The Hitchhiker's Guide to Encoding: Before we start
FridayIt's been sometime since my last post on the blog but I have tried to be as active as I could on the existing ones. I read all the posts and my thanks go to Paul Eaton who has listed many of my comments just to prove I do exist! I try to answer points raised in the blogs but unless there is a specific issue, I just can't answer each post individually.
Because there's a lot to say, I thought it would be better to serialise this blog over the next week with each episode covering a different topic. I will make no apologies for the titles and many thanks to the memory of Mr Adams.
Today: Opening and audio issues
Monday: HD encoder history
Tuesday: The EBU
Wednesday: PSNR and all that
Thursday: New encoder evaluation
Friday: Programme styles and closing thoughts
The BBC HD channel is two this weekend and in that short time we have developed one of the widest ranges of programmes you could find anywhere in a single HD package. No one is going to say it's all been perfect and in the past I have had to make a couple of very confessional blog posts when we've "just got it wrong", for example:
We take every aspect of the channel very seriously and consider you to be a valuable asset when it comes to problem solving, trouble shooting or just "plain" talking! I'm sure Paul won't mind if I quote a section from Roly Keating's reply to his email:
"...Andy Quested has himself been considering the most recent exchanges on this subject, in consultation with the BBC's R&D teams, and will be posting further messages in response very shortly. Please consider my comments in conjunction with his: I hope that between us we can at least persuade you that we are paying the closest possible attention to the debates on this subject, and working constantly to keep standards as high as possible."
I want to make sure everyone who reads this blog can understand it - well maybe with the exception of the PSNR section that even leaves me with a bit of a headache! But I hope it will be clear enough so first time readers as well as those who have been posting since the beginning have an equal opportunity to comment, so I hope some of you will understand if you find it's going over old ground or in some way is not technical enough.
Before I start, it's worth saying encoder testing takes a long time because it's not just about picture quality. Before any testing starts a new encoder is checked for full compliance with the MPEG4 standards. Then it's tested for a considerable time to make sure it's compatible with existing set top boxes from different manufacturers and with the Sky platform. Then for audio compatibility (surround sound and stereo), for audio description, for subtitles and for interactive services.
Although we test the encoders on as many receivers as possible, we can only commit to a guarantee of compatibility with the supported platforms, so for satellite that's Sky and Freesat. I know several of you use DVB cards in PCs and Macs and have had problems. However there are so many different devices, options, versions and software variables, it's just not possible to check everything and therefore we cannot support these receivers properly. If you do use a DVB decoder card, have a look at this site for more help:
The next few days will give you a glimpse of some of the work done to maintain the quality of the BBC HD Channel by BBC Research & Development, our partners and to a lesser extent, me!
So to begin... Audio!
The Holes at the End of the Chain
(Or, why do s the aud o have so many h les)
Back in 2008, Rowan led a detailed overhaul of the HD Channel's audio system to make sure the new AV Sync test signal would really be sync when it arrived at your set top box!
We made sure the Channel's AV sync was within 5ms and we check it regularly to keep it there. The number of complaints about AV sync dropped to virtually zero after the signal was first shown and, with the exception of a couple of live programmes, it's been that way ever since. We've also completed thorough training sessions with all our main service suppliers to make sure they understand the issues of distribution and transmitting surround sound signals and metadata.
Recently though, we've had a couple of weeks where nothing seemed to work properly. It started with holes in the audio during Strictly Come Dancing. Your posts suggested they were on satellite and cable, so the investigation started in Television Centre. I was convinced it wasn't the right place to start, call it professional pride but we had gone through the chain pretty thoroughly, and as I said, it's checked regularly.
So I kept listening and a couple of days later I heard holes during the live Later with Jools and in the promo but this time they were much smaller. I asked our playout people to keep repeating the clip in the promo while we listened on every receiver we could find.
This proved the holes were only on the satellite output and we quickly tracked the problem to one of the HD Channel's "resilience" circuits. As soon as it was isolated, the holes stopped. We knew what had caused the problem but are still investigating why!
For 48 hours we had nice clean audio but just when I thought I could breath again and we had audio under control it promptly failed again, not once but five times!
As the first Electric Prom went to air the holes reappeared. I was watching at home so was on the phone in about 10 seconds. This time the play-out centre could hear them and they sounded like the surround signal was not synchronous with the video reference.
The Channel director went to emergency cut mode that removes all play-out processing, stopping the holes but not curing the problem. While we investigated, I decided we should use the reprocessing unit in the central switching area put in after the 2008 Eurovision Song Contest. This cleans up signals and modifies metadata if required, before transmission. It solved the problem for the next transmissions but we had not identified the cause. Timing issues are notoriously difficult to track down especially in mobile systems, but we will find it!
There was another audio problem during Thursday's prom. The programme started well but I had been warned it would be edited on site and it would be very tight to transmission, so tight in fact that the play-out machine hadn't finished caching the audio as the second half of the edit started to go out and guess what - holes in the audio. They were "real holes" that is no audio at all, and they could be clearly heard on BBC2 as well as the HD Channel, no consolation I know but we had to put this down to machine failure and unlikely to be repeated.
Back to Strictly and a live OB from Blackpool, not the sort of show you want to mix from the back end of an OB truck no matter how big! Due to running around all day and several accidents in front of me on the M40, I didn't get to a TV in time for the start and then it was BBC One only. It didn't take long for me to find out there were serious audio level problems on the HD Channel but what was worrying was no one in the broadcast chain could hear it, to me that shouts metadata!
Later that evening I listened to the programme at home and it sounded like the set top box was being over driven. Strictly has a pretty tough mix and if the metadata isn't right it will cause major problems with the dynamic range control in the set top box. By Tuesday morning, I had the tape from the OB and could see the metadata and knew what settings the sound team thought they were mixing to and, more importantly, what they actually set.
It was quite amazing hearing the difference when I switched between the two metadata settings. We have now gone through the metadata requirements for the programme and made sure they are fully implemented.
What next? The Remembrance Day service is a very important event and due to the complexity of the OB it was going to be in stereo. When the programme started I just couldn't believe I was hearing production talkback and what turned out to be Radio 4's sound. Sending a non-broadcast audio circuit, or another networks audio to air is just wrong and should never happen. The seriousness of this incident meant an immediate investigation.
On the day the links had been established in good time and a problem with the incoming talkback circuit was identified. The stream coming back from the OB contains several audio channels and communications circuits embedded into the video signal. This is quite usual and guarantees the audio and video paths are the same and therefore always sync. The stream contained the main audio, clean audio (no commentary), Radio 4 audio and communications. To help fix the talkback problem the audio channels were "shuffled" so talkback appeared in place of the main programme along with Radio 4 - this should never be done. During the final line up phase the talkback was muted and the Radio 4 audio at that point would have sounded identical to the TV audio. In other words the channel director would have had no idea anything was wrong.
Last - and it better be! Yes, Doctor Who should have been in 5.1!
Something the dubbing mixer pointed out quite forcefully on the Monday morning after transmission. Before he contacted me I had discovered the programme was only in stereo on the server but the delivered tape had the full surround audio. After a bit of investigation we discovered someone had routed one of the server ingest video tape players to another area and changed the audio replay options to a non-standard mode. When the machine was routed back to its normal role the audio set-up was not reset and the next programme in the stack for loading was Doctor Who.
Machine or circuit failure is one thing that we all have to accept happens occasionally and we apologise if it does. But I have made it clear we must stick to agreed practice and procedures because they are there to prevent errors. I hope the action we have taken will keep the audio on the channel running smoothly with no more mistakes.
In my next post I want to talk about the Channel's HD coding in general as an opener to the main encoder testing posts.
Andy Quested is Principal Technologist, HD, BBC Future Media and Technology.