Why we need a Digital Television Group - Report by Barry Fox
The annual Summit conference held in May by the UK's Digital Television Group was short on hot news but neatly proved the need for a DTG - to create order out of the chaos which would surely exist if there were no independent body which the TV industry trusts to set standards, test products to ensure they meet standards and steer rival TV stations and set-makers into collaboration.
Intermittent muting
In the run up to this year's event, at the Kings Place concert hall in London's Kings Cross, some TV viewers had been plagued with a mysterious audio fault on all the Freeview high definition channels, but not the standard definition equivalents. On some makes of TV (notably Samsung), but not others, and not on set-top boxes (eg, Humax), the sound intermittently and apparently randomly mutes.
The conference provided an opportunity to talk with the DTG's engineers, who admitted that - like all intermittent, random faults - the HD dropout had been very difficult to isolate and make repeatable. But the DTG thinks it is finally nailed.
CODEC issues
The HE-AAC (High-Efficiency Advanced Audio Coding) audio encoders on the HD multiplex are quite separate from the MPEG (Version 1, Layer 2) encoders used for the SD channels, and earlier this year all the HD encoders were upgraded to tweak their AAC compression with a system called perceptual noise substitution (PNS). This saves bits where wanted audio sounds like unwanted noise. So the coding is continually changing, depending on the sound. Although this was tested before the upgrade, it later emerged that the decoder chipsets used in some TVs (notably Samsung) cannot cope with some levels of compression, and respond by muting.
The DTG is now co-ordinating a temporary rollback of the encoder upgrade to fix the problem - until the set makers have all developed, tested and pushed out updates to the decodes chipsets locked into viewer's sets. The rollback started in May, just before the DTG conference, and is progressing across the country.
The DTG is currently holding 'plugfests' to try and pre-empt what could have been even more widespread problems if broadcasters press ahead with plans to use HDR (high dynamic range) picture coding. Both Netflix and Amazon have gone ahead with the launch of HDR programming on their 4K Internet services, without waiting for an industry standard, and each uses slightly different systems. Sascha Prueter, Head of Android TV at Google, confirmed that the BBC is now working with YouTube on another system.
LG has adopted the proprietary Dolby Vision system, while Panasonic is catering for the Netflix and Amazon systems along with HDR10, the Open HDR system used for Blu-ray discs. Disney wants to use HDR, but with conventional 1080p High Definition video, rather than 4K.
The opportunity for chaos is obvious, with HDR video material likely to look worse than non-HDR pictures if displayed on screens that cannot correctly decode the metadata that travels with the picture signal to match the display capability with the source material.
Participants in the DTG's two recent HDR Plugfests, one in Berlin and another in London, told how TV manufacturers at the events were frantically phoning their software labs in India, Korea and Japan to describe problems encountered at the event. Because the events were spread over two days tere was no time to get back revised software down a line and try it on the spot.
Subtitle 'triumph'
The DTG's Summit at Kings Place was notable also for proving that live TV subtitling does not have to be as poor as it usually is. The current technique is for a titler to listen to the TV sound through headphones and re-speak the words into a microphone connected to a PC which is running voice recognition software. This overcomes the problem that computer voice recognition cannot (yet) cope with different dialects or accents or mumbling. But inevitably re-speaking creates a significant delay and the software still injects recognition errors, which can only be corrected when the programme is recorded and repeated or replayed later from a Catchup Service such as iPlayer.
The recognition errors are often so gross and hilarious that watching a muted TV screen in a club, pub or airport waiting zone has become a whole new form of entertainment. The errors are also deeply frustrating for the prime target audience, the deaf and hard of hearing.
At the DTG Summit, some members of the audience gradually realised that the subtitles for the on-stage talk which were continuously displayed live on screens alongside the stage were remarkably accurate and only slightly behind the speech; and this was despite the fact that many of the speakers were talking very fast, and using Tv jargon.
How is this happening so extraordinarily well, we asked; how can re-voicing and software recognition be so rapid and accurate? Up on screen, to spontaneous applause from the hall, came a direct reply from the titler: 'I am a steganographer'.
The DTG later explained that it had hired a 'verbatim' court steganographer with a dedicated keyboard that captures words as shorthand keystroke combinations which computer software then outputs as plain text. The DTG says it is now co-operating with the UK's telecoms regulator Ofcom on a report, which asks why TV subtitling is so bad, and whether it has to be so bad. Hopefully, someone from Ofcom was at the DTG Summit to see first hand evidence that it doesn't have to be so bad. But the cost will increase because it takes literally years of training to learn to capture in real time.