H O W D O I M A K E ( G O O D ) S T R E A M I N G M E D I A ?
HOW MUCH COMPRESSION IS ENOUGH? UNDERSTANDING BANDWIDTH MATH
45
In order to be sure your streaming media plays
THE RECOMMENDATIONS
HELPFUL TOOLS
smoothly, you'll need to determine how much
Check with the provider of your streaming
You could use a calculator and graphing software
data you can send to your audience, without
architecture to ascertain what bandwidth is
to carefully plot out kilobits vs. time... But, for
clogging up the system. The good news is that
recommended. For example, Real recommends
tunately, clip analysis tools in software such as
there are tools you can use so you don't really
using 34Kbps for 56K modems, while Apple
Adobe Premiere and Terran Interactive's Media
need to do any math, but it's a good idea for you
(QuickTime) recommends using 53Kbps.
Cleaner will graph the data rate of your movie for
to understand the underlying principles.
THE MATH
you. Using these tools, you can check portions of
THE FUNDAMENTALS
your program as you develop it, to be sure you
Now let's suppose you've got 50KB of video to
File sizes are measured in K or KB short
stay within the required bandwidth limitations.
stream to a 56K modem. To determine how much
for kilobytes (1,024 bytes; usually rounded to
time the sequence needs to stream:
HELPFUL HINTS
1,000 bytes). Since a byte is 8 bits, a kilobyte is
1. Convert the storage measurement (kilo
Don't forget to include the audio in your
(approximately) 8,000 bits.
bytes) to the shipment measurement (kilo
bandwidth calculations.
Transmission rates are also measured in K ,
bits) by multiplying by a factor of 8, since
Keep up with the latest documentation for
but the K here is short for Kbps kilobits
there are 8 bits to a byte.
the streaming format you are using different
per second not kilobytes, but kilobits. Note
50KB (kilobytes) X 8 = 400Kb (kilobits)
formats handle bandwidth in different ways.
that there are exactly 1,000 bits in a kilobit.
2. Divide your shipment by the bandwidth rec
Buy some time put lightweight graphics
Don't mistake kilobytes for kilobits. A 56K (i.e.,
ommended for the targeted connection. We'll
and text that consume little bandwidth up
Kbps, or kilobit per second) modem won't
use Real's recommendation of 34Kbps for a
front, to buy loading time for heavier weight
download a 56K (i.e., KB, or kilobyte) fi le in
56K modem.
material that comes later. Your audience will be
one second. And it gets more confusing
400Kb 34 Kbps = 12 (11.76) seconds
distracted while content is being loaded and
THE LIMITATIONS
buffered in the background.
3. Divide your shipment by the bandwidth rec
A 56Kbps modem is rated to move 56 kilobits
Don't disregard CPU capacity. Like most
ommended for the targeted connection. We'll
of data per second. That's 56,000 bits. Divide by
video or graphics professionals, you probably
use Real's recommendation of 34Kbps for a
8 (the number of bits in a byte) and you get
work on a relatively fast, powerful computer.
56K modem.
7KB (kilobytes) per second a more familiar, and
Everyone in your audience may not. Even if you
400Kb 34 Kbps = 12 (11.76) seconds
therefore more meaningful measure of fi le size
are careful to spread out the data rate, complex
for most of us.
So it will take a little less than 12 seconds to
transitions or effects can bog down the client
stream the 50KB. If your video is comprised
computer especially if the user has other
THE VARIATIONS
of three different images, each about the same
applications running in the background.
According to the quick calculation we've just
size, and your transitions are equally spaced at
Test the limits. Don't throw away or donate
done, a 56 KB image fi le would need at least 8
4 second intervals, it should stream quite nicely.
those old dial up modems or slow machines.
seconds to stream over a 56K modem...
But what if your images are different sizes? For
Keep them on hand for testing the limits.
Why did we say at least? Well, for all you math
example: 17KB, then 25KB, then 8KB... If we do
How low can you go without completely
whizzes, a 56K modem won't download a 56K fi le
the math, everything will be fi ne for the fi rst four
compromising your content?
in 8 seconds, because, as you'll recall, a modem
seconds. But, for the next image, we may start to
K is exactly 1,000 bits, while a fi le K is actually
RULES OF THUMB
get into some trouble:
1,024 bytes and, with start bits, stop bits, and
Even if you were to calculate and graph precisely
modem compression, a byte can use more or less
25KB X 8 = 200Kb
how much bandwidth a fi le requires, the result
than eight bits of bandwidth.
200Kb 34Kbps = 6 (5.88) seconds
would be only an approximation of the actual
Furthermore, 56K modems do not consistently
Trying to push 6 seconds worth of material
data rate as it streams over the Internet. The
provide 56 kilobits of throughput 56K is the
through the pipe in 4 seconds could clog up the
bottom line is that you need to use rules of
upper limit. The actual bandwidth fl uctuates
pipe and/or choke the streaming player...
thumb and leave some slop. See the chart
continually, frequently dropping well below
below for Safe bets using Apple QuickTime.
46
56 kilobits per second.
Connection
Rated
Typical throughput
Safe bets
28.8 modem
28.8 Kbps
2.4 KB/second
2 KB/second
56K modem
53 Kbps
4.8 KB/second
4 KB/second
Dual ISDN
128 Kbps
12 KB/second
10 KB/second
DSL
384 Kbps
35 KB/second
30 KB/second
namic Medianamic Media
T1
1.54 Mbps
150 KB/second
50 KB/second
yy
Cable modem
6 Mbps
300 KB/second
50 KB/second
Intranet/LAN
10 Mbps
350 KB/second
35 KB/second
100base T LAN
100 Mbps
500 KB/second
50 KB/second
obe Dobe D
dd
Kbps = kilobits per second Mbps = megabits per second KB/second = kilobytes per second
AA
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45 The term Bandwidth Math and examples cited in this section derived from Don't Be Scared of Bandwidth Math, by Tim Kennedy, Streaming Media World, May 26, 2000,
