I have sometimes got questions about why video from the dance reports do not play. Although the subject strictly is not one of those covered in this site, I will try to give some explanations and advices.
|If you use a computer at your work, try to find out if video is filtered out by your employer's firewall|
|If you have no reasons to believe that video is filtered out before it reaches you, then consider installing the latest version of the Media Player through Windows Update or through the Microsoft Web site. If you are prompted to install a codec when you attempt to play the video, verify that Microsoft is the publisher and then accept the update - otherwise you will not be able to play the video.|
|If no of these hints help, then try to open the video directly from the Media Player|
Video is demanding. An ordinary video for VHS video players, using the European PAL format, has 25 pictures per second using approximately one half of the resolution of the TV itself, or close to 320 x 240 pixels. This is the same resolution as the best videos that the Dance Reports currently use. The source for these videos are mostly Mini DV, which is capable to store the same resolution as the TV can display, and with stereo sound as well.
A high bandwidth is required to send such pictures and video in the source quality without any compression, much more than internet and computers can deal with. One single second of uncompressed video would use about 30 MBytes. Compare this with the broadband connections to internet available today, mostly giving from 0.5 to 10 Mbit, and dialled connections perhaps giving 0,05 Mbit at ideal conditions. As these are bits while the file sizes are in bytes, you need to divide the transfer rate by approximately 10 to get the number of bytes per second.
And then add to this that these transfer capabilities are somewhat theoretical, in many situations the actual transfer rate can be much lower due to limitations somewhere along the route from the source to your own computer.
To address this problem, video is compressed before it is published. The compressed contents is transferred to your computer, and - providing that the software to decompress it is available on your computer - it is decompressed again there before it is played.
Most of us have met compressed information. Music compressed to MP3 is widely spread, and pictures compressed to JPG are not only used on internet, but almost anywhere where pictures are handled digitally, including most digital cameras.
For video as for music and pictures, there are however more than one alternative for the compression. To mention a few, pictures can be compressed to PNG, music to Ogg Vorgis and video can be compressed using many different compressors.
The video is mostly compressed both for each image with methods similar to JPG for photos, and between adjacent images so only what is changed between the images is stored and sent. If for example a static object like a house is filmed using a tripod, all frames will be more or less identical, and much resources can be saved by just sending what has been changed between each frame. But also in a such situation the lightening might be changed by clouds, and the difficult question is to decide how much of the changes should be ignored to balance between video quality and the size of the rendered file.
The compressing techniques used for internet almost always decreases the quality of the video, because of the need to make the video file size smaller. The following methods are used:
|Decreasing the original image size|
|Removing information when compressing. With a higher compression the file size will decrease, but so will the quality|
|Compressing or omitting the sound|
There are many programs available for compressing both video and sound. A compressing/decompressing program is often called codec.
Although sound can have similar problems as video, the sound problems are less complicated. As Dansglädje so far hardly use any sound - basically to avoid eventual conflicts with the creator's copyright to the music - I will here concentrate on video.
Below there are some examples of a very short video compressed with different compressing techniques. If you have problems with video, it might help to try them. The files have been kept at a similar size - about 200 kBytes, to make the efficiency of the compression viewable.
The image size has been decreased to just 52 x 39 pixels to make it fit within 200 kB. This sequence, as all others except the DV at the bottom, has no sound and contains 34 frames (a little more than one second). No codec is needed to play this file
This is an old codec that might work also on old computers. It is rather inefficient, and has the image size of 160 x 120.
There are plenty of compressing programs available. Here is one example from the middle category. The image size is 280 x 208.
This encoder gives the most efficient compression among those compared here. To be able to play this, it might eventually be needed to install Media Player 9, and at least to install the codec if the notification about this appears. It can probably also be downloaded from www.microsoft.com.
The image size is 640 x 280,. and despite this the file size is a little smaller than those above.
The source to the videos in Dansglädje can be the simpler digital cameras or as in this case mini DV. The sequence shown above has 34 frames, with a playing time of a little more than one second.
DV has full TV quality, and has a fixed compression giving about 3 MB per second including stereo sound. Within 200 kB it is only possible to include one single image with sound, that is 1/25 second, with the European format PAL (720 x 576 pixels). To be able to play this, a codec for DV is needed.
For those not able to play the DV format, one image is here shown stored as a picture for comparison.