Summary
There’s a reason physical media enthusiasts prefer 4K UHD Blu-ray discs over streaming, and it’s not nostalgia. 4K UHD Blu-ray content will look better than the same 4K content delivered via a streaming service. But picture quality isn’t the only part of a movie or TV show that suffers from streaming’s bitrate limitations. The audio is also compressed, whereas Blu-ray discs can include lossless surround sound audio encoded in the Dolby TrueHD format. It’s a major upgrade over the lossy Dolby Digital Plus codec used by streaming services. Dolby has a new audio codec, Dolby AC-4, that aims to flip the script. It’s a highly efficient audio codec that’s designed to deliver better quality at lower bitrates, and audio that sounds like lossless at higher bitrates. Streaming has a bitrate problem A 1080p Blu-ray might outperform 4K streaming due to bitrate alone Bitrate affects both how content looks and sounds. Although both 4K UHD and streaming formats feature the same 4K resolution, it isn’t delivered in the same way. Specifically, streaming services typically deliver content at a lower bitrate (bits per second) than a Blu-ray disc and a 4K UHD player. The lower the bitrate, the less digital data can be shared in a single second. The same distinction applies to the audio tracks attached to a movie or TV show. For instance, the same movie could have Dolby Atmos audio embedded for object-based surround sound playback, but it’ll be delivered differently via streaming versus a 4K UHD Blu-ray disc. Streaming services use the lossy Dolby Digital Plus codec, even for Dolby Atmos audio. Meanwhile, 4K UHD Blu-ray (and regular Blu-ray) discs use Dolby TrueHD, a bit-for-bit lossless codec. Blu-ray discs can hit a maximum bitrate of 18Mbps for Dolby TrueHD audio, and this figure crushes streaming. The technical maximum bitrate for Dolby Digital Plus is 6Mbps, which is already a third of that of Dolby TrueHD. However, that number is only achieved in perfect conditions using a Blu-ray disc. While streaming an audio track with a show or movie, you’re lucky to hit a bitrate of 1Mbps. Instead, Dolby Digital Plus will likely be streamed in the kbps range, as the limited bitrate has to be shared between the video and audio data. Typical bitrates range from 384kbps to 768kbps. This highlights the disparity between the audio quality of streamed content compared to movies or shows decoded directly from a Blu-ray disc. AC-4 is all about efficiency Dolby can’t stream higher bitrates, but it can make more efficient codec Dolby AC-4 is an audio codec designed to deliver quality that imitates lossless Dolby TrueHD with efficiency that makes it a suitable replacement for lossy Dolby Digital Plus. At its core, Dolby AC-4 is built to use half the bitrate of Dolby Digital Plus while providing the same or better Dolby Atmos quality. It scales, so Dolby AC-4 will sound even better than Dolby Digital Plus when bitrates get high enough. Dolby tries to do this by offering three AC-4 bitstreams purpose-built for specific use cases:
- AC-4 IMS: This Dolby AC-4 profile delivers surround and immersive audio, and is designed for earbuds, headphones, and inbuilt stereo speakers.
- AC-4 Level 3: This Dolby AC-4 profile is made for broadcast and over-the-top (OTT) delivery, and supports both surround sound and immersive AV experiences. It’s intended to replace Dolby Digital Plus with Joint Object Coding (DD+JOC), the format that bundles spatial audio data with standard DD+ audio. AC-4 Level 3 provides higher-quality audio than DD+JOC at lower bitrates.
- AC-4 Level 4: This is Dolby’s most advanced AC-4 profile, and it includes the full range of Dolby Atmos metadata. That means this profile has the data needed for spatial audio playback on both loudspeakers and headphones. It has “the highest spatial and audio fidelity available today,” according to Dolby. Dolby outlines the differences between the three AC-4 profiles in the table below: Efficiency is the name of the game for Dolby AC-4. At lower bitrates than Dolby Digital Plus, AC-4 can deliver the same quality. But what’s more interesting is how AC-4 performs at higher bitrates. Dolby says that AC-4 is “perceptually lossless” at higher nitrates. Make no mistake, Dolby AC-4 is still a lossy codec like Dolby Digital Plus, but Dolby is betting you can’t hear the difference between high-bitrate AC-4 and Dolby TrueHD. Additionally, AC-4 has an increased object count, improving the realism and immersiveness of spatial formats like Dolby Atmos. It’s not just Dolby making these claims — in a blind listening test, Immersive Machines reached similar conclusions. AC-4 is already here It might be the high-quality streaming codec we’ve always needed The major streaming services still rely on Dolby Digital Plus for delivery, but Dolby AC-4 adoption is already starting. Peacock has committed to being the first streamer to offer Dolby AC-4 and Dolby Vision 2, both of which are “anticipated” to arrive later this year. In the meantime, NBCUniversal’s Telemundo streamed World Cup 2026 coverage on Peacock using Dolby AC-4. It marked the first time a company used Dolby AC-4 for widespread commercial delivery of a broadcast. Streaming clearly has a bright future thanks to Dolby’s AC-4 codec. If Dolby’s “perceptually lossless” claims about Dolby AC-4 convince casual viewers and audiophiles alike, it’ll be a hit. Even without audiophile adoption, Dolby AC-4 will be a major upgrade over Dolby Digital Plus. And one thing’s for sure — Blu-ray isn’t going anywhere. The physical media format has an abundance of benefits compared to streaming, even if Dolby AC-4 boosts audio quality on mainstream streaming services.