Dolby Laboratories and TDF announce the launch of experimental 4K UHD Digital Terrestrial Television broadcasting in the metropolitan areas of Paris, Nantes and Toulouse. Viewers with a recent television set* will be able to enjoy a 4K UHD picture and Dolby AC-4 sound on DTT channels 81, 82 and 83. To access this service, users will need to run an automatic or manual channel scan on their TVs.
*televisions with DVB-T2 reception, Ultra HD resolution, HDR10 and Dolby AC4 compatibility, with or without Dolby Atmos
These real-world tests will help refine the UHD DTT specifications and confirm that the migration toward UHD is now well under way. They contribute to the modernization of DTT as defined by the Conseil Supérieur de l’Audiovisuel (CSA) in its roadmap, supported by TDF and the entire sector.
“This experimental phase lets us give viewers a preview of what tomorrow’s television will be, with picture and sound quality unmatched until now,” explains Anaïs Libolt, Head of Broadcast & Home Entertainment at Dolby France. “Sound is a key component of the entertainment experience. Thanks to this partnership, when audiovisual content is encoded in Dolby AC-4, the TV will reproduce it to the best of its abilities, and it will also be possible to activate improved dialogue intelligibility. This step foreshadows the audio quality viewers will enjoy on their TVs in the near future.”
Franck Langrand, Managing Director of TDF’s Audiovisual Business Unit, states: “Building on the experiments carried out by our teams, I welcome the collaboration with Dolby, which represents a major step toward bringing UHD to DTT. In terms of sound, we are reaching the desired quality level that will be offered to the general public by 2024. These innovation phases are essential to drive the modernization of Digital Terrestrial Television, unanimously supported by the ecosystem. The DTT platform proves every day that it is indispensable for the French public.”
The programs currently broadcast on these channels are:
- (Ch. 81) The Explorers trailer in 4K UHD HDR10 with a stereo audio track encoded in Dolby AC-4.
- (Ch. 82) a series of video sequences: of the city of Dinard in HDR10 in 4K UHD or HD formats with immersive Dolby Atmos audio encoded in Dolby AC-4 in two languages (French and English); of the city of Paris in HDR10 in 4K UHD or HD formats with stereo sound encoded in Dolby AC-4 stereo.
- (Ch. 83) The film Aphorism in 1080p HDR10 with immersive Dolby Atmos audio encoded in Dolby AC-4 in two languages (French and German).
Dolby AC4 is a multichannel audio format using lossy compression, the successor to Dolby AC3, better known under the commercial name Dolby Digital. Compared to Dolby AC3, Dolby AC4 offers:
- more efficient compression
- higher playback quality
- up to 12 channels instead of 6
- integration of Dolby Atmos metadata
- interactive features that enhance the listening experience
Dolby AC4: compression technologies used
Dolby AC4 uses two different encoding technologies for the original signal. First, Advanced Joint Channel Coding is applied at lower bitrates and only allows up to 5.1 streams. The typical bitrate of a Dolby AC4 5.1 stream is 192 kbps, compared with the 384 to 640 kbps required for Dolby AC3 (Dolby Digital). Next, Advanced Coupling technology makes it possible to carry additional metadata (such as Dolby Atmos) and include up to 12 channels. The typical bitrate of a Dolby AC4 stream with Dolby Atmos 7.1.4 is 288 kbps.
Dolby AC4… compared with Dolby Digital Plus
So far, there is nothing very different from another multichannel audio format developed by Dolby, Dolby Digital Plus (or Dolby EAC3). DD+ already offers much better compression than Dolby Digital and can carry Dolby Atmos metadata. In fact, it is the audio format Netflix uses to stream all its movies and TV series. In practice, Dolby AC4 brings two additional features related to playback loudness and dialogue clarity.
Dolby AC4: Real Time Loudness Leveler
A Dolby AC4 audio stream includes metadata to adapt the signal’s dynamic range to the device playing it. With a television or smartphone, volume swings are greatly reduced for better listening comfort. With a home theater amplifier, the full dynamics of the signal are preserved. Users can still set their preferences. Recall that Dolby AC3 (Dolby Digital) included dynamic range compression profiles, but these were optional at the encoding stage and might or might not be honored by the playback device. With AC4, listening comfort becomes a core requirement.
Dolby AC4: dialogue enhancement
Another improvement concerns the reproduction of the center channel, where dialogue lives. Its level can be adjusted manually by the user if intelligibility is insufficient during a stereo downmix. In addition, home theater amplifiers and televisions compatible with Dolby AC4 will be able to enhance dialogue from stereo programs or 2.0 surround.
Dolby AC4: let the user create their own mix
Dolby AC4 should introduce à la carte sound. In other words, users will be able to listen to only one or several tracks of a multichannel stream. Take a football match, for example. If you prefer not to hear the commentary and only want the sound from the pitch and the stadium, AC4 will allow it whether you have one speaker, two speakers, or more.
Dolby AC4 is not intended to replace Dolby TrueHD, which should remain the format used on Blu-ray and Ultra HD Blu-ray discs. Dolby TrueHD uses lossless compression and, along with DTS-HD Master Audio, is currently the best multichannel audio format. However, Dolby TrueHD cannot be used for streaming or DTT because its bitrate is equivalent to that of a 4K video stream (Amazon Prime Video, Netflix, etc.).
