Teletext Tutorial (4)
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Teletext Tutorial (4) |
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 Many people are overwhelmed by the prospect of programming their home video recorders for future recording; wrong programmes are often recorded due to programming error or re-scheduled transmissions by the broadcaster. Programme Delivery Control (PDC) is a system which permits simple programming and recording control of VCRs using Teletext technology. The main benefits of the system are:
It is very much simpler to programme VCRs to record. Typically all the viewer has to do is position a cursor against the desired programme on the teletext TV Guide page before pressing a program button. This is much easier than the multiple keystrokes commonly required with manual VCR programming where a minor entry error can mean a lost programme.
Unlike some other quick VCR programming systems, programmes are recorded even if the broadcaster changes the transmission times due to over-runs, schedule changes, etc. It is quite common for scheduled programme times to change due to overrunning live events e.g. sports, concerts, etc. Some broadcasters regularly start programmes a minute or two earlier than published: this, combined with slow reorder start up times, can lead to frustrating losses. Under PDC, the recorder is under the control of the broadcaster who transmits accurate recorder start and stop information with each programme.
Under PDC the VCR can be programmed to look out for and record certain types or categories of programme. In addition to on/off recording control information, data is also transmitted about programme categories and intended audiences. General categories include sport, music, leisure, etc. Intended audience data identifies different age groups, disabled people etc. For example it is possible to programme a VCR to record all programmes featuring rock music or athletics.
For PDC to work, the broadcaster should be transmitting PDC data and the viewer should have a VCR capable of making and controlling PDC selections. PDC can be considered in two parts:
There are a number of methods of preselecting programmes. However all methods require that the receiving equipment is set up with a code or label that identifies the programmes to be recorded. This label could be entered manually using a keypad or bar code reader or it can be derived from teletext. There are two methods of transmitting programme preselection data via teletext: Placing and reading data on a viewable teletext page. This is known as transport method A. Placing and reading data in hidden or ghost packets in the teletext signal. This is known as transport method B. Transport method B preselection data is carried in a hidden teletext line known as Packet 26. One teletext packet comprises 45 bytes of data transmitted in a television line within the TV Vertical Blanking Interval. Packet 26 data contains data on each programme listed on the TV schedule page, identifying channel, programme data and start time and also the position of the programme information on the TV Guide page. The viewer uses a remote control handset to position a screen cursor on the selected programme. The cursor position for the selected programme is linked or mapped to the appropriate Packet 26 preselection data which is used to programme the VCR timer. Using this system the TV Guide information can be in any format and located anywhere on the page provided an obvious selection area is available for each programme title.
There are various methods of transmitting recording control data. One method is to insert data in hidden or ghost teletext packet defined as packet 8/30 format 2. (See section 7 for more details). In this method of recorder control, the broadcaster sends a label within the packet accompanying and identifying each transmitted programme. If the transmitted label information matches that stored in the VCR by user preselection, recording begins and continues until the label changes indicating the start of the next programme. The label normally appears a preset time before the start of the programme (typically 30 seconds) to enable the VCR tuner to detect it and place the machine into record mode. It should be noted that the VCR may have to scan a number of different channels while in standby or record mode. | |||
Programme Delivery Control (PDC)