Los Gatos, CA (May 2, 2019)—Netflix has announced that it is now distributing “studio quality” and adaptive audio streaming, bringing sound up to the company’s established video quality benchmark, largely in response to the demands of content creators.
“We’re really proud of the improvements we’ve brought to the video experience, but the focus on those makes it easy to overlook the importance of sound, and sound is every bit as important to entertainment as video,” write Guillaume du Pontavice, Phill Williams and Kylee Peña—on behalf of Netflix’s streaming algorithms, audio algorithms and creative technologies teams—on a May 1 post on Medium.
The drive to improve audio quality was inspired by critical feedback from the Duffer brothers, the producers behind hit Netflix series Stranger Things. The brothers reported a reduced sense of sound localization in the 5.1-channel stream, as well as audible degradation of high frequency content. In working to correct those specific issues, Netflix began a larger conversation regarding audio quality.
“Series mixes were getting bolder and more cinematic with tight levels between dialog, music and effects elements. Creative choices increasingly tested the limits of our encoding quality. We needed to support these choices better,” the trio write on the Netflix Tech Blog.
Netflix has been streaming 5.1 audio since 2010 and began streaming Dolby Atmos in 2016. But while the company has supported video technology initiatives, including HDR and Netflix Calibrated Mode, to maintain creative intent from production to viewer, the same could not be said of audio.
Netflix refers to the newly unveiled improvements simply as “studio quality” or “high-quality audio.” The audio stream is not lossless, but based on listening tests conducted by Netflix, together with data from Dolby Labs and other scientific studies, the company determined that for Dolby Digital Plus at and above 640 kbps, audio coding quality is perceptually transparent.
“In addition to deciding 640 kbps — a 10:1 compression ratio when compared to a 24-bit 5.1 channel studio master — was the perceptually transparent threshold for audio, we set up a bitrate ladder for 5.1-channel audio ranging from 192 up to 640 kbps. This ranges from ‘good’ audio to ‘transparent.’ At the same time,” state du Pontavice, Williams and Peña, “we revisited our Dolby Atmos bitrates and increased the highest offering to 768 kbps. We expect these bitrates to evolve over time as we get more efficient with our encoding techniques.”
Until now, Netflix has streamed audio at a constant bitrate determined by network conditions at the start of playback. “However, we have spent years optimizing our adaptive streaming engine for video, so we know adaptive streaming has obvious benefits.”
In their Medium announcement, the Netflix team take a deep dive into the challenges of adaptive streaming, which center on achieving a balance between audio quality and video quality while avoiding rebuffering. With its large subscription base, Netflix also had to consider the wide variety of TV devices in the field and their respective CPU, network and memory profile differences.
“We had to assess this by testing adaptive audio switching on all Netflix supported devices. We also added adaptive audio testing in our certification process so that every new certified device can benefit from it,” the Netflix team writes.
Now, they state, “If your network conditions are good, you’ll be served up the best possible audio, and it will now likely sound like it did on the mixing stage.” So if you’re being chased by the Demogorgon, you’ll know exactly where it is.
Netflix Tech Blog • www.medium.com/netflix-techblog