This article covers the following frequently asked questions on Dolby AC-4:

• What is Dolby AC-4
• Is Dolby AC-4 Backwards compatible?
• Is Dolby AC-4 more complex to encode than Dolby Digital or Dolby Digital Plus?
• Is Dolby AC-4 more efficient than Dolby Digital or Dolby Digital Plus?
• Is Dolby AC-4 the same as Dolby Atmos?
• What are the main advantages of Dolby AC-4 from a broadcast systems perspective?
• What standards currently support Dolby AC-4?
• Which third-party professional equipment vendors support Dolby AC-4?
• Is Dolby AC-4 backwards compatible to AC-3 and what is required to implement it on legacy networks?
• Where can I get more information about muxing of Dolby AC-4 in MPEG-2 transport streams?

 

FAQs

What is Dolby AC-4?

Dolby AC-4 is a Next Generation Audio codec that uses cutting edge compression delivering equivalent experiences at half the bitrate of E-AC-3. It is able to carry channel based configurations and Dolby Atmos. AC-4 matches the delivery method with the most optimal configuration, enabling encodes tailored for broadcast or streaming and catering for headphone or speaker playback. It is also capable of delivering enhanced, user configurable, and accessible experiences. The AC-4 coding system utilizes new aspects of object audio beyond what is already available with Dolby Atmos in other Dolby codecs for features like dialogue enhancement or commentator substitution. For more details on AC-4 bitstream types (IMS, L3, and L4), benefits of AC-4 L4, and how to encode/play back AC-4 L4, see this article.

Is Dolby AC-4 backwards compatible?

Dolby AC-4 is implemented in TVs and Set Top Boxes in such a way that the output is always compatible and optimized for your downstream device, be that connecting into a PCM 2.0 output, a Dolby Digital AC-3 system or something Atmos capable. Although the AC-4 decoder is now available in the vast majority of new UHD TVs there is still a necessity in many cases to deliver alternative codec options for non AC-4 end points. This may be E-AC-3 in some broadcast use cases and commonly an AAC flavour is available in most streaming platforms to ensure maximum compatibility. Optimizing the availability of premium audio experiences is something Dolby are happy to help with if needed.

Is Dolby AC-4 more complex to encode than Dolby Digital or Dolby Digital Plus?

Dolby AC-4 can support a new range of complex use-cases, the encoding process can be more computationally expensive. However, when comparing similar use cases, for example 5.1 channel-based encoding, the increase in computational load is modest. Depending on the processor employed, the increase is somewhere in the region of 10-50% compared with encoding Dolby Digital Plus. This means that existing equipment supporting Dolby Digital Plus professional encoding can often be upgraded, in software, to support Dolby AC-4 encoding. The user experience for setting up an encoder for a legacy use case can be very similar to our older codecs, some lesser used controls may even be hidden in newer UI layouts to make things easier to setup.

Is Dolby AC-4 more efficient than Dolby Digital or Dolby Digital Plus?

AC-4 can deliver equivalent channel based audio quality at roughly half the bitrate of Dolby Digital Plus. The increased efficiency though can be considered even greater by utilizing the new Next Generation Audio encoding options. By delivering a common Music and Effects (M&E) bed and separate mono objects you can deliver full mixes in alternative languages or for accessibility listeners at only the cost of a single mono audio track at 48kbps. This means that everyone can benefit from the full top tier production and not having to substitute in stereo experiences to cover the needs of your end users.

Is Dolby AC-4 the same as Dolby Atmos?

Dolby Atmos is the brand for Dolby's immersive audio experiences. These can be delivered with various different audio codecs. The AC-4 codec is Dolby's latest platform for bringing Atmos to listeners. It's a cutting edge system, extremely efficient in bitrate and also goes beyond what was possible with previous generations of codec. As a Next Generation Audio Codec we are able to do more with object audio than was previously possible bringing new personalization and accessibility opportunities to service providers.

What are the main advantages of Dolby AC-4 from a broadcast systems perspective?

At a high level the main features are:

• Audio/Video Alignment
• Advanced Loudness Management
• Improved Dynamic Range Control
• Accessibility
• Efficiency
• Atmos compatibility

What standards currently support Dolby AC-4?

Dolby AC-4 is currently specified or supported in the following standards:

Codec specification standard:

• Digital Audio Compression (AC-4) Standard; Part 1: Channel based coding (ETSI TS 103 190-1 v1.3.1 - 2018-02)
• Digital Audio Compression (AC-4) Standard; Part 2: Immersive and personalized audio (ETSI TS 103 190-2 v1.2.1 - 2018-02)

Delivery standards:

• DVB
  • ETSI TS 101 154
  • ETSI EN 300 468

• ATSC 3.0
  • A/300
  • A/342-1
  • A/342-2
  • A/331

• MPEG
  • ISOBMFF
  • DASH

Guidance and practical information can also be found here at ott.dolby.com. 

Which third-party professional equipment vendors support Dolby AC-4?

The following vendors provide solutions that support Dolby AC-4:

• Harmonic
• DekTec
• Linear Acoustic
• DS Broadcast
• Interra

Is Dolby AC-4 backwards compatible to AC-3 and what is required to implement it on legacy networks?

The Dolby AC-4 format is not backwards compatible with Dolby Digital (AC-3) or Dolby Digital Plus (E-AC-3). Dolby AC-4 supports legacy channel-based use cases and can be deployed in existing channel-based broadcast audio systems.

Where can I get more information about muxing of Dolby AC-4 in MPEG-2 transport  streams?

Please see our article An overview of Dolby AC-4 in Transport Streams.