Dolby AC4 is a multichannel lossy audio format and the follow-up to Dolby AC3, better known under the trade name Dolby Digital. Comparatively, Dolby AC4 offers:
- Improved compression technology
- Superior restitution quality
- Up to 12 channels, as opposed to 6
- Integrated Dolby Atmos metadata
- Interactive features for an improved listening experience
Dolby AC4: compression technologies
Dolby AC4 uses two different signal encoding technologies. The Advanced Joint Channel Coding is applied to the lowest bit rates and is limited to 5.1 streams. The typical bit rate for a Dolby AC4 5.1 channel is 192 Kbps, compared to the 384 to 640 Kbps needed for Dolby AC3 (Dolby Digital). The Advanced Coupling technology can incorporate extra metadata (Dolby Atmos for example) and can support up to 12 channels. The typical bit rate of a Dolby AC4 stream for Dolby Atmos 7.1.4 restitution is 288 Kbps.
Dolby AC4… compared to Dolby Digital Plus
So far, there aren’t many differences between Dolby AC4 and Dolby Digital Plus (or Dolby EAC3), another multichannel audio format developed by Dolby. DD+ already offers a much superior compression rate compared to Dolby Digital and can include Dolby Atmos metadata as well. In fact, it’s the audio format used by Netflix to stream movies and TV shows. In practice however, Dolby AC4 provides two extra features related to sound restitution and dialogue clarity…
Dolby AC4: Real Time Loudness Leveler
A Dolby AC4 audio stream incorporates metadata in order to adapt the audio signal’s dynamic range to the equipment used for the restitution. When it comes to televisions or smartphones, the variation in volume is largely minimized, allowing improved listening comfort. With an AV receiver however, the signal’s entire range is restituted. Nevertheless, it will be possible to adjust the restitution according to your own preferences. Keep in mind that Dolby AC3 (Dolby Digital) featured dynamic compression algorithms, although these were optional during the encoding process of the production and weren’t necessarily taken into account by the display. With the AC4 format, listening comfort will be key.
Dolby AC4: dialogue enhancement
Another of the improvements concerns the central channel – the channel used for dialogue. If the user decides that the legibility is inadequate during stereo downmixing, they’ll be able to adjust the volume of the channel’s restitution. Furthermore, AV receivers and TVs compatible with Dolby AC4 will be able to enhance dialogue restitution from stereo or 2.0 surround programs.
Dolby AC4: tailored by the user
Dolby AC4 should introduce customizable sound. In other words, the user will be able to choose whether they want to listen to one or multiple tracks from a multichannel stream. Take a football match for example: AC4 will allow you to cut out the commentary so that you can only hear the noises on the field and in the stadium, whether you have a setup with one, two, or more speakers.
Dolby AC4: when will we be able to reap the benefits?
The Dolby AC4 format is intended for DTV and satellite television broadcasting along with streaming. Its release is planned for the USA first and includes the application of the TV ATSC 3.0 broadcast standard. It’s probable that it will be the only format available. The AC4 format should then be available in Europe in the next few years, with the implementation of the DVBT-2 standard. Moreover, the majority of recent UHD TVs are AC4 compatible. It is likely that Netflix will be pioneering this new format as it is the only streaming service currently using Dolby Digital Plus with Dolby Atmos metadata. Netflix will probably be the frontrunner once again when it comes to making full use of the Dolby AC4 format’s potential, even more so given that it will allow the streaming service to save server bandwidth.
Dolby AC4 wasn’t developed to replace Dolby TrueHD, which should remain the preferred format for Blu-ray and Blu-ray UHD discs. Dolby TrueHD uses lossless compression and is to this day, the best multichannel audio format along with DTS HD Master Audio. Furthermore, Dolby TrueHD can’t be used for streaming or DTV broadcasting as its dataflow is equivalent to streaming a 4K video (Amazon Prime Vidéo, Netflix…).