Newsletter #135 - January 27, 2007
the Bitrate
of Stories
If you need to review your bitrate math, and brush up on your
understanding of CBR versus VBR, here's a quick brief...
Bits and bytes and bitrate...
What's bitrate? If you watch a video file that is 100 MB
(megabyte) in size and is 10 minutes long, the math says it's
100,000,000 bytes (or 800,000,000 bits, as a byte = 8 bits) over 600
seconds.
Smooth transmission and playing of such a file means
the data flows at an average rate of 133,333 bits per
second, or 133.3 Kbps (Kilobits per second).
The Profile Editor for Movie Maker and Photo Story gives you
a choice of CBR (Constant bitrate) or
VBR (Variable bitrate)...
When you plan a long car-ride and use an average of
60 miles per hour for the whole trip, that would be a constant rate.... but in
the real world you go faster on better roads and slower on city
streets... you use variable speed to adjust for
conditions.
A video file of constant bitrate makes for nice easy
calculations, but in the real world with variations in visual scenes, the
overall quality is better if more bits are used when there's a lot of visual
changes going on, and fewer bits when there's little change from one frame to
the next. It's like you can save or budget your bits
for use when most needed. In the long run the video average might be
the same.
For a Movie Maker movie you can choose either CBR or
VBR. But when making a profile for Photo Story, the Image codec will
only work if you select VBR. Like it or not, we use VBR for
stories.
When doing a file download, the buffering of bits
offers the luxury of having some saved-up bits ready to use when the
bitrate needs demand them. Streaming video on the other hand, demands
that your player use the bits as provided... and when the flow of bits
can't keep up with the demand, the video goes to blackness. We'll see that
illustrated in the example for this issue.
Newsletters #130 and #131 were about making a story from a
video clip, and then notching it up in size to higher-def ones
that still play smoothly online... over a good broadband
connection....
This week takes all the successes to make my
'best-ever story'... the first good draft of a Florence story, one of
my Europe-301 vacation projects. In doing it I found some things that
worked, and others that didn't. As I explored the failures I found
myself deeply into bitrates.
Here's the link to the finished 9-1/2 minute story (or
click the picture). The 'video-like' segment that I thought would make a good
opening clip is in it, but it starts 1-1/2 minutes into it.
It's the most complex story I ever made... with 270
pictures... 187 of them are those pictures of the river with the
moving boats, pictures being flipped through at the rate of 5 per
second to make a 'video-like' segment.
It turns out the segment needs a lot of saved up bits. They were
when playing the file-download video, but not the streaming
file.
What didn't work?
-
The story plays smoothly as a file
download from my 1&1 website server... but the video-like
segment goes to blackness when viewed from the mydeo streaming server. If
you want to compare them yourself, here's the link to the
story
on mydeo to contrast to the file downloading from the first
link.
-
The complexity of the story was such that it wouldn't get
through the rendering step on my laptop with XP and 2 GB of RAM. I resolved it
by doing the rendering on my Vista system with its 1 GB of RAM. The
laptop renderings kicked into gear after a few days, so the complexity
and available memory may be going into and out of borderline territory.
Neither XP or Vista could render the story to a size of 1280x720, so I settled
for a still respectable size of 852x480.
To check why the story would download smoothly
but not stream, I took
WMSnoop out of
my toolbox and studied the bitrate frame by frame.
The modest overall video bitrate of 108.4 Kbps says it should play
well... but the batch of 187 pictures going through
quickly needs almost 3 Mbps for a 40 second period.
The mydeo streaming starts into the segment, but the overall
bitrate can't flow through the pipeline between mydeo and me fast
enough to satisfy the playing, and the visual turns to blackness. It
recovers by itself when the story returns to a normal bitrate after that
segment.
Why doesn't the file download run into the issue? Because
the long buffering before starting saves up enough bits to get over the
hill smoothly....
... pause
for a few notes...
Notes...
Reader's Comments about Last Week's
Newsletter...
One
provided info about the c:\Windows\Prefetch folder, with
things that auto-start at bootup... an interesting
place I'd never explored.
I looked in my
prefetch folder, and sure enough, it explained why I had an iPodService.exe
and an iTunesHelper.exe running even though the services list in Computer
Management shows iPod Service as a Manual startup type. The prefetch
list is what turns it on at startup. I'm using 3,852 K of memory for my
laptop as it waits for me to plug in my iPod, but I
don't own one. I changed the setting from 'Manual' to
'Disabled' and it turned it off, so the prefetch item no longer 'manually' turns
it on. Turning off iPod Services also turned off the iTunesHelper.exe
process, which used another 5,032 K.
Another
reader passed around the comments
about laptop batteries to some 'in-the-know'... who
say the limited number of "cycles" for Li-Ion batteries is in the order of
300-500 IIRC (if I recall/remember correctly), but leaving it plugged in when
shut down shouldn't be cycling it. He also provided a link to a site with
useful info about getting great battery life.... charge often and alternate
batteries.
Vista Corner...
The transition from the
marketing team to the tech support team happens in a few days.
Vista scored again in
better memory management, saving my final story when my
laptop XP system with twice the RAM couldn't.
I think highly of
TripAdvisor... mostly for the useful trip planning info we've gotten from
their site, but partly because they've rejected me three times so far.
-
When I wrote about the
hotel we stayed at in Florence, they edited out the link to my website
page where readers could see videos.... this was before they started
accepting videos themselves
-
When I uploaded two videos about
Florence, they let one go online but stopped the other one about the B&B
we stayed at, on the basis of it being too commercial looking... it
was!!
-
I submitted an application to become
an affiliate and add appropriate links on my Europe 301 pages,
which are still in progress.... they said to try again someday
when the website was more finished
We did a lot of trip
planning last year using their website, and will be doing more this year... if
they are that careful screening my stuff, they must do it to everyone,
a clue that info on their site is highly credible.
While bleeping and
rejecting, they have been very quick to turn around each item,
and responsive to my questions and comments... and they recently
started accepting videos, which adds a whole different slant on our possible
relationships.
Lesley at TripAdvisor, who
I've been working with lately, uses a Mac, so she couldn't view the
online Photo Story for this newsletter, and didn't know if they could accept
such a file. She checked with their engineers, who use Linux machines and
said they couldn't view it either... they asked if I'd upload it
to see if their conversion process would handle it... conversion to
flash format.
... and it did. Here's the
link to my playlist at Trip Advisor... this week's story is
As a widescreen story, it's
got the right aspect ratio, but I suggested moving it up to the middle
vertically, more normal letterboxing.
.... back to the main
topic...
the Rest of the Story for this Story
CBR versus VBR...
A video produced to play at a Constant Bitrate (CBR) has a fairly even
pattern... here's a picture of what the story looks like when I run it through
Movie Maker and save it to a 2.1 Mbps CBR file. Relatively nice and even
throughout, at least compared to the VBR story pattern.
Note the average file bitrate of 1.907 Mbps versus the 108.4 Kbps of the
story file... the movie is 18+ times the bitrate of the story, too much
for even good broadband internet pipelines to deliver a steady
streaming file..... file download is possible, but it's a long time between
starting the download and being able to start viewing a smoothly playing
video.
Photo Story, using the Image 2 codec, only does VBR
mode.... which rules out the bitrate leveling of a CBR
setting.
Frames per second and keyframe
intervals
My starting profile used 30 frames per second with keyframe
intervals of 8 seconds.
I made changes to the profile to see what effects changes to
them would have. You can see from the file list at the right that the changes
had minimal effect on file sizes.
And the bitrate patterns in WMSnoop were remarkably similar.
To check what the wave patterns were saying, I uploaded the 24 fps story
with keyframes every second to mydeo.... not only did it not resolve the
blackness in the video-like segment, but the viewing was terrible
throughout. It was easy to see that reducing the frame rate or
the interval of keyframes isn't the way to resolve the streaming
issue.
The quality of the local playback for all of the files was too
close to call. I have a personal preference for online viewing at the same rate
as movies, 24 fps... but I'm not seeing enough reduction in file sizes
to adopt or recommend it as anything more than a personal
choice.
My last shot at a frame rate change was to try 5 fps... more
curiosity about the bitrate pattern than the visually jerky playback.
The playback was as far from smooth as you'd guess, and the file
size and bitrate pattern aligned with the other changes.
Conclusion and Closing... and What's Next?
For large-size high quality stories, go with a file-downloading
process and not streaming video... I wrote to mydeo a few days ago asking
for any comments, but haven't heard back yet.
I started into the next layer of exploring, but didn't do
enough except to say where I was heading. It's the profile settings which you
don't see or tweak when using the Profile Editor... could lines such as these in
the profile mean anything?
bitrate="1000000"
bufferwindow="20000"
reliabletransport="1"
They can be changed manually using Notepad... and I noticed that
if I temporarily toggle to CBR mode in the Profile Editor, change the
bitrate setting that isn't an option when the mode is VBR, and then
switch back to VBR... the setting change in the profile remains where you
left it, so you can change the setting (but does it do anything or does the
quality setting drive it all?).
Have a great week!!
PapaJohn
