Don’t Forget The Kitchen Sync
Posted by: Rowan in BBC HD Audio, tags: BBC, High Definition, Sync, Tech, Work
A couple of weeks ago I told you about the work we’ve been doing on the synchronisation of audio and video (lipsync) in our surround sound signal chain. However, no matter how much work we do, there’s one thing we can’t control, and that’s the equipment in your front room. You might not know this, but your shiny new flat-screen TV (LCD or plasma) introduces somewhere in the region of 40 to 100 milliseconds of delay, which means that if your audio isn’t delayed to match, the sync between the two is quite considerably wrong. Worse still, the audio is ahead of the video, which is much more noticeable than the sound being late.
Why? Well think back to your GCSE (or O-Level!) Physics lessons. Sound travels much slower than light, so you are used to hearing the sound of an event slightly after seeing it, particularly if it happens far away. Just try going to a gig in a stadium, and you’ll notice that if you’re at the far end of the stadium from the stage, the singer’s lips will be moving well before you hear the sound. So audio being late, whilst undesirable on TV, is much more tolerable than the video being late. The latter scenario looks very unnatural indeed with even quite a small error.
Most HD set-top boxes allow you to adjust the audio/video sync on the outputs, usually in 20ms steps. However just by watching TV programmes, it’s hard to judge when the sync is correct. As such, we wanted to do something to help our viewers to get their own setups correct – some of you have been asking for this in comments on our blog posts, and we suspect more will be interested in trying it when given the opportunity. So we’ve been devising a sync test which will be broadcast a few times during each day on BBC HD, starting in a week or two. The test signal is based on work from Andrew Mason, Oliver Haffenden, David Kirby and Alastair Bruce at BBC R&I, and it should allow you to adjust your set-top box to an acceptable level of sync just by watching the sequence and tweaking your settings.
If you want to get even more precise, you can make your own sync test device! The signal has been designed to be easy to monitor with a simple electronic circuit, using a light probe to sense a flash which appears on the screen once a second and an audio input to listen for a ‘clap’ which happens at the same time. The device can then tell you the time offset between the two and hence you can adjust your set-top box to compensate. Details will be posted on the BBC website. The ‘clap’, incidentally, is a recording of two bits of wood being snapped together; believe it or not we found this at least as effective as more ‘high-tech’ alternatives such as a brief burst of tone.
My job in all this has been to ensure that the test signal is in-sync when it gets broadcast, which is trickier than it sounds. Of course we aim to ensure that everything we transmit is in-sync however it’s absolutely impossible to ensure that sync is perfect, not least because the delays introduced by some equipment vary over time. Within a small tolerance of a few milliseconds, the difference is imperceptible anyway so it’s not a problem. The EBU defines standards for sync that say that when a programme is delivered to a broadcaster like ourselves, audio should be within +10 to -20ms (i.e. no more than 10ms ahead or 20ms behind the video). Of course, to ensure that the signal is within sensible tolerances when a programme actually gets broadcast, each step of the chain has to have sync errors much smaller than this, otherwise the delays could add up to a much larger a total error. When transmitting a signal whose express purpose is to be in-sync we’d like the sync to be even tighter than usual, which means making sure that every step of the chain is as close to ideal as it can be.
The signal started life on an edit suite, and so job number one is to ensure that when it comes off the edit suite and on to tape it stays in sync. Along the way we have the audio encoded to Dolby E, so we’ve already got 3 bits of equipment in play: the edit system, the Dolby encoder and the tape deck. So we do that, check for any offset, correct it in the edit suite and then get back on to tape again. But wait… how do we check the sync? I’m armed with our sync-checker, but in order to use it, we have to decode the Dolby E and play the video out to a monitor, both of which could introduce their own delays! So we have to ensure we know the delays of each component in the chain, isolating them one by one, before we can rely on our measurements. Rule number 1 of testing is that you must ensure that your test equipment isn’t affecting the results, or at least that the effect is known. In this case we can’t test without having an effect, but isolating that effect allows us to get accurate measurements. It means we have to be really careful though – forget one source of sync error and you mess up all your results! That’s my justification for the terrible pun that titles this post…
Then there’s the signal chain to get the test sequence to air. We ingest the tape onto our playout servers (this process could, of course, introduce sync error), then play it out though a presentation mixer and some processing gear then down some fibre optics to Television Centre. From there it goes on through to the Coding and Multiplex centre where the signal is prepared for digital broadcast. All that could introduce a sync error, as could the coding process itself; the audio and video are coded separately and multiplexed back together, then multiplexed in with other channels for broadcast. Argh! So I’m going to work with staff from Red Bee and Siemens (our technology partners in these areas) to run the signal through the off-air chain, a backup set of equipment and connections used if the main signal chain goes down, and also in testing situations like this. We’ll measure the sync error through this whole set of equipment, and if necessary adjust the offset at the encoders to get everything back into sync.
I’m fairly confident that there shouldn’t be a big sync error in this signal chain, as it’s been tested before and we’re broadcasting with it every day, so major errors would be noticeable. However as I mentioned previously, our tolerances for broadcasting this test signal are much tighter than usual, so it will be a great chance to check that everything’s working as well as it can possibly be. Hopefully all will go well, but you never know – I’ll let you know next week!
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Hi Rowan. So if LCD/Plasma displays introduce that amount of delay, how do broadcasters ever get to the situation where there is no lip sync error? I have a new Sony Bravia LCD, and have had no such problems when watching BBC HD, and also with most other HD channels on the Sky platform. However, Sky Sports 1HD is a different matter – the sound always seems to lag slightly behind the picture. So what’s the likely cause of that – are they over-compensating with the audio delay? I’m not very technical, but your opinion would be appreciated.
Hi Mark. Thanks for reading. The simple answer is we don’t get to a situation where there is no lip sync error – all we can ever hope to do is transmit in-sync; if your equipment messes that up then there’s nothing we can do, as everyone’s equipment is different so we could never correct for it. However, the thing that must be understood is that small lip sync errors aren’t really noticeable, so for many people it won’t be a problem anyway. People vary in how sensitive they are to sync error; some people claim to be able to see less than 10ms of error, but I’m very dubious about that, whereas I’m personally not particularly sensitive to it and find anywhere up to 60ms or so acceptable for normal viewing. (Though if I start watching for it I can see 30-40ms of error usually.)
Another issue which I may not have made properly clear is that this issue generally only applies if you’re connecting the audio to a separate amplifier and speakers. If you play the audio through your TV’s speakers (usually over HDMI), there’s unlikely to be a sync error introduced as most decent TVs will automatically correct the sync to compensate for their own delay.
So between those 2 factors, that will hopefully explain why you haven’t generally had a problem with BBC HD. As for Sky Sports 1HD, I very much doubt that they would be attempting to correct for the delays introduced by people’s TVs, because as I said before, everyone’s setup is different and many will introduce no noticeable delay. If there’s a consistent problem with audio delay on one particular channel, I would consider contacting the broadcaster (in this case Sky), however it might be best to look around the web a bit and see whether any others have had the same problem first, as there may be helpful information out there.
Finally, as a quick update, I can confirm that we did a one-off broadcast of our sync test earlier this week, and measured it as in sync to 0.8ms, which is very good indeed. We had one small problem with it which we’re currently working on resolving, but you can expect the sync test to be broadcast more regularly soon, probably next week. If you’re interested Mark, you’ll then be able to have a look and see just how in-sync your telly is
Rowan
Rowan. Thanks for getting back to me. My interest is not just because I’m fairly sensitive to lip sync errors, but also because I’m doing a computing and ICT course with the OU, so I’m just interested in things digital! I use HDMI and use the TV ’s phono output to pass the sound through my hi-fi, which is miles better than the TV’s speakers. I’ve used them together, and could not discern any difference i.e. TV speakers and hi-fi both output the sound at the same time. My understanding is there is most likely to be a problem when using home cinema systems which use an optical audio output. I have contacted Sky, but there technical support people are a bit opaque – they say they are aware that some people are experiencing slightly delayed audio, and state that they are continuously updating the software of the HD boxes. I’m not sure I’m convinced – surely if it’s a software problem at the customers end then the problem would be apparent across all HD channels (which it’s not). Furthermore, I haven’t noticed any problem with SD channels, including Sky Sports 1. Will look out for the sync test broadcast.
Mark
Hi Mark. Whilst I couldn’t be certain without checking specifications, I would expect your TV to be compensating for any delays if you’re using its phono outputs – the issue generally only comes when connecting your audio to the phono outputs of a Sky/Freesat box, or as you say using optical. With regards to Sky Sports, I’ve actually been in a meeting with some of their technical staff today, and I gather they did have a known issue a couple of weeks back with sync on that channel, which has now been resolved. Have you been watching recently? Failing that there’s not a lot I can suggest at the moment… Rowan
Hi Rowan. Very kind of you to get back to me regarding this. I’ve been watching it today (India v England) on Sky Sports HD1, and although it’s not actually an HD broadcast, the problem is still there – very noticeable during the discussions in the breaks, which are from Sky’s UK-based studio. I spoke to their tier 2 technical support, and they acknowledged that my problem is probably not due to the HD box software, but most likely an issue with the broadcast itself. He said he would report it to their engineers.
Mark
Cool, hope it goes well.. Rowan
Dear Rowan
Thanks for the informative article. Just to clarify, I have a 32″ flat screen Toshiba with 1080p screen connected to a Panasonic DVD HDD recorder. I use freeview and normal DVD’s. I connect these to my old hi-fi with phono leads. The sound from the TV is out of sync with my hi-fi sound (I never had this prob with my old CRT TV). Are you saying there is no cure except turning the TV sound off ? My brother has a similar flat screen set up and does not suffer this problem. Many thanks in advance for your help
Hi Jon
So do you normally have both the TV sound and the hi-fi sound on? If so then yes, I would expect these to be out of sync, and the cure will basically be to have the TV sound turned off – I’d do this anyway for sound quality as mixing the (probably relatively rubbish) sound from TV speakers with hi-fi sound will leave you with unbalanced reproduction. If your brother’s set-up works better, perhaps there is something different that you haven’t spotted… maybe he takes his sound output from the TV rather than his set top box directly (in this case the sound output from the TV would be delayed to match its video processing) for example?
Hello.
Fascinating stuff here, but I’m puzzled why myself and others (writing on avforums.com) are finding that the lip sync is out on BBC HD transmissions – but the picture is arriving first! (And thus, with amps only having an audio delay feature, there is no way of putting things right.) Last week’s Hustle was virtually unwatchable; last night’s Hunter was slightly better, but still nowhere near perfect. (My own setup is that I have a direct feed into my Pioneer KRP-500A plasma, and then an optical output to my Onkyo receiver, but others have noted the same problem via their Sky HD boxes.)
I look forward to your thoughts!
Many thanks
Martin
Hi Martin. Well the first thing to say is that *if* lipsync is out on BBC HD specifically, then using your box’s audio delay to fix it would be bad anyway as it would ruin the sync on other channels. We actually made a change to some infrastructure last week and we’re investigating whether this could have caused a sync problem – I’ll report back when I know more…!
Thanks for reading! Rowan
Thanks for the quick response; I must stress again that what I’m experiencing is the picture coming well before the audio anyway, so any (audio) delay that I could add via my receiver would just make things (even) worse. BBC HD is the only HD channel I watch (I only have an SD Sky box, but can pick up BBC HD via my plasma’s inbuilt (HD) satellite tuner), so I have nothing to compare it to, although the really odd thing is that it appears inconsistent within the one channel. I’ve just watched about twenty minutes of the HD preview and the audio sync looks bang-on, whereas, as mentioned, last week’s Hustle was so bad (picture first, then audio) that the whole family commented on it.
(Not sure if this helps, but after Hustle had finished last week, I switched to BBC1 on my (SD) Sky to watch the news – the trailers for other shows looked a bit out as well, but as soon as the news started, everything seemed to click back into sync again. I’ve also noticed Top Gear appear to be out of sync on (SD) BBC2!)
I appreciate it’s terribly complicated – as mentioned, I came to your page (indirectly) via a forum that was discussing audio sync problems with the various manufacturers of Sky’s HD receiver. But I’m sure that I’m getting pictures first, then audio, which is all rather peculiar…!
Thanks again
Martin
http://www.bbc.co.uk/blogs/bbcinternet/2008/12/a_christmas_present_from_the_h.html
Comment from Andy Quested today suggests that there was a BBC HD-specific problem which would have affected Hunter. I wonder if that also affected last week’s Hustle (which I didn’t see in HD so can’t say whether I had the same problem, although I had a major problem with Hunter – sound was well delayed from pictures.)
Hi all
As Jez pointed to Andy’s response, we did find an issue with some maintenance done last week. Martin – in your setup (and yes, I realise you are absolutely right about only being able to add audio delay which is the wrong way round for your problem) I would expect your equipment to keep sync correctly as you are taking the audio from the TV’s internal tuner, which I would expect to compensate for its own internal delays. Sadly, this can’t be guaranteed, but most reputable manufacturers seem to do this.
Which leaves us with a problem on the channel itself. As I said, we have now found and corrected a frame of sync error, so hopefully that will solve your problem. Let me know…
Thanks, Rowan
Many thanks for your response. Hustle last night was much better; I’m not sure I’d say it was perfect, but at least I wasn’t constantly aware of the sound delay. Over the weekend I’ll see if the HD lipsync test throws any light on the matter…!
Best wishes
Martin
PS. Have now watched the audio sync test, and it was bang-on. However, by half an hour later, and for the Barbara Cartland clip, the sound was off; you could clearly see the actors forming the words before the audio arrived. Watched Jonathan Ross last night, and the sound was delayed (though nothing’s been quite as bad as Hustle a couple of weeks back!) – and I wasn’t the only one in the house to notice. We switched to SD because the delay was proving distracting; a shame, as the pictures were so crisp!
Any thoughts on the matter gratefully received!
Hi Martin
I watched the most recent Hustle myself here on our (carefully calibrated) equipment and I have to say it looked spot on to me. As for Jonathan Ross, I’ve just taken a look, and while my playback method involved a software decoder – meaning the sync isn’t 100% reliable – it looked good. Andy Quested says it looked fine for him at home too, as he saw it on Friday whereas I didn’t.
If you say the sync test was spot on, then I’m confused as to how other things could be so different. The same broadcast chain is used – and in fact the Barbara Cartland clip you mention is part of the same file being played from the same server! It is of course possible for an individual programme to be delivered to us out of sync, but our own quality checks should pick this up and I have no reason to believe that anything recently broadcast has slipped through the net.
Have you tried things like tuning to a different channel and back again, or turning the unit off and on again? Sad though this is, we know some receivers do drift sync over time (and my own SD receiver from a popular media company who shall remain nameless does this a fair bit), so I’m inclined to look to that as a possible cause.
Sorry I can’t be more help! Rowan
Thanks again for getting back to me.
We were late watching Hustle last night (and only have SD Sky+ – we get BBC HD via the plasma), so watched in SD, and even on that the sound was clearly delayed. (And, again, my lip-reading wife noticed it, so it wasn’t just me going nuts!) So either there’s a problem with both my Sky+ box AND my month-old Freesat HD-equivalent Pioneer plasma, or there’s something wrong with the programme itself…!
I’m thoroughly confused now…
Cheers
Martin
Hi Martin. Last night’s Hustle was indeed quite badly out of sync. Trying to work out why as we speak… it’s confusing us too right now!
Hustle was appalling last night. It was so far out of sync to be almost unbearable (watching on HD via HDMI and sound through the TV speakers). BBC channels seem particularly badly affected. Box is a Pace and I just rebooted it last week and had seen what I thought was an improvement – until last night!
Hi John, thanks for reading. However as I’ve already said, we’re aware of the problem on Hustle last night and are looking into it. Generally the HD channel sync has been good over the last few days, but I’ll let you know if we find out why that particular programme went wrong…
Hi Rowan,
Really good that you and Andy are prepared to keep in touch with us all via these pages. It must be a bit of a pain as presumably 90% of what you get is effectively “things aren’t right”, but thanks. It’s good when there are problems to see that you guys are working on things, rather than being kept in the dark.
Thanks Jez – I appreciate the appreciation…as it were!
Right, well the answer about Hustle is that it seems to be a compound error. The programme itself was delivered to us with a slight sync error it seems, but small enough to fall within our technical policies. Perfect sync simply isn’t a realistic requirement – remember for example that many programmes will have ADR/dubbed audio, so fractional errors will always exist. Additionally, there seems to have been another small error (less than a frame) within the stereo path of the broadcast chain. (i.e. affecting stereo programmes like Hustle but not surround sound programmes.) Applied to most programmes it was small enough to not be noticeable, however, applied in addition to the pre-existing error on Hustle, the combined effect was enough to be clearly visible. We apologise for that – we were certainly very unhappy with it.
What we’ve done is made a temporary fix which probably won’t get everything perfect, but should improve things to within tolerable levels for now. I’m then arranging the most comprehensive tests of sync we’ve ever done on the channel, which will take place next week, so we hope to get much closer to perfection then.
Have a good weekend everyone, and I hope you’re able to enjoy some great programming on BBC HD without major sync problems!
Thanks for that, Rowan. I do appreciate you taking the time to get to the bottom of these issues.
Best wishes
Martin
No problem Martin! Thanks for the comment
Just realised I hadn’t updated you guys. Basically, the tests we had planned for Monday due to snow and people not being able to make it. I hope to get this stuff done tomorrow, so expect a full report either as a comment here or maybe even as a new blog post. (Since I haven’t written one for quite a while…)
Cheers
Rowan
Nice one Rowan, thanks for the update.
I know this may be outside your area of expertise, but do you know if the BBC are considering ever broadcasting MotD in HD, or does that require too much equipment to too many football grounds on a Saturday?
Hi Jez. Simple answer is yes it’s out of my area of expertise, but my suspicion (and this is by no means an official line from the BBC) I wouldn’t get your hopes up any time soon. It would be a lot of HD kit in a lot of places, and I suspect the difficulty in getting half-decent small HD cameras could be a contributing factor too. If I hear anything though I’ll let you know. R
FWIW, we had terrible lip sync issues with the BAFTAs last night (on Sky SD, but I also had the same problem on Freesat) – picture first, as before, it seems. And not just a little bit out, either – probably the worst lip sync I’ve yet seen since that episode of Hustle in January. During one film clip, someone got slapped – the arm moved, and there must have been a second-long pause before the sound came.
Could of course be some oddity of my system, but I tried turning everything off and back on again (and, as mentioned, we switched from Sky to the Freesat-compatible tuner in the Pioneer), and it made no difference. As always, it seemed to be back in sync in time for the news…!
Looking forward to hearing the results of your test(s).
Cheers
Martin
[...] you read the comments on this blog, you’ll recall that we’ve been having problems with the lipsync of stereo [...]
[...] Last time I told you about the efforts we’ve been making at BBC HD to get an A/V sync test to your TV in order that you can measure the synchronisation between audio and video in your home TV setup. You’ll be very pleased to know that we’re done and the test has made it to air! Andy Quested has posted in his blog about how you can use the sync test – and its counterpart the test card – to line-up your equipment. I therefore won’t repeat that here, but I wanted to give you a bit more detail about what we’ve achieved and how. [...]