Audio Engineering Society UK

Title: ‘Who’s the bad guy now? Maintaining audio/video sync in today’s broadcast environment’
Location: Royal Academy of Engineering, London
Description: Lecture by Andy Quested, Head of Technology, BBC R&D
Start Time: 18:30 for 19:00
Date: Tuesday 12th January 2010

To complain that “the audio is out of sync” was, in the past, doing audio an injustice. The use of visual effects units, time base correctors and other digital processing in the video chain, while the audio continued to pass through an analogue signal path, meant that it was, in fact, the video which was usually out of sync. However, the move to digital audio processing, and in particular surround sound broadcasting – which often  requires six channels to be passed through a two-channel infrastructure – has significantly moved the goalposts. The advent of HD, with its more clearly defined imaging, has exacerbated the problem. Andy Quested will highlight some of the audio/video synchronisation issues that the BBC HD channel has had to deal with, and will outline the measures it is taking to put audio back into its rightful place.

Andy’s BBC blog provides some more background: http://www.bbc.co.uk/blogs/bbcinternet/2009/12/the_hitchhikers_guide_to_encod.html

The lecture recording is available to download here (45MB MP3)

Meeting Report

Andy was joined for the lecture by a colleague from BBC Future Media & Technology, Rowan de Pomerai, who provided details of BBC HD’s audio/video transmission infrastructure and the points where sync errors can be introduced. This was comprehensively illustrated by slides showing block diagrams of the various elements in the chain, many of which can be found in an excellent white paper on the EBU’s website: http://tech.ebu.ch/docs/techreview/trev_2009-Q1_HD-Audio-Delays.pdf. Andy’s contribution was more anecdotal, highlighting the actual problems encountered, and this report will focus primarily on his part of the lecture.

Andy opened with some statistics on HD adoption in the UK. Sky has 1.8m HD subscribers, Virgin has 280,000 and 48,000 watch HD via Freesat. Freeview HD is launching and is expected to become the biggest single platform. In 2009, Wimbledon and Torchwood attracted HD audiences of 1.75m. Overall, 2009 was not a bumper year for sport but there will be plenty in 2010, including the Winter Olympics and the World Cup. Launched in April 2009, the HD iPlayer is now the most successful version of the BBC’s catch-up service.

In a recent survey, viewers were asked what they considered to be the most important elements of an HD channel. Not surprisingly, picture quality was placed top by 56 per cent, followed by choice of programming by 48 per cent. Sound quality was fourth at 34%, a figure that hasn’t really changed since BBC HD was launched in 2007. Part of the problem is that, unlike cinemas which have laid-down standards for audio replay, home speaker layouts can vary enormously, particularly in the placement of the centre speaker. This can make it difficult to predict the listening experience.

Moving on to the specific topic of audio sync, Andy noted that the BBC HD channel suffered from several audio sync and metadata problems in the early days. Programmes affected were the Proms, Electric Proms, Olympics and Strictly Come Dancing.

One of the earliest instances of a major problem involved the 2008 Eurovision Song Contest. Andy was watching at home and immediately noticed that there was no music track on the HD broadcast, only vocals from the centre speaker. Somehow what should have been a 5.1 track was actually 1.0, which shouldn’t happen because the BBC HD channel is locked to 5.1 even when broadcasting stereo in order to prevent clicks or mutes which happen when some AV receivers switch modes.

Andy phoned the broadcast centre, which was unaware on the problem – they were hearing 5.1 all the way through the chain. Andy suspected a metadata issue but where was the problem occurring? The broadcast chain includes many elements, not helped by the BBC’s outsourcing policy which means that there are several companies involved (see Rowan’s white paper). The decision was made to switch to an upconverted BBC 1 feed with stereo audio while the problem was investigated because taking audio only would have resulted in sync problems.

At Andy’s request the Dolby encoder was checked and it was found to be set to disable the metadata (a option that has since been removed by a software update). With no metadata the Dolby decoders in the set-top boxes revert to their default mode, which is 1.0. This is a legacy from Dolby systems in cinemas where the centre dialogue channel is the most important element and is therefore the most logical default.

With regard to maintaining sync, BBC HD has taken the approach that audio and video should be in sync at every stage of the chain – known as in-sync encoded. However, this hasn’t stopped numerous complaints about audio/video sync from viewers.

Rowan de Pomerai explained that many of the problems are due to delays created within set-top boxes and flat panel displays, the latter creating a video delay of up to 100ms. Hearing audio before the video is counter-intuitive because light travels faster than sound and we’re therefore used to hearing the audio delayed relative to the picture, not vice-versa. Many set-top boxes have a delay function, but this has to be configured. The BBC has developed a sync test to assist in setup which is broadcast a regular intervals during the daytime. (For a full description of the test see Rowan’s white paper.)

Providing a sync test is a great idea but for it to work correctly it’s essential that the audio and video signals arriving at the set-top box are in sync. The broadcast chain was measured all the way through to the broadcast encoder and adjustments made for minor sync errors introduced throughout the system. A duplicate system at BBC R&D Kingswood Warren was also measured to verify the figures. However, the only way to check categorically that everything was OK was actually to broadcast a test.

The final problem was how to measure the sync off-air. A set-top box was not reliable enough so the solution was to record the MPEG transport stream, decode it offline and measure the analogue waveform and video frame numbers. The BBC’s was aiming for ±5ms – a quarter of the EBU’s recommendation – but the result of this test was measured to be ±2ms. “So, it’s no longer just ‘OK leaving me’, it’s also ‘OK arriving at you,’” Andy noted, adding that servers do drift so ±5ms is BBC HD’s target as an average. This is still an excellent figure when taking into account that there’s around 8ms sound delay between a TV and the viewer.

Before transmitting this test BBC HD received 20-30 complaints a week regarding sync but after the test these dropped to zero. The only complaints received since were for one live broadcast that actually was out of sync. In that instance BBC HD knew the feed was out of sync because they had the confidence that the broadcast chain was 100 per cent in sync.

In conclusion, Andy stressed that audience education is essential. The BBC receives about 90,000 hits on its website and 3,000 calls per month about HD. The days of just plugging everything in and it all working are gone. Users need to understand about 5.1, adjusting audio delay and speaker positions, and – very important – removing the SCART lead. Countless viewers are watching HD programmes in SD because pin 8 on their SCART has switched the TV from HDMI to the AV input!