BBC ONE HD: test transmissions
So... many of you have found the test transmissions!
I've also noticed there's been a lot of speculation about what we are up to for the last few days and we thought it would be good idea to let you know what's been happening and will be happening before BBC One HD launches on Wednesday.
We are, as you may have guessed, in test mode at the moment. Testing is not just about delivery of sound and pictures going through the chain from our playout provider through to the encoders. It is to check the service works on all platforms especially all the DTT transmitters and about testing Subtitles, Audio Description, Audio Channel Continuity, AV sync, Green Button services, Surround/Stereo switching, Up and Down Conversion ...
Our playout service is run by Red Bee Media, Siemens are responsible for managing the encoders and Arqiva manage the DTT transmitters. Virgin Media handle their own encoding so we send an uncompressed HD SDI signal from Red Bee to their coding site. Finally Siemens manage the satellite transmission bundle for Freesat and Sky.
The Red Bee engineers are currently rebuilding the BBC One playout suite in two stages. Stage 1 goes on air on the 3rd November and once this is up and running the tough job of rebuilding BBC One's complete playout suite in HD begins! The reason for doing a two stage rebuild is to get the channel on air in time for Christmas.
In the Stage 1 build, the HD service is a separate path that piggy-backs onto BBC One SD. The SD channel is completely unaffected by the changes in Stage 1. Because it is a piggy-back service some of the functions will be limited, for example some of the junctions between programmes will be simpler on the HD version than the SD. When the Stage 2 build is completed, BBC One will be HD; the SD channel will be derived from a down conversion of the HD version.
What are we up to then with the tests?
Subtitles - During Stage 1, BBC One HD will use the subtitling data from the SD service. To make sure this works properly we need to test the link and the timing between the subtitles and the images. This is why you have seen some random subtitles from BBC One SD over the HD promo!
Audio Description - We have been using Doctors to check how the audio description service works. The timing of AD service is critical as the AD audio MUST be in sync with both the HD and SD services irrespective of the timing difference between the two signals (some HD processing takes longer than SD especially if we have to compensate for Dolby E encoding and decoding.
Audio Channel Continuity - seems obvious but we need to make sure Left Surround comes out of the Left Surround Speaker etc.! We also check Stereo/Surround switching and the Stereo Mix-down (when you listen to a surround sound programme via a TV's stereo speakers).
AV Sync - this is a subject close to my heart. As many of you know I was an editor for many years and can see AV sync errors very quickly! Some of you may have noticed the sync signal interrupting the HD Channel's promo for long periods. I am using the opportunity to check the HD Channel to find out why we have some small differences between stereo and surround programmes. It's only a few milliseconds but we need to understand what's causing the difference. As a reminder the EBU standards are +10 to -20ms for the equivalent processing and we are well within that.
For BBC One HD we have the added complication of the delay caused by up-converting SD programmes. This adds a yet another 40ms of video delay to one of the possible paths through playout
A word of warning here; as we are adjusting the AV sync on BBC One HD, I cannot guarantee the AV sync signal is correct at the moment; we may be adjusting AV delays while it's going out and if you make any changes during the tests you could end up with up to a 40ms (1 frame) error. The signal on BBC HD is correct and measured at about -4ms in the transmitted transport stream.
Up-conversion - BBC One HD, just like ITV 1 HD, C4 HD Sky 1 HD etc, will be a simulcast channel so all SD programmes will be up-converted for the HD service. Up-converters don't just change the video from SD to HD; they also have to move timecode from the SD position usually in the vertical sync interval (VITC) to a VANC (Vertical ANCillary) data package. Up-converters may also have to change any embedded subtitle packages and move (shuffle) embedded audio tracks to cope with stereo and surround track positions.
Down-conversion - Viewers of BBC One SD will see SD programmes up-converted then down-converted. The criteria we set for the up/down converters was based on the quality of an SD source shown on the SD service - in other words, what was the most transparent process!
STB Compatibility - the BBC HD and BBC One HD channels must work on all DTT and D-Satellite so quite a lot of testing is about making sure the service is totally compatible and all the additional services work.
Over the next couple of days we will be tidying up and loose ends before the service becomes visible on the EPGs. Please remember to check and re-do any series links as we are changing the channel some programmes are transmitted on (BBC One HD from BBC HD) and changing the channel number of BBC HD.
Andy Quested is Head of Technology, BBC HD & 3D, BBC Future Media & Technology
N.B: You can also follow Danielle Nagler, Head of BBC HD on Twitter @naglerhd