The 17-year-old making films fun for deaf children

"Seeing my son be able to sit and understand the movie and see things he's not seen before in eight years, it's amazing."

For eight-year-old Toby, who is deaf, watching films or TV on streaming platforms can sometimes be a bit pointless - because so many of them don't have sign language versions.

"We have captions but they don't really do anything for him because it goes quite fast. He would just watch and not get much from it," his dad Jarod Mills tells Radio 1 Newsbeat.

But now, Toby has some help thanks to an app developed by a 17-year-old A-level student.

Mariella Satow, who has dual UK-US citizenship, lives in the UK but has been stuck in New York since summer 2020 because of Covid travel restrictions.

In that do-something-new phase of lockdown, Mariella created a signing app called SignUp.

She got the idea when she was teaching herself American Sign Language (ASL) - one of hundreds of sign languages used across the world.

Mariella wanted to watch TV shows to help her learn, so was disappointed to discover how few had signed versions.

According to research from The Big Hack by disability charity Scope, many streaming platforms offer closed captions and audio description, but not sign language.

It's taken a year for Mariella to develop the technology, with lots of help from ASL teachers and the deaf community.

The app is available in the US as a Google Chrome extension - with an interpreter appearing in a box once the film starts playing.

It only works on Disney Plus films at the moment, because that's where Mariella thought she could help the most children.

"Me and my sister were avid movie watchers when we were younger, and I couldn't imagine that not being a part of our childhood," she tells Newsbeat from New York.

'Level playing field'
Jarod, who works in Kentucky at a school for deaf children, says it was "exciting" watching Toby use Mariella's invention.

"The app creates a level playing field," he says.

"Kids are getting that understanding and information like any hearing child does - they learn a language even before they go to school."

Deaf people in the UK face many of the same barriers when it comes to cinema and TV.

"A hearing person can spontaneously go to the cinema. Whereas people who rely on subtitles have to make sure there's a suitable viewing at a time we're free," says 27-year-old Stacey Worboys from Cardiff.

"I'd like to be able to go anytime without having to change my plans or book time off work."

Stacey says she hasn't previously come across any signing services "which will allow for an interpreter to be added to interpret the whole film".

She has learned British Sign Language (BSL), and is comfortable using subtitles when watching TV shows and films.

But she feels having an interpreter would make things "more inclusive", especially for someone who might struggle with subtitles.

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Stacey and Toby aren't the only people to be welcoming Mariella's app. It's now got thousands of users, and she's working long days to juggle it with her A-level studies.

The time difference from New York to her school in Rugby, Warwickshire, means she sets her alarm for 5am.

She admits it's "a bit tough" - but the silver lining is it gives her time to work on SignUp once lessons are over.

The positive reaction from teachers and parents has kept Mariella motivated to face the early starts.

"The most meaningful comments are when it's the first time a child has had full access to a movie. The numbers don't really matter, it's the messages," she says.

Stacey says a UK version of an app like SignUp would make film and TV "more accessible" for the deaf community, especially as sign language is often deaf people's first language.

Mariella is up for the challenge - and hopes to make a British Sign Language version of her app for other streaming platforms such as Netflix and Amazon Prime.

"There are more than 300 sign languages used worldwide, so it'll take a long time to get all of those versions out," she says.

Disney hits such as Frozen, Moana and The Incredibles - Mariella's favourite film - have all now been signed for SignUp, and Mariella's had requests to do hundreds more films.

"I can't believe how big it's become," she says.

"I had no idea what I was launching into the universe."