TV distribution

Andy Quested, head of technology at BBC HD & UHDTV, explains the journey a programme takes from broadcaster to viewer

TV viewers are often unaware of what happens to a programme between getting made and going live. This article provides a brief overview of the processes a programme goes through, from when the producer delivers it to when it appears on home TV screens. 

The current system

When a pre-recorded programme is finished, it is copied to tape. High definition programmes are copied to a format called HDCamSR, while standard definition programmes are copied to Digital Betacam. Interestingly, the distant ancestor to both these formats was Betamax and although VHS won the format war in the home video market, Betamax ended up much more popular as a broadcast format.

The tape is sent to a quality control (QC) area where the technical quality of the programme is measured and it is watched through to pick up any errors that aren’t detectable during automated testing. A report is made, and if there are any problems the programme is sent back to be repaired. One test that every programme has to pass is the photo sensitive epilepsy (PSE) test. This test looks for flashing images that could cause problems for people with epilepsy. It is difficult to do the tests on live programmes, but a test unit can monitor rehearsals to give the production team an idea of the level of flashing. If there is concern, a warning will be given before the programme starts; this is quite common on BBC News, since many stories contain a lot of flash photography. 

Digital delivery

The BBC is beginning the move from delivering programmes on tape, to transporting them via digital file. However, this is a gradual process, limited by the sheer volume of material the BBC deals with and the varying connection speeds from outside programme makers. The BBC takes delivery of over 500 pre-recorded programmes a week; assuming the average programme length is 30 minutes, this totals over 25TB of programmes a week. If everyone is trying to deliver at the same time, and some suppliers are on slow upload speeds (under 10Mbs), problems will soon arise.

As any internet user knows, file transfers sometimes fail or need restarting, and it would not look good if the BBC had to apologise for Eastenders not being ready because the programme hadn’t finished downloading. Therefore a day’s programming has to be delivered up to four times faster, which makes the BBC’s delivery capacity requirement equivalent to 100TB a week. It’s also impossible to know how many programmes will be getting processed at the same time – or how fast the connection speed from a sender will be. The BBC is working on ways to make the process as smooth and as fast as possible, and the move away from tape delivery will be well underway in 2014.

The BBC is a member of the Digital Production Partnership (DPP), a group of UK broadcasters who have come together and agreed on a common file format for programme delivery and exchange (buying or selling programmes). It is called AS11 and allows the BBC to have a single format for HD programmes and a way to store the data needed to transmit the programme.

After a programme passes the QC process, assuming it’s not going to be transmitted in the next few hours, the programme goes to the archive where it remains until called for transmission. From this point, several parallel processes are carried out, usually without the need to move the programme from its shelf (physical or digital).

Subtitles and audio description

All BBC programmes, including live broadcasts, are subtitled. Obviously, with non-live programmes adding subtitles is less time-constrained. A copy of the programme is sent to a subtitler, who will go through the programme with the script and make a subtitle file that’s sent back to the production team before transmission. The subtitler has time to make sure the wording and spelling is correct and that the subtitles fit the shots without masking critical information. They can also re-position subtitles so they are near to the person speaking and assign different characters their own text colour. However, even with non-live programmes, things don’t always run smoothly. Sometimes programmes have to be re-edited after they are delivered because recent events make certain content inappropriate, new information is found or better images/sound is found and the producer wants to change the programme. This means the subtitle file may have to be updated right up until the time of transmission.

Some BBC programmes have audio description, an alternative audio track that can be selected at home to add narration to the programme audio. Like subtitles, this is prepared in advance and the description script can be adjusted so it doesn’t overlap the programme’s dialogue. When the process is finished, the final narration is sent back as a file with a digital signal that tells the TV or set top box to fade the main audio while the narrator is speaking and to raise it again after each section. Unlike with subtitles, it is not so easy to add or change the audio description if the final version of a programme is delivered too close to transmission. All these component parts eventually end up in their respective stores, where they remain until the scheduling system calls for the programme to be sent to the BBC’s playout provider.

Scheduling

The channel scheduling team, along with the channel controller, build the schedule starting with the fixed points – the News bulletins, major sporting events and so on - and then build the day’s and week’s programmes into the channel. The trails and promotions that go between the programmes are scheduled too and all this is built into the final plan that is sent to the playout provider.

Red Bee Media is a playout provider which runs the channels on behalf of the BBC; when Red Bee receives the schedule they start to ask for the programmes to be delivered. At this stage, not all the programmes will be available, so the channel will build blocks of time labelled either Red – meaning ‘programme not available’ - or Green – ‘programme ready to playout’. As the day of transmission approaches, alerts will be sent back to find programmes that haven’t been delivered and more exact times for live programmes will become available.

As well as the programme and its accompanying subtitles and audio description, Red Bee needs data that tells them the channel, date and time of transmission, the duration of the programme and technical details – such as the aspect ratio (4:3 or 16:9), whether the programme is in HD or SD, stereo or surround sound. Red Bee also requires details that help guarantee the identity of the programme and that the correct episode and version (for example, pre- or post-watershed versions) has been delivered. They also receive instructions about iPlayer scheduling. For BBC One, the schedule states if the programme is for the whole of the UK or if any or all of the Nations (Scotland, Wales and Northern Ireland) will be showing their own programmes. 

A television channel like BBC One functions like one single 24 hour-a-day programme that has various pre-recorded and live sections, all linked together by an announcer, trails and graphics. As with any programme, there has to be a central point of control where all the parts come together with a script (the schedule), and this is where Red Bee come in.

The Red Bee control room

Playout 1 is the area in Red Bee where the network directors and announcers are based. The directors make sure each programme is available and that they can communicate with live sources such as news studios and outside broadcasts. They also prepare the junctions and make sure all the graphics and promotions are ready and correctly spelt. The director and the announcer work on scripts for the voiceovers between programmes. The director is also responsible for the (now very rare) technical fault captions and announcements, and needs to be prepared for programmes that overrun or breaking news stories that either extend the news bulletin or interrupt scheduled programmes.

The whole lot comes together on a well-protected cable: there are several for each channel. Red Bee’s responsibilities end at this point, but there are several other playout areas the programmes go through before they are sent to the transmitters. 

Catering to the whole UK

A key part of the BBC’s television output is the programmes made for the Nations, which produce their own national versions of BBC One and BBC Two. The 15 English Regions are able to opt out of the BBC One network schedule for local programmes. Red Bee provides a 'sustaining feed' of BBC One and Two that allows the Nations and Regions to leave and join the main output. The Nations and English regions then send their localised output to the coding and multiplexing area where they are encoded and sent to the transmission platforms. 

The diagram below is an overview of how the BBC broadcasts to the digital terrestrial platform Freeview and the digital satellite platform Freesat and Sky. The HD channels are sent by fibre link to Virgin Media to be added to the cable platform.

Diagram

In total, there are 18 versions of BBC One in standard definition and four of BBC One HD: a pan-England HD version with no local programmes, BBC One Scotland HD, BBC One Wales HD and BBC One Northern Ireland. There are four versions of BBC Two in SD and a UK-wide version of BBC Two HD. 

The digital television platforms also carry some of BBC’s radio services. Eleven network stations are sent to coding and multiplexing from Broadcasting House and six stations for the Nations are sent from Scotland, Wales and Northern Ireland. 

In Coding and Multiplexing, all of the BBC’s TV channels are aggregated into a single format suitable for transmission to the home. There are a number of aggregation centres which generate all the signals for all of the Freeview transmitters and for satellite. 

The video feeds for each channel are coded and compressed to a size suitable for transmission, then the multiplexing takes the compressed channels and combines them together. There are different multiplexes for different regions, for Freeview and satellite and for HD and SD. In total there are: 

  • 18 Freeview SD multiplexes
  • eight Freeview HD multiplexes
  • four satellite SD multiplexes
  • and two satellite HD multiplexes

 

Diagram

Finally, the multiplexes are sent to the various platform transmitters or up-links. 

The 18 Freeview SD and eight Freeview HD multiplexes are sent to around 80 transmitting stations around the country; these provide about 90% population coverage. The BBC’s Freeview transmissions have 98.5% population coverage; to reach the extra 8.5% over 1000 additional transmitters are required. These 1000 transmitters pick up a signal from one of the 80 core transmitters and retransmit it. 

The BBC’s four satellite SD and two satellite HD multiplexes are sent to an Earth Station where they are uplinked to SES’ Astra satellites for Freesat and Sky receivers. 

Most SD cable transmissions are taken from the BBC’s satellite transmission and the relevant local version is then added to the cable distribution network.