Pro-Codec User Manual

Table Of Contents

4.2 xHE-AAC 4 SUMMARY OF CODEC FEATURES AND APPLICATIONS

4.2 xHE-AAC (Fraunhofer)

xHE-AAC (the combination of Extended HE-AAC and MPEG-D DRC), primarily developed

by Fraunhofer IIS and the latest addition to the MPEG AAC codec family, bridges the gap

between speech and audio coding. It provides consistently high-quality audio for all

signal types, such as speech, music or mixed content, at all bit rates – starting as low as

6 kbit/s for mono and 12 kbit/s for stereo services, up to 500 kbit/s and above.

xHE-AAC playback is supported by Android, iOS, macOS and Windows 11 and xHE-AAC

is currently used by Netﬂix and other large content providers. xHE-AAC provides several

beneﬁts over prior generations of AAC:

• It extends usable bitrates down to as low as 12 kbit/s stereo, allowing streaming

playback to continue if a listener enters a region of diﬃcult network conditions.

• It allows a very wide range of bitrates to be included in a DASH or HLS adaptation

set, allowing seamless switching between bitrates without any pre-conditioning of

the encoding or bitstreams.

• At the higher bitrates, audio quality is slightly better than AAC-LC. The transparent

range of xHE-AAC begins about at 200 kbit/s for demanding content.

• xHE-AAC oﬀers playback-side loudness normalization. All xHE-AAC content plays

at a consistent loudness. This is based on ITU BS.1770 loudness measurements

and is similar to the normalization now performed by all the large music and video

streaming services. These loudness normalization and dynamic range concepts

have been used for decades in TV and ﬁlm production.

• xHE-AAC includes dynamic range control. A user can select reduced dynamic

range when he is in a noisy environment – such as a subway station, airplane ﬂight,

or car with windows down - and still hear the original dynamic range when listening

at home or on isolating earbuds/IEMs

In basic terms, xHE-AAC is designed to supersede all previous versions of AAC,

removing their limitations.

Mastering of xHE-AAC should focus on the desired sound and dynamics, as there is no

longer any possibility of creating a louder track by compression, limiting, or “crushing”.

Of course, the mix may be compressed for artistic eﬀect or norms of a genre. There is no

need for concern about codec clipping or overshoots, as xHE-AAC includes processing

and protection limiters to prevent this.