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Blogtalk Downunder » Broken Data

Blogtalk Downunder
   May 19-22, Sydney

Blogtalk Downunder

Broken Data

To be able to serve only parts of a file the server hosting the content (in this case video) must support byte serving, which is not commonly the case, while the application requesting the data must also be able to refer to the specific bytes required. In this prototype, while I might wish to play from 01:01:24 (this is standard time code and refers to minutes:seconds:frames) to 01:020:00, this must be translated from within QuickTime into a byte request so that only the data corresponding to this section of the video is sent to the client and then played within the movie. However, this is further complicated (and largely broken) in QuickTime because most web published material audiovisual is what is known as ‘fast start’ which means crucial movie data is placed at the beginning of the file. This is what allows a movie to start playing before all of its data has arrived, but if it were being byte served then such a data structure for a movie file may in fact render partial data unusable.

Perhaps a simpler illustration is in order. Imagine viewing an image on a web page, but instead of requesting and viewing the entire image you only wanted to top left corner. Your client could request this part of the data, and the server send it to you, however all the necessary data to draw the top right portion of the image does not reside in a common location in the file format of that image. In other words it isn’t just a case of sending data from byte x to byte y. There is data that describes the colour values of each pixel, the location of that pixel, as well as such things as luminance and the like.

These issues are resolved if time based media is published using the Real Time Streaming Protocol (RTSP). This format allows for the serving of parts of larger files, without having to download all of the data in the file. However, in videoblogging RTSP is not used because the protocol is designed for live broadcasts and does not resend lost packets (lost information). Hence if you care about how your video looks when it is viewed, RTSP is not a good format simply because if 50% of the data is lost on the way to the client, then it is lost, and your video will look appalling (imagine trying to listen to music where only half of the music can be heard, and where that missing 50% is random!)

This does not make such a task impossible, but it does suggest that, outside of text, it might be simply too difficult to achieve. Hence in videoblogging, if we want or feel the need to develop practices (informal or otherwise) that support the quoting of each others video then we probably need to pay careful attention to the scale of our video, it’s size in relation to the network. What I have been characterising as its granularity.


note: This page forms a part of a hypertext essay by Adrian Miles. The homepage for this essay is located at:
http://incsub.org/blogtalk/?page_id=74
A long version of this paper (containing some but not all of the text contained in the hypertext version) is available at:
http://incsub.org/blogtalk/?page_id=76

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