I get served ads on my smartphone based on what I watch on TV. How does it work?

Smart TVs use Automatic Content Recognition (ACR) and shared network data to track what you watch and target related ads to your mobile devices. While regulators require consent for this cross-device tracking, users should also manually review privacy settings on their TVs, streaming apps, and smartphones to limit data collection.

We’ve all been in the uncomfortable situation of watching a program about a specific holiday to a specific location on your television, then opening Instagram on your phone and finding yourself being advertised flights to that exact same place. That uncanny ability to understand what we’re looking for can feel hugely disconcerting when it happens – and it happens quite regularly.

It can feel like your phone is listening to the television. It’s not quite that sinister, but you’re not going crazy: advertisers can and do connect what you watch on one screen with the devices you use on another.

One way they manage to do so is by a technology installed on many smart TVs called automatic content recognition (ACR). This takes short samples of what’s playing on your TV screen at regular intervals, creates a digital fingerprint, and then compares it against a massive database of content it has access to. The extent to which ACR operates depends on the manufacturer, but it can include fingerprints from image or audio data.

Uncanny valley

Researchers from University College London and the University of California, Davis demonstrated way back in 2024 that the technology could operate even when Samsung and LG televisions were being used as displays for devices connected through HDMI.

Once the company knows what’s on your TV, their job is only half done. The advertiser also needs to work out which phone belongs to the same household. An easy way they can triangulate is the shared Wi-Fi connection and public IP address that are generally used within an individual household. That allows advertising companies to infer a connection. But it’s not foolproof, so they also use accounts, advertising identifiers, and other data to more accurately link devices closely in what’s called identity graphs in the industry.

Samba TV, an advertising technology company, markets its ability to blend television viewing with web behavior and to reach audiences across multiple devices. LG similarly acknowledges that its optional interest-based and cross-device advertising services can use viewing information, subject to separate consent.

Whatever the specific service, it means that advertisers can identify households that watched a particular sports event or saw its commercial on television, and then use that to try to reach those households again on mobile.

ACR isn’t the only game in town when it comes to this kind of connection making. Popular streaming services and TV platforms also collect information about what users select and watch through their apps and accounts, which – depending on the service and the privacy choices the user makes – can be used to build a better picture of who you are and what you’re interested in.

Regulatory crackdown

Those approaches have attracted the attention of regulators who worry about overly generous information gathering. In 2017, US regulators said Vizio had collected viewing histories from 11 million televisions without consumers' informed consent and brokered a $2.2 million settlement. In the UK, the Information Commissioner's Office says that using ACR for advertising requires valid consent under privacy rules.

But in addition to regulatory intervention, it’s always worth taking action yourself to ward off such tracking. So how can you stop your television from following you around?

First things first: dig into the privacy settings. On Roku televisions, ACR can be disabled under Settings, then clicking on Privacy and Smart TV Experience. LG users cannot agree to or withdraw from the Viewing Information Agreement and a separate agreement covering interest-based, cross-device advertising. If you’ve got a Samsung TV, head for Viewing Information Services and personalized advertising controls.

But bear in mind that even after doing all that, turning off ACR doesn't necessarily stop other services from collecting information – so you also need to review the privacy settings of streaming apps and the advertising preferences on your smartphone.