A   S t r e a m i n g   M e d i a   P r i m e r :  
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
31
31
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,       
http://smw.internet.com/symm/tutor/bandmath/index.html
46  QuickTime for the Web, by Steven W. Gulie, May 1, 2000, Morgan Kaufmann Publishers
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