Facebook wants to spy on you by hiding inaudible messages in TV ads.

These secret messages would force your phone to record audio of your private conversations without you knowing, according to a patent application by the firm.

Clips taken of your background conversations or movements across a room would help advertisers determine whether you are watching their promotions.

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Facebook wants to hide secret messages in TV adverts so it can spy on your watching habits a new patent application reveals. This image from the application details how Facebook would use your television (top) to send messages to applications on your phone (bottom)

According to the patent, spotted by Metro, the system would use 'a non-human hearable digital sound' to activate your phone's microphone.

This noise, which could be a sound so high-pitched that humans cannot hear it, would contain a 'machine recognisable' set of Morse code-style beeps.

Once your phone hears the trigger, it would begin to record 'ambient noise' in your home, such as the sound of your air conditioning unit, plumbing noises from your pipes and even your movements from one room to another.

Your phone would even listen in on 'distant human speech' and 'creaks from thermal contraction', according to the patent.

TV advertisers would use this data to determine whether you had muted your TV or moved to a different room when their promotional clip played.

A patent filed the company reveals it has designed a system that uses high-pitched sounds inaudible to humans to force your smartphone to record audio of your home. This illustration shows a sound wave in an advert masking a high-pitched message to smartphones

This would help them to better calculate the number of people who watched their advert.

A muffled code sound could mean you have moved away from your television, while if they trigger is picked up loud and clear you are likely still in front of the screen.

Facebook would store the ads people tuned into to help it show content that is better-honed to their preferences in future, the patent says.

HOW DOES FACEBOOK PLAN TO TRACK USERS THROUGH TV ADS? A Facebook patent filed in June reveals the company has designed a system to track its users through TV adverts. The system uses 'a non-human hearable digital sound' to trigger your phone's microphone to record background 'ambient' noises in your home. The trigger noise is hidden underneath the normal audio of adverts and could be so high-pitched that humans cannot hear it, Facebook said. It would contain a 'machine recognisable' set of Morse code-style sounds. If you phone hears the noise, it would begin to listen in to noises in your home, including 'distant human movement and speech'. TV advertisers would use this data to determine whether you had muted your TV or moved to a different room when their promotional clip played. Advertisement

Facebook has repeatedly stated that its patent applications should not be taken as evidence for its future product plans.

'Most of the technology outlined in these patents has not been included in any of our products, and never will be,' Allen Lo, a Facebook vice president and the company's head of intellectual property, told the New York Times.

But experts argue that a company's intellectual property is a clear indicator of the direction it is headed.

'A patent portfolio is a map of how a company thinks about where its technology is going,' said Jason Schultz, a law professor at New York University.

Facebook, fronted by controversial CEO Mark Zuckerberg (file photo), insisted most of the technology outlined in the firm's patents does not make it into its products

Facebook has repeatedly denied it uses audio taken from smartphone microphones to target its adverts.

The firm most recently spoken about the practice in a bumper document presented to Congress in April following the Cambridge Analytica data scandal.

In answering whether the Menlo Park firm ever captures microphone or camera data without a user's knowledge, a spokesman said: 'No, Facebook does not engage in these practices or capture data from a microphone or camera without consent.'

Cambridge Analytica whistleblower Christopher Wylie said in March that he believed companies like Facebook use phone microphones to help tailor their adverts.

WHAT IS THE CAMBRIDGE ANALYTICA SCANDAL? Communications firm Cambridge Analytica has offices in London, New York, Washington, as well as Brazil and Malaysia. The company boasts it can 'find your voters and move them to action' through data-driven campaigns and a team that includes data scientists and behavioural psychologists. 'Within the United States alone, we have played a pivotal role in winning presidential races as well as congressional and state elections,' with data on more than 230 million American voters, Cambridge Analytica claims on its website. The company profited from a feature that meant apps could ask for permission to access your own data as well as the data of all your Facebook friends. The data firm suspended its chief executive, Alexander Nix (pictured), after recordings emerged of him making a series of controversial claims, including boasts that Cambridge Analytica had a pivotal role in the election of Donald Trump This meant the company was able to mine the information of 87 million Facebook users even though just 270,000 people gave them permission to do so. This was designed to help them create software that can predict and influence voters' choices at the ballot box. The data firm suspended its chief executive, Alexander Nix, after recordings emerged of him making a series of controversial claims, including boasts that Cambridge Analytica had a pivotal role in the election of Donald Trump. This information is said to have been used to help the Brexit campaign in the UK. Advertisement

Wylie, a former employee of Cambridge Analytica, the political consultancy firm that harvested the data of 87 million Facebook users via a loophole in Facebook's privacy settings, told a Commons committee he believed the social media giant was able to decipher whether someone is out in a crowd of people, in the office or at home.

He said: 'On a comment about using audio and processing audio, you can use it for, my understanding generally of how companies use it... not just Facebook, but generally other apps that pull audio, is for environmental context.

'So if, for example, you have a television playing versus if you're in a busy place with a lot of people talking versus a work environment.'

'It's not to say they're listening to what you're saying. It's not natural language processing. That would be hard to scale.

'But to understand the environmental context of where you are to improve the contextual value of the ad itself' is possible.'

Allen Lo, VP and Deputy General Counsel, Head of Intellectual Property of Facebook, said: 'It is common practice to file patents to prevent aggression from other companies.

'Because of this, patents tend to focus on future-looking technology that is often speculative in nature and could be commercialized by other companies.

'The technology in this patent has not been included in any of our products, and never will be.

'As we've said before, we often seek patents for technology we never implement, and patent applications should not be taken as an indication of future product plans.'