Audio Engineering Society UK

Title: ‘Synchronising the synchronisation standards’
Location: Royal Academy of Engineering, London
Description: Lecture by John Emmett
Start Time: 19:00 for 19:30
Date: Tuesday 16th February, 2010

Download recording of lecture here (20MB MP3)

Lecture Report

Dr Emmett opened the lecture by summarising the audio-video synchronisation challenges encountered when putting together a television programme. It is better to correct synchronisation problems as they occur in the broadcasting chain than to attempt to correct them all immediately prior to transmission, as the former practice greatly simplifies video editing. With this achieved, attention turns to keeping audio synchronised during broadcast transmission and reception. This is particularly important for human speech: humans are exquisitely sensitive to lip sync. We develop this facility almost as soon as we can see, and the psychological need for lip movement to be attached to speech is so great that each Dalek must display a light that pulses in sync with speech, in order to bond dialogue to a particular character.

A number of techniques were employed in the days of purely analogue transmission to ensure that audio and video were kept in sync. It was not unusual for a programme’s video signal to be relayed via satellite and its audio via telephone, and a compensating audio delay had to be inserted to offset uplink and downlink delays. An example of this was used in ITN in the early 1980s. An in-band masked ‘bong’ was timed to follow any video cut in the programme by exactly one second. It was possible then for engineers to adjust the audio delay manually to maintain sync, even where this varied during the programme. Similar timestamps must still be maintained in digital systems, although this facility is now generally accommodated within the channel code.

It is increasingly common for audio and video to be streamed by piggy-backing on a packet-based protocol and transmitting via existing IT infrastructure. This works as long as there is sufficient bandwidth. Otherwise, heavy-duty interleaving is required to compensate for dropped packets, which increases transmission delay, and the chances of sync loss and system failure. As with real piggy-backs, the heavier the payload, the slower the system, and the greater the likelihood of collapse.

Consider what the word ’standard’ means: this is where problems are compounded. The word has two distinct meanings. It can refer to an outgoing or obsolescent paradigm (such as ’standard definition’), or to standard-bearing in its original sense — at the technological vanguard. We frequently encounter problems when it is necessary to choose between a plenitude of competing standards of different ages, some of which have yet to be adopted, and many of which should not. Standards are necessary only when the current best practice is unclear, so clues for choosing ‘good standards’ were suggested. A good standard must be fit for purpose, timely, and robustly defined: if the plug fits, the signal should work. There are also caveats, because not all standards are intended to be friendly (DRM systems were cited as an example), and even de facto standards undergo sudden and complete changes. Finally, although a standard needs to be owned by a company or committee to avoid obsolescence, it should contain no element for revenue generation.

The emergence of competing delivery standards in broadcasting has brought the synchronisation problem into the home. Many digital multichannel audio transport layers can be conveyed over S/PDIF channel code using IEC 61937 (Dolby Digital; DTS; linear PCM), and a home cinema amplifier may typically accommodate sixty connectors and a dozen multichannel formats. As for the picture, high-definition video formats such as 720p and 1080p co-exist with conventional 625-line 4:3 and 16:9 broadcasts. There are a number of video interconnection formats with different costs, advantages, and limitations. Any of four digital video broadcasting standards are in use in different regions throughout the world, encompassing several standard frame rates. Meanwhile, individual consumer products are designed for world markets, and are simultaneously compatible with many of these standards. In fact, UK broadcasters have been unable to rely on viewers possessing ’standard’ receiving equipment since 625-line broadcasts began in the 1960s.

Now that it can take half a day for a professional engineer to set up a new television, it is quite likely that a set-top box in a typical home may be configured to down-convert 720p video to standard definition, and transmit this signal over RGB SCART to a plasma television, which will then up-convert it to 1080p. Audio-video synchronisation is then at the mercy of equipment manufacturers.

Dr Emmett summarised his lecture with advice from Antoine de Saint-Exupéry: ‘No design is finished until the last superfluous item has been removed.’

Report by Ben Supper