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My digital TV has a menu containing a guide to several days鈥� worth of forthcoming programmes on each channel. When I first bring up the guide, there are lots of blanks in the schedule. Over the next few minutes these gaps are gradually filled, seemingly at random. How does this happen and what determines the order in which the holes in the schedule are filled?
鈥� Digital television is sent as a series of packets of information, each containing a block of compressed video, audio or the TV programme data your questioner mentions. These packets are interleaved to produce a stream that can be picked up quickly after selecting the channel.
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The amount of compressed data generated by audio and particularly video varies: slowly changing scenes need less bandwidth then fast-changing ones. So the TV programme data is chopped up and transmitted when there is spare capacity left by the other data streams. Since it is effectively transmitted one line at a time, it does not have to be sent in any order.
Alec Cawley, Newbury, Berkshire, UK
鈥� The answer depends on the type of box you have to receive television programmes. Free-to-air boxes are probably receive-only devices so cannot request specific data. I suspect the whole schedule is broadcast in repeated chunks and the box has to wait for the relevant data to be received before showing it, similar to waiting for a page to come round in the UK鈥檚 teletext system.
With cable boxes, the transfer of data from a server to your box takes time. If you had to wait for this information for every page you wanted to skip before reaching your desired page of TV programmes you would quickly get frustrated by how slow it is. To combat this, when you move to a new page a request for that page鈥檚 data is sent in the background. If you are still viewing that page when the data arrives it is displayed. If, however, you have moved to another page, the data for the previous page is either discarded or cached locally in case you go back. This is a technique called 鈥渓azy loading鈥�.
If you have a cable box and don鈥檛 see all the programmes on the page appear together then this suggests poor implementation, probably because the code to update the display is inefficient.
For both box types the order in which the information appears on the screen depends on the way the server鈥檚 software is written. The order that TV programme information is sent in the data packets could be determined by factors such as the order in which it was originally added to the database, or which data the database server has previously cached to improve performance.
Peter Morris, Birmingham, UK