be saved as a file.
To capture from a VHS tape, you plug the video and audio outputs from your
VCR into a video capture card on your computer. You then use a capture program
to digitize the analog signal. Many capture cards include a simple capture
program. Most of these save the digitized media as an AVI file, which is a
standard Microsoft video file format. After the video is saved as an AVI file,
you use a Media Encoder to convert it into a Media file that can be streamed.
You can also capture directly to a Media file by using Media Encoder,
Adobe Premiere, Terrains Cleaner, and more.
Figure 1. The process of capturing and converting audio and
video
When you capture audio and video to your computer, it is no longer sound and
pictures. It is data. Analog information from your VCR is converted into a
digital stream of zeros and ones, or bits. It helps to think of bits as a
long series of instructions. The bits aren't the audio or video itself, but
rather a series of instructions for how to recreate the audio or video in analog
form.
With your movie digitized to a file on your computer, you can distribute it
just as you would any other data. For example, you can make copies on floppy
disks, CD-ROMs, or other storage media, and then send them in the mail to your
audience. You can also distribute the data over a network, such as the Internet.
A network is created when two or more computers are linked so that data can
be exchanged between them. Corporate intranets can consist of hundreds or
thousands of networked computers; the Internet consists of millions.
There are two types of computers on a network:
A client requests data from a server, and a server handles the request and
sends the data back. One of the advantages of Microsoft Windows® operating
systems is that a computer can be either a client or server depending on what a
user wants to do. For example, a computer can be used to copy files from another
computer, and another computer can be used to copy files from the first
computer.
Digital media can be delivered to clients over a network using one of two
methods:
The Media file format is optimized for streaming, but the files can be
downloaded as well. The best way to stream Media files is by hosting them on a
server running StreamCanada Media Services.
After encoding your digital media to a Media file, you host a file by copying
or publishing the file to a location on the server. This location is called a
publishing point. When a request is received from a client, Media Services
accesses the digital media from this publishing point. Then, assuming the server
computer has a high-speed connection to an ISP and is properly registered on the
Internet, users can play the digital media.
The final step is accessing and then converting the bits—the digital
instructions—back to analog form so the movie can be viewed. An end user does
this with Media Player. A digital media file can be opened and played directly
if it is on the end user's computer or another computer connected through a LAN
(Local Area Network). An end user can also play Media files by streaming them
from a Media server. This is done by entering the URL (Uniform Resource Locator)
of the digital media in the player.
The URL for accessing Media files and streams from a Media server is similar
to the URL for accessing Web pages from a Web server. The main difference is the
protocol used. An end user opens a Web page with a URL such as http://WebServer/default.htm.
An end user opens a movie on your Media server by entering a URL using the Media
protocol, such as mms://WMServer/MyMovie.wmv.
End users can open a Media file by entering a URL in Media Player, but
typically, the digital media is accessed from a link on a Web page. For example,
an end user can come to a page for a radio station, read the information on the
page, and then click a link to some Media-based content. When the user clicks
the link, Media Player opens and initiates a connection to the file on the
server. After the connection is established, the digital media starts to play.
There are three main ways to incorporate Media in your Web design. You can
put a simple link on a page that opens Media Player outside the browser and
initiates the connection between the player and the digital media. This design
enables the end user to continue to surf with the browser while listening to the
stream. Another method is to embed the player in your Web page using OBJECT tags
in your HTML. This method enables you to integrate the design of the player with
that of your page. The third method uses a simple link but opens the digital
media in the Media Player radio toolbar that is built into Microsoft Internet
Explorer 5.
The complete Media system involves capturing, encoding, distributing, and
playing digital media. In the following sections, we take a closer look at
downloading and streaming, and what happens during the encoding process.
When you download a file, you copy the file from one computer to another over
a network. This is similar to copying a file locally from a floppy disk to your
hard disk drive.
When you want to play content that is available for downloading, you click a
hyperlink on a Web page that points to a file on a Web server. Your browser
initiates the process of copying the file from the server. After the file has
been copied to your hard disk drive, you can open and play it by using software
such as Media Player.
Downloading files takes time because you have to wait for all of the data to
be copied from the server to your hard disk drive before you can play the file.
Copying time is directly related to the available bandwidth of the
network and the communication speed of your modem (or NIC). Network bandwidth
can be compared to a water pipe. If you connect to the Internet by using a modem
and telephone line, the size of your pipe is very narrow—only a limited quantity
of data can get through in a given amount of time. A file is like a tank of
water. A very large tank of water takes a long time to go through a small pipe.
A large file can take several minutes to download over the Internet. However, it
may only take a few seconds to copy locally from a CD-ROM to your hard drive,
for example, because the bandwidth is much higher.
Figure 2. Bandwidth available with a slow Internet
connection versus a local transfer
When a user plays digital media, Media Player uses the instructions contained
in the bits to recreate the original sounds and pictures. The more instructions
you have, the better the rendition, and the higher the quality; the more bits,
the bigger the file, and the longer the download time.
What if you could skip the download process and simply play the data as it is
being received by your computer? Instead of waiting for the bits to be copied,
they would be converted and played as soon as they arrive. This is the concept
of streaming. The bits are played as they are being received from a server on a
network and are usually not saved on your hard disk drive.
With streaming media, you get instant gratification—there is no download
wait. Streaming an audio or video file is like playing a CD or tape. You have
all the same playback controls, like play, pause, stop, and rewind. The only
thing you don't have is the physical media. That is, the tape or CD. You can
choose what you want to listen to and when you listen to it.
Streaming also enables you to do live broadcasting, just like a radio or
television station, except over the Internet. You can eliminate the need for
files altogether. Media Encoder sends the bits it creates directly over a
network to a player, instead of to a file.
For streaming to work, the bit rate of the media must be lower than the
bandwidth of the network. Bit rate is the speed at which data is sent across the
network. Returning to the plumbing metaphor, if bandwidth is the size of the
pipe, the bit rate is the amount of water—or data per second—that can travel
through the pipe. Because you are playing the digital media as it is being
received, if the network bandwidth is lower than the bit rate of the media, the
media will not play properly.
If you download a still image over a slow connection, the image quality will
not be affected. It will just take longer for all the bits to get to your
computer. The still image itself does not have a bit rate. On the other hand,
streaming media does have a bit rate. As long as the content is playing in Media
Player, the bits are streaming at a steady and continuous rate. The player must
receive a stream of bits continuously or the picture and sound will either stop
or will play back unevenly. Think of it this way: when you encode a file for
downloading, file size is important and bit rate is irrelevant. When you encode
a file for streaming, file size is irrelevant and bit rate is important. You can
easily stream a very large file, even one that has an undetermined size (such as
a live stream), as long as the bit rate is within a client's bandwidth.
The bit rate of high-resolution, full-frame, broadcast video is about 128
megabits per second (Mbps). To download one second of broadcast video over a
28.8 kilobit per second (Kbps) connection using a modem would take one hour and
14 minutes. Streaming this type of video would be impossible over a network. To
recreate every detail of a video frame would require so many instructions that
most computers couldn't even play the video. It would also require a huge amount
of storage space for the file. Streaming Media Technologies handles this problem
by using compression. Compression lowers the bit rate while maintaining
the best possible quality.
A compression algorithm analyzes the data and removes or changes bits so that
the integrity of the original content is maintained as much as possible, while
reducing the file size and bit rate. Media Encoder compresses the data when you
create a Media file, and Media Player decompresses the data when you play it.
There are two types of compression: lossless and lossy. As the
names suggest, with lossless compression, data can be compressed and
decompressed, and the decompressed data matches the original data exactly. With
lossy compression, data is lost during the compression process and cannot be
recovered during decompression. The amount of data lost depends on the quality
of the compression algorithm and the amount of compression applied to the data.
To achieve the low bit rates necessary to stream audio and video over the
Internet, streaming media compression and decompression algorithms (called
codecs) are lossy.
Streaming Media Technologies has been engineered to get the highest quality
possible at a number of bandwidths and to automatically adjust the stream to
accommodate unevenness in available Internet bandwidth. When you select a
quality option in Windows Movie Maker or Media Encoder, the digital content is
compressed so that it does not exceed a given bandwidth. Therefore, you can
create content with a bit rate of 20 Kbps, for example, that will play correctly
on a computer that is receiving the movie over a 28.8 Kbps connection. You can
also create a movie at 384 Kbps that will stream over a high-speed connection,
also known as a broadband connection, such as when using ISDN, DSL, or a
cable modem. A consequence of reducing the bit rate is a reduction in file size.
A file that has been encoded for streaming may also be suitable for downloading
or copying to a disk.
The Media Audio and Media Video codecs are used to compress your digital
content. Aside from enabling high-quality playback of audio and video, they are
designed to help the stream withstand highly variable bandwidth conditions. For
example, when data is lost in transmission, the video codec attempts to fill in
the missing parts of the frame. The audio codec enables you to get very good
audio quality at relatively low bit rates. For example, you can get close to
CD-quality audio at a small fraction of the bit rate and file size of
uncompressed CD audio. A standard CD can hold a little over an hour of music. If
you compress the music with the Media Audio codec, you could have near
CD-quality playback and fit 10 or more hours of music on one CD.
When Media Encoder converts a file or captured audio and video, it first
compresses the data, and then encodes it with the Media Format. After digital
media has been compressed and encoded, it can be saved as a Media file. If the
digital media is a live broadcast, the media is delivered in real-time to a
Media server, from which it is streamed to players that are connected to the
server.
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