I’M enjoying Vigil, the new BBC thriller set on a nuclear submarine deep beneath Scottish waters, but when I tuned in at first, I couldn’t make out a word of the muffled dialogue. Not wanting to blast the neighbours or wake up my son, I ended up putting on the subtitles to make out what Martin Compston and the rest of the talented cast were saying.

I wondered if my hearing was at fault as it’s not the first TV drama that I’ve found impossible to follow, while other programmes are clear as a bell. It turns out that there are a host of reasons that we either squint at subtitles and miss the acting nuances, or rewind over again to decipher a key piece of dialogue, like a bunch of radio signals operators trying to make out an enemy code.

The BBC in conjunction with the Royal Institute of the Deaf and the Voice of the Listener and Viewer Association did a study to find out the cause of viewers’ problems making out dialogue on TV. The study revealed that it's not just older people who don't always understand what's being said – 70 per cent of all adults in the 20,000-strong survey said they'd had trouble, compared to 76 per cent aged over 65.

One of the reasons for this problem is modern technology. Our fancy internet-ready tellies may have high-definition pictures, but they are so slim it’s impossible to fit decent speakers in them. So, while the picture has improved in recent years the sound quality is worse than it was in old-fashioned boxy ones, a problem highlighted by magazine Which? in a consumer report.

READ MORE: Vigil star Lauren Lyle on submarine drama

The solution is to get gadgets called variously a sound bar, a sound projector, a subwoofer, or a sound base, that sit next to the set and give extra depth. There’s also surround sound, where you buy five speakers to give a home cinema experience. But having spent a lot of cash on a TV not so long ago, it seems a bit of an ouch to have to shell out on more tech to improve what should be essential to a new TV – being able to hear it.

However, as I could hear another recent drama, The White Lotus, perfectly well without subtitles, there must be another reason – could it be the mumbling actors?

The 2009 BBC survey found that an actor or presenter's speech clarity was one of the main causes of not being able to understand what was going on in a drama. Since then, the BBC has been on the receiving end of complaints about mumbling dialogue and The Royal Television Society say difficulty hearing dramas is a recurring irritation for audiences.

In 2013, the then BBC Director-General Tony Hall highlighted the difficulty of hearing dialogue in dramas as one of the problems he was keen to address: “Actors’ muttering can be testing. I don’t want to sound like a grumpy old man, but I think muttering is something we could have a look at.”

In 2014, there were more than 2,000 complaints about the mumbled conversations in Jamaica Inn and it lost a third of its viewers as a result. Adaptor Emma Frost admitted it “sounded like listening through mud.”

Viewers also hit out at the poor quality of sound during the otherwise excellent 2016 drama Happy Valley. Following that, the BBC’s Controller of TV Channels, Charlotte Moore, also pledged to tackle the issue. She said the BBC had taken episode one back to the editing suite to improve sound levels, infuriating veteran sound engineer Chris Ashworth who most recently worked on The Crown. He blasted back that it was all down to actors mumbling their way through the script. “ Actors turn their back or head or speak quietly. Some directors like the fact that the audience is straining to hear what is said.”

READ MORE: Alison Rowat's TV reviews: Vigil; Stephen; Mortimer and Whitehouse, Gone Fishing; The Chase

Meanwhile, Simon Bishop, Chair of the Institute of Professional Sound, puts the blame on directors who refuse to listen to sound experts when they report muffled dialogue, and to modern production techniques where producers watch rushes on an iPad or iPhone, which don’t give an accurate appraisal of the sound, whereas 20 years ago the sound would be checked through loudspeakers.

Diederick Santer, former producer of EastEnders and Grantchester, sometimes resorts to subtitles because the sound is so bad – so I’m not alone. At the final mix he said, after adding the music, “as you sit in the audio suite surrounded by big speakers, you can hear ever chirp of bird song, every pluck of harp, every lip smack, it all sounds rich and lovely.

“What I do is wheel in a crappy telly and play the whole thing in the way that replicates the experience at home, and not compel the audience to switch to subtitles, which I am so often forced to do by badly recorded and mixed shows.”

With Vigil a prime example – I had to resort to subtitles within minutes, unable to make out a word of Compston’s shouty, garbled outburst against submarine background noise – the problem clearly persists.

After decades of carefully enunciated diction in screen dramas, Marlon Brando was one of the first to adopt method acting and his mumbling has been much parodied. Actors swallowing their words as people do in real life may be more naturalistic, but it’s frustrating for the viewer. If we could hear what they were saying, we could enjoy dramas like this a whole lot more.

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