Netflix has added support for the advanced xHE-AAC audio codec for content streamed via its Android apps to allow consistent loudness and fidelity of dialog and music between TVs and mobile devices.

The information affirming that the Fraunhofer-licensed codec is now a part of the Netflix streaming experience was first revealed Thursday in a Netflix Technology blog.

The codec is an extension of the now commonly deployed HE-AAC that has been used in TV, radio and streaming world wide offering bit rates of 48 to 64 Kbit/s STE stereo, 160 Kbit/s for 5.1 surround. xHE-AAC profile decoders are capable of decoding all AAC versions including AAC-LC, HE-AAC and HE-AACv2.

One of the key features of xHE-AAC is mandated support of MPEG-D loudness and dynamic range control metadata, which allows service providers to embed content- and endpoint-specific metadata in the audio bit stream.

The metadata can be used during playback to present a consistent loudness level and optimal dynamic range across supporting devices.

In Android TV v.9 and later sets (like some Sony, Hisense, TCL, Philips, Skyworth, Konka, and other models) xHE-AAC and MPEG-D DRC are now natively supported. It’s also supported in Amazon’s Fire OS 7 (based on Android 9 Pie) operating system, and on Apple’s iOS 13, macOS 10.15 Catalina, tvOS 13, iPadOS, watchOS 6 and later.

The xHE-AAC codec will enable hearing a consistent level of audio quality between different types of devices, so that a sound track that is robust in the multi-channel home theater remains clean and clear when played through a mobile device. MPEG D uses a single stream to address both applications, while xHE-AAC further improves coding efficiency at low bit rates.

Thus, xHE-AAC provides reliability for streaming services even under the most challenging network conditions. xHE-AAC is said to be the only perceptual audio codec that covers the entire bitrate spectrum.

XHE-AAC streaming apps and streaming radio players may switch to very low bit rate streams and offer a continuous playback level even while the network is congested. Once more bandwidth becomes available on the network again, the xHE-AAC client application can request a higher bit rate version and seamlessly switch over the full range.

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When first developed, AAC was being encoded at discrete bit rates. So, for example, music would be purchased at 128 Kbps or 256 Kbps for the audiophile versions, and that’s what was presented regardless of bandwidth. But xHE-AAC offers bit rate switching, improved coding efficiency at low bit rates, and the ability to scale up to perceptually-lossless quality at high bit rates for smoother use with varying levels of available bandwidth.

Thus, issues like stuttering or momentary hiccups will be eliminated playing high bit rate content and at low bitrates audio quality is maintained so music sounds good even when listening in quiet environments. XHE-AAC can switch on the fly the bandwidth of a song or show between 12 Kbps and 500 Kbps to always match the available bandwidth speed.

“Netflix specifically aims to play all dialogue at the same level,” the Netflix Blog said. “This is consistent with the North American television broadcasting standard ATSC A/85 and AES71 recommendations for online video distribution.”

The article continues that “When a show has a high dynamic range, a member may ‘ride the volume’ to turn down the loud segments and turn up the soft segments… Volume change interactions are noticeably down for xHE-AAC. This indicates that DRC is doing a good job of managing the volume changes within shows. These differences are far more pronounced for titles with a high dynamic range.

“On mobile devices, most Netflix members use built-in speakers. When members switch to headphones, it can be a sign that the built-in output level is not satisfactory, and they hope for a better experience. For example, perhaps the dialogue level is not audible. In our test, we found that members switched away from built-in speakers 7% less often when listening to xHE-AAC. When the content was high dynamic range, they switched 16% less.”

The xHE-AAC codec makes other improvements including speech quality. As bit rates go down with a traditional perceptual audio decoder like the AAC codec family, problems with raspiness of speech can result, because speech doesn’t sound as natural at a bit rate that is good for music. This occurs in radio broadcasts at times when music sounds fine but when an announcer’s voice comes on it sounds distorted, due to the low bit rate. This is solved in xHE-AAC using adaptive coding of two audio codecs–traditional AAC for music and a speech coder for voice.

Importantly, although xHE-AAC provides universal loudness control at playback, the system will support loudness variations to preserve artistic effect such that listeners won’t be cranking up the volume to listen to one program and then find themselves being blasted out when the content changes.

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By Greg Tarr

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