AT&T today said it will begin limiting the quality of mobile video for cellular data customers in early 2017. A new feature called "Stream Saver" will throttle video to DVD resolution of about 480p. Customers will be able to opt out of Stream Saver, but it will be enabled by default—even for customers with unlimited data, AT&T told Ars.

AT&T will notify customers once Stream Saver has been activated and provide instructions for turning it off and back on, the company's announcement said. Customers on limited data plans may appreciate the feature, as it could help them stay under their data caps. But AT&T's decision to enable video throttling by default on unlimited plans that were sold without any mention of such limits has little benefit for customers. It could have some benefit only because AT&T reserves the right to throttle unlimited data plans when customers exceed 22GB a month and connect to a congested cell tower. Using less data for video will help keep "unlimited" customers under 22GB.

"It’s your choice. You can still stream video in higher resolution, when available, whenever you choose," AT&T said. "You control Stream Saver and can turn it off or back on for any qualified line at any time at myAT&T or Premier for business customers. There is no charge to disable or enable Stream Saver."

For multi-line accounts, Stream Saver can be turned off for some lines but not others. It will be available both for AT&T's postpaid plans and AT&T GoPhone prepaid plans.

Stream Saver will limit the quality of most video, but "Due to the way some content owners deliver video streams, Stream Saver cannot detect and then optimize all video," AT&T said. Stream Saver only applies to cellular data, so video will stream in normal quality on Wi-Fi or home Internet connections.

Stream Saver is similar to a T-Mobile system that reduces video quality. On T-Mobile's older plans with monthly data limits, customers can turn the video throttling on or off without it affecting their bills. But on T-Mobile's new "unlimited" plans, unlocking HD video costs extra. Even then, customers can only switch HD video on for 24 hours at a time.

With AT&T, at least, disabling Stream Saver isn't a temporary thing—your preference is saved until you change it again.

Sprint also recently began selling an unlimited data plan that reduces quality of video, gaming, and music. AT&T only sells mobile unlimited data plans to customers who also subscribe to DirecTV or AT&T's wireline TV service, with the exception of customers who bought unlimited data years ago before AT&T started imposing data limits.

AT&T's newest limited data plans throttle customers to 128kbps for the rest of the month after they reach their data limits unless they switch to a more expensive data plan. With earlier data plans, customers were automatically charged overage fees.

AT&T's announcement did not say whether the company will limit the quality of DirecTV video. AT&T owns DirecTV and has exempted DirecTV video from mobile data caps while charging other video providers for the right to bypass customers' caps. The Federal Communications Commission yesterday told AT&T that this arrangement may violate net neutrality rules.

We asked AT&T whether it will let DirecTV stream in high quality by default. We will update this story if we get an answer.