So instead of using its clock, the video uses the PDC signal to decide when to start and stop recording. This way you don't have to worry if everything is running late - the programme still gets taped.

The video sits and looks out for the PIL some time before the start time it is programmed with. (A programme can start early as well as late!) If it is programmed to record from more than one station, the video will channel hop, as necessary.

PDC videos can be programmed in the same way as ordinary machines: "manually" or (if available) with VideoPlus+. The BBC recommends you program the video by taking the published start time or VideoPlus+ number from a weekly listings magazine. It's usually these times that are contained in the broadcasters' PILs. (The end time isn't so important for programming, since it is not used in the PIL.)

Some PDC videos can also be programmed by selecting the TV show directly from the teletext listings page with a cursor. These machines tend to be more expensive because they contain a teletext decoder. This method does have the advantage that it's easy and you don't have to have a magazine to hand. It's also far more accurate.

When programming a PDC video manually the video determines the PIL it will look out for from the date, channel and start time you give it. Similarly with VideoPlus+, the PlusCode is decoded to determine the PIL that the video will use. This can cause problems. For example if a programme was originally scheduled to be broadcast at 20:00 when the broadcaster had determined its PIL, then 20:00 will be the start time used. If however, the show was then set to be moved to 20:30 before the weekly TV magazine was published, its pages will show 20:30 as the start time. So, no matter if the show actually starts at 20:30 and you program your PDC video with this actual start time, your video will not tape the programme because the PIL broadcast contains the time 20:00. To get around this problem, the Radio Times always indicates the start time (and corresponding VideoPlus+ code) used by the broadcaster to determine a PIL wherever it differs from the latest known actual start time.

With a StarText/PDC video recorder, you don't have to worry about knowing a VideoPlus+ code or whether the listings contain the same start times as used by the broadcasters when they determined the PILs. What happens is when you program using the teletext page, the PIL for the programme you're taping is actually looked up. The PIL for each programme is hidden from view, below the bottom line of the teletext page, where it can be accessed by the video during programming. This way, the teletext page can show the latest known start time whilst the video can determine the correct PIL containing the original start time. The drawback of this system is that because the teletext services only carry listings for today's and tomorrow's TV, you cannot program your video a week ahead, say.

Of course, if your video is equipped with PDC, but the TV station is not broadcasting a PDC service yet, then whatever programming method you use, the video will only be able to turn on and off at the times it is programmed with, thus missing parts of programmes that run late.

The most common mistake to make when recording with PDC is to forget that it works on a programme-by-programme basis and that it ignores the programmed stop time.

Remember: if you want to record two or more consecutive programmes you must set the video for each programme individually.

For example, if you want to tape Friends from 21:30-22:00 followed by Frasier from 22:00-22:30, you might set the start time to be 21:30 and the stop time to be one hour later, 22:30. Normally you would expect a video would tape both programmes. With PDC enabled however, the video would assume the start/stop times applied to a single programme and since it ignores the programmed stop time, you would only get Friends on the tape and not Frasier.

If you use VideoPlus+ or StarText to program your video, it's not possible to make this mistake, since both systems work on a programme-by-programme basis.

VPS (Video Programming System) is used on terrestrial channels in some European countries (e.g. Czech Republic) and also on some of the channels on the Astra satellites. (See below for a list.) VPS videos usually have a 4-character display, which shows the channel name.

The VPS data is in the same format as that of PDC, but is broadcast in a different way. (For more details on this see the technical section below.) PDC is a newer and slightly more advanced system than VPS.

Some, but not all, of the PDC video recorders sold in the UK will be able to decode Astra's VPS signals too.

Currently in the UK, BBC One, BBC Two, Channel 4, Five and seven ITV companies - Border, Grampian, Granada, Scottish, Tyne Tees, West Country and Yorkshire are broadcasting a full PDC service.

This means viewers in Scotland now have a PDC service on all five terrestrial stations (although the BBC service remains experimental there). But the story for viewers in Wales is not so good - S4C have no plans to broadcast PDC and HTV talk of the system being obsolete.

Both BBC One and BBC Two are now broadcasting a fully operational PDC service in the UK, except for Scotland where it remains experimental.

• PDC on BBC Two began officially on Saturday 9th December 1995, although PDC tests had taken place intermittently on The Learning Zone only during November.

• The BBC One service began nearly two years later on Tuesday 4th November 1997.

According to Radio Times the BBC's full service (non-experimental) began on Saturday 29th November 1997.

Before the Ceefax re-launch in November 96, page 697 forecast that BBC1's PDC service would begin in the Autumn of 1996. The same page had already unsuccessfully forecast Spring and Summer. The PDC page has now been removed from Ceefax and replaced with one on Digital Radio.

The main reason that BBC1 has taken much longer to begin its PDC service is to do with the greater number of regional opt-outs. Extra equipment is needed to cope with broadcasting a different PDC service for Scotland, Wales and Northern Ireland where they have their own regional programmes and, more importantly, time shift material (i.e. they show a programme at a different time from the main BBC schedule). The PDC codes for opt-outs are generated locally, and so extra time was required to fully test the equipment and train the local operators.

BBC2 also has regional opt outs of course, but this was originally dealt with by putting out the Timer mode flag (see below), which forces the video to revert to its internal clock for the duration of the programme.

According to the BBC's Promises for 1996:

The BBC is in the process of implementing this service. We intend to make it available on both our television channels as soon as possible.

On Thursday, 23 October, 1997, after a meeting with BREMA and other broadcasters, the BBC issued this statement:

The BBC believes that PDC can offer viewers with suitably equipped VCRs a convenient and reliable way of recording programmes, even if the programmes transmission time is delayed for any reason. There is currently an experimental service operating on BBC2 and an experimental service is planned to start early next month on BBC1, with a full service following shortly after.

Apparently, now that the service is fully operational on both channels, more care will be taken to prevent some of the problems seen with BBC2's experimental service - essentially more checks will be made to ensure that the correct PDC codes are being transmitted.

At one stage the ITV companies were planning a network-wide PDC system with regional opt-outs. It now seems as though the plan is for every company to fend for itself. So far, less than half of all the ITV companies have done this. Westcountry TV began its PDC service in early 1996 followed later in the year by Scottish TV. Yorkshire and Tyne Tees began at the same time in late 1997. Granada commenced its service in early 1999. The latest companies to begin transmitting PDC are Border and Grampian.

I have been told that the PDC indicator is lit when a PDC video is tuned to the following ITV stations: Anglia, Meridian, Central and Carlton London, even though none of these stations is broadcasting a PDC service. What they are doing is broadcasting a continuous PDC stream containing the timer mode flag. (This tells a PDC video to behave like a normal video and use its own clock to start/stop taping. See below.) Sometimes this is shown on the ITV teletext service p6AA. (Explanation below). This could be an encouraging sign - it means the companies involved have at least got the equipment necessary to broadcast a proper PDC service.

Some ITV companies explain their position on PDC on their teletext services on p695. I've written to the ITV companies who are not yet providing a full PDC service in the UK. Their replies are reproduced below.

It would appear that some ITV companies are waiting to get their systems in place for Digital TV before providing a PDC service (because it's very much easier that way). With Digital TV now up and running let's hope that's the end of the excuses. The bad news for viewers "served" by HTV is that PDC might never happen there, and they appear very reluctant for me to tell you that!.

Channel 4's experiments with PDC began back in late 1991. The Channel is proud that it was not only the first broadcaster to transmit PDC in the UK, but that it also appears to have the most reliable service. This is attributed to its keeping the implementation as simple as possible.

4-Tel did finally get around to putting together a few words on C4's PDC service on teletext p390, but this was then removed. Later two more pages explaining PDC were added. An introductory explanation could be found on p393, with a more detailed explanation on p394. However, at the end of September 2003, Teletext Ltd took over the C4 ancillary text service contract and its revamped service no longer includes the pages on Engineering, widescreen and PDC.

Teletext Ltd have also completely removed the teletext service from the digital version of Channel 4 available to Sky viewers. This action will have also ended the accompanying PDC service.

I e-mailed S4C back in 1997 about their plans for PDC and they told me:

S4C does not carry a PDC service and at present has no plans to implement one, although the matter is being kept under review, partly taking into consideration letters like this and partly the policies of other broadcasters.

I e-mailed them again in 1999 to see if their plans had changed. Gill Griffiths of the S4C Viewers' Hotline said:

I regret that S4C has no plans at present to introduce a PDC service.

If you'd like to see S4C broadcast a PDC service, you could e-mail them at hotline@s4c.co.uk or call them on 0870 600 4141 or write to them. The address is available at the S4C Web Site.

Channel 5 (now branded as Five) began broadcasting on 30th March 1997 with an experimental PDC service. Minor bugs were sorted out within a month or so of this date, and since then the Channel has regarded its system as fully operational.

Five's PDC signals are also carried on their cable and satellite transmissions.

Some satellite stations are sending out PDC data. So how is it possible to use PDC on satellite and cable? There are two difficulties: first a PDC video needs to change channel to look for the PILs - most videos cannot change the channel number of the satellite/cable receiver. Second, a channel numbering agreement would be needed. However, at least two manufacturers have overcome the problems of using PDC on satellite.

• Nokia make an analogue satellite receiver which can control your video and which has PDC built in.

• Grundig also had a clever video that could control a satellite receiver via one of the pins in the SCART cable. Brian Hamilton Kelly e-mailed me to tell me his GV250 is attached to a motorised satellite dish and the manual warns not to use PDC on channels from different satellites in case the motor is worn out whilst the video hops between the channels looking for the PDC signals! Unfortunately, the Grundig GV250 is no longer on sale.

• There may also be PDC video recorders with built-in satellite decoders.

Hywel Williams used to receive Channel 5 from his analogue satellite system: the terrestrial signal is non-existent.

... I tuned in C5 on my video recorder and set it up to record a programme with the PDC flag turned on. Sure enough, instead of using the timer, it was activated by the PDC stream...

As using a UHF channel for the satellite would mean that I'd only get a mono sound channel I decided to see if I could get PDC to work on the AUX input from the SCART. Again I programmed the VCR to record a programme from C5 but this time told it to record it from channel AV1. Since this isn't recognized by the VCR as C5 in the initial autotune I set the PDC flag on manually. Once again, the VCR used the PDC stream to initiate recording, proving that PDC can be used to start up recordings even on the SCART input!

Unfortunately my Panasonic NVH-S900 doesn't have a satellite control facility so I have to program the satellite receiver separately. I had a quick glance in Sevenoaks HiFi recently - it looks like the next generation of Panasonics will have this feature which could make PDC on satellite more interesting. I wonder if it'll flick between 2 satellite channels looking at their PDC codes ? This would make watching the satellite a little difficult before the show starts...

Philip M Reynolds has been experimenting with PDC on BBC2 and Channel 5 through his (analogue) Diamond Cable box using a Mitsubishi HS-541V video.

... The video has an infra-red output, which controls the [cable] box. When using PDC on cable, you see the box hop channels as necessary. It works as well as it does in the ordinary way, although the video's infra red output is on the right, the cable box's sensor on the left, so I've had to perch the box on a spare video tape.

My experiment with BBC2 and Channel 5 ... completed OK.

It seems that the video does not understand SAT1's VPS signals. I have been unable to find out about PDC on other channels. Sky say they have no plans to use it.

According to BBC Reception Advice, the PDC feature seen in many modern video recorders will not function on digital television. However, according to the ITC it may be possible that the EPGs (Electronic Programme Guides) for digital will feature a "reminder" service to tell you a programme is about to start as well as some way of automatically video-taping programmes you select.

Contrary to what the BBC says, Five (but sadly no longer Channel 4) are broadcasting a teletext stream on digital satellite that includes a working PDC service. Viewers with SkyDigital boxes may therefore be able to use PDC on the digital version of Five.

When Digital Terrestrial launched, the equipment wasn't designed to decode traditional teletext services. The idea was to replace teletext with enhanced digital text services. Hence if you are recording from an iDTV or ONdigital/FreeView set-top box, you won't be able to use PDC.

With digital cable, the availability of PDC will depend on whether the channel carries the full teletext stream (as with SkyDigital).

Many of the foreign channels on the Astra analogue satellites support VPS. Viewers in the UK with non-digital satellite receiving equipment and videos that support VPS can therefore use this system to tape from these channels.

Darren Nadin reports that his Ferguson FV 91 LV identifies the following Astra stations as supporting VPS. (They are listed with the Channel IDs which show up on the front panel display of the video.)

PDC is a European standard and has been available in mainland Europe for some time, where it began life as VPS.

Austria use a simpler variant of PDC called VPS - Video Programming System.

• The Belgian public broadcaster VRT (formerly BRTN) have two channels called TV1 and TV2 (formerly BRTN 1 and BRTN 2, also known as BRT1 and BRT2), both of which broadcast a PDC service. These channels are also available in Holland.

  None of the Belgian commercial channels, VTM's VTM and Kanaal2 nor VT4 have PDC.

  All of these channels are Flemish channels. (Flemish is the same as Dutch except for a difference in the accent and is spoken in Flanders, the Dutch/Flemish-speaking part of the country.) They are all national channels, available on cable.

• RTBF1 and RTBF2 are the public channels in the French-speaking part of Belgium, Wallonia, also available in some parts of Holland. They do not carry PDC.

  Wallonia has two commercial channels, RTL-TVI and Club RTL. We don't know whether they carry PDC.

In the Czech Republic, there are four public TV broadcasters.

• CT1 (Czech Television One) and CT2 both carry VPS and have done since 1994. Additionally, since late November 1997, CT1 has also carried a PDC service.

• Prima and Nova are the commercial channels. They don't carry VPS or PDC.

In Denmark there are two national broadcasters, both are transmitting PDC information.

• Danish Radio carries PDC on DR1 and DR2.

• TV2 supports PDC on TV2.

The Finnish public broadcasting company YLE has two channels, TV1 and TV2, both of which are broadcasting a PDC service.

YLE state on their web site that PDC transmissions began in 1994. They constructed their PDC hardware themselves, based on the Domestic Video Programme Delivery Control System (PDC) EBU SPB 459 Rev. 2 February 1992 specifications.

Normal PDC transmissions are partially automated; live broadcast are handled manually. Camillo's impression is that PDC signals are usually reliable. "Occasionally when programs are delayed, the start time is not reset, but the end time practically always is. This I have seen once, i.e. the beginning of the recording was from the previous program, but the end was correct."

Additionally, YLE says that the start signal is transmitted 30 s before the actual beginning and the stop signal 10-15 s after the actual end. Channel hopping is yet unimplemented and program classes (PTY - see below) are not transmitted except for the news and sports news.

YLE's web site is mostly in Finnish, but there are some English pages. The site includes a PDC info page, again in Finnish only.

France 5 claims it and Arte are the only French broadcasters using Le PDC.

More details about the service are available at France 5's Web Site, but only in French.

Germany use a variant of PDC called VPS - Video Programming System. It is also in use on Germany's Astra satellite channels. (See above for a list.)

• In the Netherlands, the three public broadcasting channels, Nederland 1, Nederland 2 (also known as TV2) and Nederland 3 all support PDC.
  
  Hendri Hondorp wrote to tell me about a page on the NOS Teletekst service, which is linked to the PDC data and informs the viewer what is on the three channels now (Nu) on one sub-page what's on next (Straks) on another.
  
   You can also see these pages on the NOS teletext web gateway. What's on now (Nu in Dutch) is on p222 sub-page 1. What's coming up next (Straks) is on p222 sub-page 2.

• None of the 5 Dutch commercial channels support PDC.
• In Holland you can also receive the VRT channels TV1 and TV2 (see above).
  
  You can also receive BBC programmes in Holland and it's possible to tape programmes from BBC2 using PDC if the signal is strong enough. To use PDC you need to program the start time using UK Local Time. So, for example, if the Dutch TV guides give a starting time of 19:00, then you need to remember that the UK local time for the programme (and hence the time contained in the PIL) will be 18:00. So if you use PDC to tape the programme, you need to set your video to start taping at 18:00, otherwise set your video for 19:00. Watch out for the change over to summer time, too!

In Switzerland, videos are equipped with a simpler variant of PDC called VPS - Video Programming System.

Switzerland currently has 6 national TV channels: TSI1 and TSI2 (Italian), TSR1 and TSR2 (French), SF-DRS1 and SF-DRS2 (German). All of these use VPS. Two German stations, TV3 and Tele24, are also available, but these do not provide a VPS service.

Just about every house has cable as standard, because the mountains make normal aerials useless. The cables carry over 30 different stations.

ORF1 and ORF2 (Austrian channels) and 3:sat (German) use VPS, as do Sat1 (German) and RTL2 (Luxembourg). Arte (French/German) uses VPS as well as PDC.

Given that stations cause the PDC indicator to light even when there's no PDC service in operation, how can you tell the difference? Here's an idea that might work - tape a programme, but set the turn off time to be only five minutes after the turn on time. If the station is broadcasting PDC, then you should get the whole programme, otherwise you only get the first five minutes.

Alternatively, if you have a video which can tape the current programme using PDC (this function may be described as "record to (next) stop (code)") then try this out and see the video begins recording.

• PDC isn't restricted to use on VHS/S-VHS video cassette recorders. Many new devices that have analogue TV tuners but record onto hard disks and/or recordable DVDs also support PDC.
   
• The Thomson TiVo is a clever gadget that learns your taste and then automatically records onto its hard disk things it thinks you might like. When you are watching television through TiVo, it is always recording the channel you are tuned to. This enables it to rewind and pause live television. But while it is doing this, it cannot also look out for the PDC signals from other channels. Hence TiVo does not support PDC.
   
   A full explanation of this can be found in the pages of TiVoPortal.co.uk.
   
• Sky+ is similar to device to TiVo. It records from channels broadcast on digital satellite and stores them onto its hard disk. Like TiVo it does not use PDC. However, some channels, such as BBC One and Two, use "triggers" in the Sky EPG to provide a similar function. What happens is that the EPG is updated so that a programme is listed as being on "NOW" only when it has started. Hence Sky+ should always record the whole programme, even if it starts late.

Most new videos on the market, especially those with PDC, also have VideoPlus+ (known as ShowView in Europe and as VCRPlus+ in the USA). The two systems can be used together.

If you use the Radio Times to set your video, you will occasionally find that the odd programme will have a different start time for use by PDC video owners. This start time will also have its own VideoPlus+ number which you should use if you are taping using the PDC facility.

Gemstar have a VideoPlus+ help-line which customers can call if they have any problems with the system, and lately they have found an increasing number of calls are about PDC. The company has been working closely with the broadcasters in order to try and alleviate the confusion and problems to do with PDC.

 Further details on VideoPlus+ and PDC are available at Gemstar's web site.

StarText is a name for the system of programming a video recorder by selecting the programme from a teletext listing page. A StarText/PDC video looks up the PIL codes from the lines hidden below the Fastext line in the teletext TV listings pages.

However, it is also possible to buy videos that you can program from teletext pages, but which do not have PDC capability; they do not look up the PIL from the page. These videos also use hidden codes in the teletext listings pages on ITV/C4. These codes can be seen if you press REVEAL on your remote control handset.

Both systems (with and without PDC) may be known as VPT (Video Programming by Teletext).

Originally, the PDC specification allowed for a unique code for each programme. This code did not rely upon knowing the original scheduled start time. However to use this system, TV magazines and newspapers would have to have published this PDC code alongside the programmes and it was thought unlikely that they would be willing to do this since they were already publishing VideoPlus+ codes. Also, the system was delayed enough as it was, again VideoPlus+ was already available and so for PDC to stand a chance of becoming established, video machine manufacturers had to start making PDC-enabled machines as soon as possible.

When programming manually or by VideoPlus+, the best place to get the start times is weekly TV listings magazines (or your free local newspaper). However, it's possible that last-minute changes have been included in these publications too, after the PDC code has already been decided. The Radio Times states that when this happens they will amend the programme details with a note for PDC users to show the original start time and its corresponding VideoPlus+ number. Radio Times is the only publication that has undertaken to do this.

Teletext listings are the worst place to get the start times from, since they are likely to show last minute schedule changes (and hence not the original start times). The PDC spec. recommends that when this happens broadcasters should show PDC times on teletext listings, but none of them do. C4 get around this problem by broadcasting two PDC streams - one with the original start time in and one with the start time shown on the listings page. It is the only broadcaster to do this, to my knowledge.

With a StarText/PDC video, you don't have to worry about knowing the correct original start times, simply program the video using the teletext pages.

The video will hop between different channels as necessary checking for the PDC signals. On some videos there's a visual indication that this is happening.

See answer below.

Priority is given to the show already taping. So you'll get all of show A and after that the video will start looking for the PDC code for show B and start taping that. You will miss the first half hour.

This is undetermined. It may depend on the video manufacturer's specification. What's likely to happen is that whichever show has its PDC code broadcast first gets taped. If both codes are transmitted simultaneously then the show will be recorded from whichever channel the video was looking at during its channel-hopping search for the PDC codes.

The PDC specification states:

• For a programme scheduled between 04:00 and 23:59, the PIL is valid from 00:00 to 03:59 the following morning.
• For a programme scheduled between 00:00 and 03:59, the PIL is valid from 20:00 the previous evening to 03:59 the following morning.

However, PDC does allow for re-programming of your video, so that if it is waiting for a PDC code to be transmitted but the programme is rescheduled outside of the above limits (or indeed the programme is moved to another channel), then the video can switch to using a new PIL (containing revised date/time/channel information). This is done using what's called the LUF (Label Update Flag).

No! Even if you have a StarText/PDC video which allows programming directly from the teletext listings pages, the teletext decoding equipment is already in your video.

It's a pity because PDC is an ideal system for skipping ad breaks. The Japanese are already looking at ways of building videos that will cut out the commercials in more complicated ways.

• If you have a teletext set with Fastext, on Five select p598 and then press the RED Fastext button. The page which then appears, p5AA, shows the PDC streams currently being broadcast by Five and the date and time of the Programme Ident Label (PIL).

• Similarly, on some ITV stations, there's a status page on teletext p6AA. (Again, go to the last page of the magazine, p699, and press the RED Fastext button.)
• Similar information used to be found on Channel 4 by going to p399 and pressing RED. Additionally, you could press GREEN for p3BB, which showed whether PDC was working in each of C4's advertising regions. But at the end of September 2003, C4's ancillary teletext service, which had been provided by Intelfax, was taken over by Teletext Ltd, who revamped the service. In the process, the index page was moved from p300 to p400 and at the same time the pages explaining PDC and the "hidden" status pages were removed.
• The BBC do not broadcast PDC status information on Ceefax.

To find out what all the numbers on these status pages mean, see below.

For the real answer to this question, you need to read the PDC specification (and be able to understand it). It's over 80 pages long. (See below for how to get hold of a free copy.) But for a feel of what the PDC code is, in layman's terms, read on...

You can now see for yourself what a PDC signal consists of thanks to Robin O'Leary who has a computer connected to a teletext decoder connected via radio modem to the Internet. He feeds the PDC information (decoding it on the way) to a web page.

You will see on Robin's page that the PDC signal for controlling video recorders comprises the Programme Ident Label (as discussed above) plus some other data.

In most cases the date and start time obviously correspond to the programme currently being broadcast. This tells the video to tape the programme for as long as that code is transmitted. However, there are some PDC codes which have an impossible date of 00/15 and a time of, for example, 31:63. These codes are special PDC modes. Valid modes are stop, timer pause and continue.

As explained earlier, you can monitor the four PDC streams currently being broadcast on ITV1 and Five by turning to the clock cracker page of the ancillary teletext service (p699 on ITV1 or p598 on Five) and then pressing the RED fastext button. Sadly Channel 4 stopped broadcasting its hidden PDC page at the end of September 2003.

Here are a few examples (which I've colour-coded for ease of reference):

	 from Anglia: 0 1 2C 1E 00 00/15 31:63 1
from Channel 4: 0 1 2C 11 83 02/09 19:00 1
		 from Five: 0 3 2C 02 00 17/11 19:00 1

The green number identifies the television channel. Each channel in the EBU has one or more numbers assigned to it. Anglia and LWT are using the code for the ITV Network, despite having codes of their own.

The light blue number is the Programme Type Indicator (PTY), which is explained in the next question.

The blue numbers show either the "advertised" start time (i.e. that used in the PIL), as shown in the C4 and Five examples, or they represent a special flag, as in the Anglia example. The C4 example is for Channel Four News starting at 19:00 on September 2nd. The code in the Anglia example is listed with the other codes in a table above.

Usually of the 4 streams shown on these pages, only the first will have any meaningful (i.e. non-zero) data. This is because most commercial broadcasters use only a single stream. However, sometimes Channel 4's page showed two streams in use, each with a different start time. This is a cunning technique that allows viewers to program the video using either the original start time (e.g. from Radio Times) or the latest scheduled time, if this differs, as shown on the teletext listings pages.

There is another code broadcast in the PDC data called the Programme Type Indicator or PTY. This can indicate the intended audience of a programme or the type of programme. Broadcasters can also use it to identify a particular series.

The PTY code allows a sort of "standing order" to record. So you could say to your VCR "record any science programmes", or, on C4, "record the Brookside Omnibus". This feature is supported by the Akai VS-G2200 and so, for example, if you use the video's StarText feature (i.e. you program directly from the teletext page) to tape Brookside, the video gives you the extra option of "Repeat recording (y/n)".

There is also an interesting PTY code called Alert. This is supposed to be recorded by any PDC VCR that's in standby mode! The intended use is for national emergencies.

The list of PTY codes and their meaning is given at Robin O'Leary's PDC Live web site.

Scottish Television has been using PTY codes since early 1996 and more recently has been using a number of series codes.

Anything over 80 (hex) is defined as a series code, tied up to the network identifier so that different stations can use the same codes for different programmes (as you will see).

Scottish use the following codes.

There are more codes than this, but they are only applicable to Scottish TV.

Scottish also allocate guest series codes to mini-series, for example a three-episode run of Dr. Finlay, so that viewers can catch all episodes without re-programming. In the PDC spec (300231), it says that you can re-allocate codes a month after the programme finishes - so these codes can be re-used the programme run has finished.

Channel 4 have also used series codes 81, 82 and 83, as follows.

If you adjust the vertical hold on a TV picture you'll see a black band with rows of twinkling dots. The black band is called the Vertical Blanking Interval (VBI) and the rows of dots are the teletext scan lines where the teletext data is transmitted.

Each teletext page consists of a number of rows or lines of text/graphics. Each row is transmitted in a separate packet. Additionally, each set of pages belong to a magazine. For example, pp100-199 belong to magazine 1. So the 3-digit "page number" you dial into your Teletext decoder is in fact a one-digit "magazine number" followed by a two-digit "page number". The distinction is not relevant to the end-user, but is more important to the information providers as it dictates how exactly the information may be interleaved during transmission.

Basically every Teletext scan line carries a packet. Typically the data in a packet (40 bytes) consists of all the information for one line of one page of one of the magazines being transmitted. To save space in the transmission format, the whole addressing information (i.e. magazine, page and line number) isn't sent on all packets. Only packet zero (the title line at the top of all pages) carries it all. Subsequent lines only carry the magazine number and packet number (usually written M/PP in discussions).

Packet 0

(as discussed) carries the page-number, broadcaster's ID and the date in a fixed format. It also carries some flag information relevant just to that page itself.

Packets 1 - 28

carry info which are part of the page identified by the most recent packet 0 of the same magazine. These packets may appear in any order, and may be interleaved with packets for pages in different magazines. A page is considered complete when the next packet 0 appears for that magazine.

Packets 1 - 24 are the lines of on-screen text (including the Fastext labels in Packet 24). Blank lines on-screen are not transmitted in order to save bandwidth. Packet 25 contains text to be used in place of the normal header (e.g. on ITV p888, "ITV SUBTITLES"). Packet 26 is where the PILs are transmitted. Packet 27 contains, in coded form for data integrity, the page numbers to which the Fastext labels refer. Packet 28 carries other non-visible information for a page, e.g. which language (i.e. character set) to use.

Packets 29, 30 and 31

are special. They may appear interleaved with the other packets as and when required.

Packet 29 carries information relevant to its magazine as a whole. Packets 30 and 31 carry information relevant to the TV service as a whole (for instance, PDC) and their magazine numbers are thus not really relevant. At the moment though they are always transmitted with their magazine number set to '8'.

So the answer to the question is that the PDC data is transmitted on Teletext in Magazine 8, Packet 30.

When you change channels on some teletext TVs, the station name appears on screen. This name is also broadcast in packet 8/30. It contains the channel's name (in text) plus a few other control codes like the channel's EBU identifier, what time zone the teletext service runs on, and which page number a teletext set should select as the "front page" (400 on Channel 4, 100 on ITV, for example.)

The jargon for the TV station name is Short Programme Label (SPL) and its full packet address is 8/30/1 - see next question.

Some teletext packets include a designation code, which is used to distinguish two or more packets with the same packet number but carrying different information.

This designation code effectively then produces two different types of packet 8/30, referred to as format one and format two. Packet 8/30 format one is the normal Television Service Data Packet, also given other names, that carries the time, date and other bits. Packet 8/30 format two is a packet containing PDC information. Each of the 8/30 packets is transmitted once a second. PDC has the capability of flagging up to four programmes at once. You would use two if you have an overlap, but you might also have a "programme within a programme" where you nest two separate PDC codes. In that case, there might be up to 5 packet 8/30's transmitted per second - one format 1 and up to 4 format 2's. Again, an over-simplification, but you get the idea.

Similarly, packet 26 uses designation codes - it takes about three packets to store the PILs on each of the teletext TV listings pages.

So the even fuller (i.e. full of jargon!) answer to the question of where exactly the PDC signal is transmitted is: the PDC signals are transmitted in Broadcast Data Service Packet (BDSP) 8/30/2 in accordance with CCIR teletext system B.

For the VPS system used in some other European countries, the PDC data is transmitted on a dedicated line of the vertical blanking interval, line 16. The VPS broadcast system is technically known as EBU's PDC method A, whereas the PDC system described above is known as method B.

Below I've listed some problems that owners of some PDC videos have reported. If you have a PDC video, you could contribute by experimenting with your machine to see if the same problems are found. This may help potential buyers weed out the not-so well behaved machines.

Some PDC videos don't stop taping until a short while after a programme has finished. The BBC claim that they send out the stop signal promptly, so why is it some videos keep going?

It may be that some video manufacturers have caused this because of Channel Four's implementation of PDC. Reportedly, C4 do not send a stop signal at the end of each programme. So in order to make sure that a programme has really finished and that the PDC signal hasn't simply been lost temporarily, the video keeps going for 30 seconds after the signal is lost. Since C4 were the first channel to broadcast PDC, some video manufacturers may have built their machines to work with C4 and assumed that other channels would implement PDC in the same way.

To illustrate this topic, we will take the example of BBC2 broadcasting Star Trek followed by Buck Rogers on Wednesdays. (Even if they don't any more!) Some PDC videos (e.g. Toshiba V854B, Sony SLV-E90), when programmed to record both programmes, stop taping for between 7 to 20 seconds during the opening titles of Buck Rogers.

Steve Gutteridge, from BBC Reception Advice, thinks that this problem is another example of videos not stopping promptly after the PDC code has finished. Suppose the video recorder waits for 30 seconds after the PDC code for Star Trek has finished before it actually stops taping and that the PDC stop code is sent only 20 seconds before the Buck Rogers begins. This means that the video stops 10 seconds into Buck Rogers.

Some videos retain their programs in memory for days after a programme has been successfully videoed. The BBC have put this down to their use of overlapping codes.

There can be up to four active PDC codes transmitted at once. This might be useful, say, as the BBC1 One O'Clock News is about to finish when its PDC code is still active and then the PDC code for the Weather becomes active and then so does the PDC code for Neighbours following the Weather.

BBC2 have been overlapping codes for their programmes. In our example where you have Star Trek followed by Buck Rogers, the start code for Buck Rogers is transmitted before the stop code for Star Trek.

Presumably this is because BBC programmes are closer together and the worry is that you have to give the video enough time to be running so that it doesn't miss any of the programme. C4, reportedly, don't overlap their codes. But then they have commercials and hence more time between programmes.

However, timer memory problems have been reported when taping from C4, too.

Overlapping codes might also contribute to the problem with taping consecutive programmes. After all, if the PDC stop code for Star Trek came 30 seconds or more before the PDC code for Buck Rogers (i.e. there was no longer an overlap) the gap in the recording experienced by some PDC video owners might be moved to between the programmes. This may be impossible to do with programmes that are broadcast too closely together, however.

Akai have also supplied one of its customers with another explanation for programmes remaining in the video's time memory. Peter Raggett noticed his Akai VS-G2200 was retaining programmes in memory for 24 hours. Akai told him that this was in case "the programme came back". For example, ITV often used to show a film broken up in the middle by News at Ten, and retaining the memory enabled the video to tape the film and miss out the news.

Below I've summarised the makes and models I've been told about and whether they're badly behaved or not. Remember, sometimes problems experienced are the fault of the broadcasters and not the video. Also some manufacturers may make improvements in a later revision of a particular model.

Unfortunately, details on PDC, aside from the really simple stuff and the really complex stuff, are not easily available, which is why I put this page together. The ITV companies suggest you contact your local TV/video retailer :-)

• The BBC Reception Advice (formerly BBC Engineering Information) staff have been invaluable in helping me research this page. You can give them a ring on 08700 100123 or e-mail them at reception@bbc.co.uk.
• Robin O'Leary has a teletext adaptor which reads PDC data connected to a computer which decodes that data which itself is connected via radio modem to the Internet. His site also contains a thorough explanation of teletext and the coding of PDC data. He's on-line at http://pdc.ro.nu.
• Look out for articles on PDC in the video magazines. There was a good article on PDC in the April 1996 edition of What Video, for example.
• The full PDC technical specification, ETS 300 231, is available for free download in Adobe Acrobat PDF format from the European Telecommunications Standards Institute (ETSI). Supplementary information is also available in the same format from the European Broadcasting Union (EBU).
• If you are interested in the equipment used for a PDC service, the following companies' web pages should prove useful: Saturn-Toya sro., providers of equipment to broadcasters in the Czech Republic; MRG Systems also provide teletext-related products to broadcasters.

I'm extremely grateful to those who have taken the trouble to e-mail me, especially Darren Meldrum, Hywel Williams, Ian Collier, Mike Henry, The Watcher, Marcus Bainbridge, Roger Allen, Phil George, Peter Raggett, Rod Begbie, Tim Steele, Brian {Hamilton Kelly}, THE Adrian, Peter Chidzey, Ian May, Tim Hewett, Robert Cuijpers, Peter Broekhuizen, Hendri Hondorp, Camillo Särs, James R Grinter, Patrick Limbird, Mike Ives, Philip M Reynolds, Terry Jones, Stephen Livingston, Tim Adye, Martin Rasborg, Tony Lamb, Darren Nadin, Pete Ford, Simeon Tankard, Farshan Masani, John Hewett, Philippe Vijghen, Svend Waldorff; to those who have patiently explained the technical stuff: Steve Hosgood, Ant Purvis, Robin O'Leary and Jim Main; to Leon de Beer, Technical Director, Gemstar Europe; to Francis Watson, Engineering Manager, Yorkshire Television; to Steve Gutteridge, Roderick Duncan and the team at BBC Reception Advice and to Aiden Stowe, deputy editor of Ceefax. Many thanks to you all!

If there are any errors in the text, or you have any other comments, (especially if you want to report a badly/otherwise behaved PDC video) then please feel free to e-mail me.