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About Video

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.

Advices when experiencing problems with video - summary

The problem

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:

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.

What is needed to play video over internet

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.

1. Uncompressed

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

Uncompressed 34 frames 52 x 39 Size: 205 kB


2. Cinepak

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.

Cinepak 34 frames 160 x 120 excl audio. Size: 219 kB


3. Intel Indeo

There are plenty of compressing programs available. Here is one example from the middle category. The image size is 280 x 208.

Intel Indeo 34 frames 280 x 208 excl audio. Size: 241 kB


4. Windows Media 9

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.

Wm Pal 1024 34 frames 640 x 480 excl audio. Size: 177 kB


5. DV - original quality

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.


6. One image in original quality

For those not able to play the DV format, one single frame is here shown for comparison. This frame lasts for only 1/25's second when playing the video in TV quality, which today is considered as rather low. This is a good illustration of how much information that has to be transferred when playing video.

Mosquit safari on Seskarö 2004-07-14