They use a system of codes called Content Reference Identifiers (CRID).

Basically for every episode in a series a Series Crid identifies all episodes considered by the broadcaster to be part of a series.

Each episode has a programme CRID that identifies each episode. In that way once recorded repeats are ignored.

When you set a series recording the epg is scanned for the first showing of that series this episode is stored in your schedule (Note this may be earlier than the one you actually used to set the recording)

About 15 mins before a programme scheduled start time is due, the box wakes up and watches for the broadcaster to signal the new Now start of recording. At the end of the recording the box stops recording when the current programme status changes to the next programme.

There are a few other bells and whistles like the ability to offer a HD recording when you set from the SD epg entry.

The system lacks a end of series flag so the reservation stays in the list for 13 weeks before it auto deletes. 13 weeks is the minimum gap that the broadcaster has to leave before recycling the series crid for a new series.

You can manually delete them before if you wish.

uk1 – 6 minutes ago  » 

What form does the signal take from the broadcaster to the box that the broadcast is about to start? Is it a sound frequency or visual,signal for example?

It is part of the same digital metadata that provides the EPG.

uk1 – 3 minutes ago  » 

Thanks Graham,

What form does the signal take from the broadcaster to the box that the broadcast is about to start? Is it a sound frequency or visual,signal for example?

It's part of the broadcast data stream for that channel basically what you see if you look at now/next on a satellite box.

If you look at the photo you can see my HDR-1000S is recording Escape To The Country, it will stop recording when Royal Recipes becomes the now entry.

[attachment=62089,883]

uk1 – 10 minutes ago  » 

So the EPG is actually quite fluid and is kept constantly up to date. So a pragramme originally planned due to start at say 5pm might be updated to start at say 4.59pm or 5.01pm?

Correct.

uk1 – 3 hours ago »

So the EPG is actually quite fluid and is kept constantly up to date. So a pragramme originally planned due to start at say 5pm might be updated to start at say 4.59pm or 5.01pm?

There is the epg as in what is seen in the full guide and there is the now/next epg which, as Graham says, is what most models use when displaying now and next, plus is also used for the current and next programme in the full guide, at least for those boxes that have been freeview compliant (since about 2005?).

The now and next epg is very fluid for some channels and are updated in close to real time for what is playing out ‘now’. While for other channels there are no real time updates.

In your example if a BBC TV progamme scheduled for 5pm is about to start being played out to start at 17:01:30 then the BBC appear to be sending the change to the ‘now’ at source about 20 seconds before that at 17:01:10. This allows for time for the epg in the TV data streams to be updated and the PVR to act on it. The epg for the channels in a particular multiplex are transmitted every 2 seconds while channel from the same transmitter but a different multiplex are transmitted every 5 seconds. PVRs that use a spare tuner to look for a ‘now’ to record are therefore at an advantage of they look at the epg from the multiplex that has the next wanted programme to record.

The original expectation was that for those channels that don’t update the ‘now’ in close to real time and still populate the full epg with times rounded to the nearest 5 minutes, would default the now and next epg (provided in advance by the channel’s schedule provider) to an estimate likely to be slightly more accurate than the full epg.

If you want to see some technical detail behind this you could start off with section 8.10.1 from version 7 of the d-book. It is an old version of the d-book from 2011 but this side of things is still pertinent toady, plus that version of the d-book has been available since April 2015 via wikileaks. Search for ‘D-Book 7 Part A v 1 wikileaks’.

The d-book also has references to supporting standards.

uk1 – 10 hours ago  » 

Luke, thanks for that very clear explanation which explains a lot about how it works and it’s potential. I guess that theoretically in the event that a new channel wanting to join the core channels for say purely recording, then this technology would offer them the opportunity to have a two tier service. One service could be with adverts and the other could be a service perhaps for a few pound a month a service that stops recording at the start of adverts and restarts recording at the end of adverts.

I can’t see that happening you would need to incorporate additional codes for each ad break and the capability in the pvr software to pause and unpause recording. Most likely a whole new pvr.