On a digital TV channel the guide data can be multiplexed into a sub channel on the MPEG carrier. In the normal course of events this does not need to be a very fast channels. When a TV starts up from cold it obviously needs to read this channel and store the information. I have no idea how it is decide which order to traansmit the data in. This data is being streamed so when you switch your TV on from cold it may start reading at any point in the stream. If your TV is left in "standby" or even switched off but with power connected, it should retain the guide information in memory and will only have to update any changes from the last time it was switched on.

Some TVs may have non-volatile memory and retain guide data when depowered, mine does not.

The answer is: software design and the notorious laziness of software engineers.

When I read a question like "Blank look" I wish I had "New Scientist" subscription, so I could cancel it!

Oh, I actually did cancel my subscription three years ago after reading a question about the relationship between shaving quality and the speed of an electric shaver...

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.

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

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's 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's 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 "lazy loading".

If you have a cable box and don't 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's 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

Two fine answers from Moderator, but I still have a hangup.

My DTB tuner picks up about 100 stations from multiplex channels numbered from about 21 to 68, and from at least 2 separate ground-based transmitter sites (Crystal Palace and West Berks, I think).

If I am watching, say, BBC4 on station 9, and put up the all-channel EPG at 20:58, then ALL the program details update around 21:00 as the program changes roll over, including ones on the other multiplexes and other transmitters. I still have live BBC4 in my picture-in-guide format.

Does that mean every multiplex carries program information for all other transmitters and multiplexes too? Or does it mean my digital box has a second tuner that cycles through all channels just pulling the EPG stuff?