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Dolby demonstrate their new AC-4 audio codec

Audio compression for the 21st century

by Steve Withers Jan 11, 2016


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    Dolby demonstrate their new AC-4 audio codec
    It's hard to believe that Dolby's lossy AC-3 audio codec is nearly thirty years old but in that time much has changed; compression algorithms have become far more advanced and delivery has moved away from physical media to internet streaming and download.
    As a result there is a need for a new audio codec that addresses the needs of the modern world. AC-4 has been designed from the ground up as Dolby's next generation audio codec that offers far greater efficiency. Currently a Dolby Digital 5.1-channel soundtrack can be encoded with AC-3 using 384Kbps but with AC-4 that same track can be encoded at only 96Kbps with no perceivable loss in quality. In fact the compression is so good that a stereo track can be encoded at just 32Kbps, making AC-4 incredibly efficient and significantly reducing the amount of space needed when streaming a soundtrack. We heard a demonstration of a Dolby Digital 5.1-channel soundtrack encoded using AC-4 and there certainly appeared to be no noticeable drop in quality.

    However AC-4 isn't just about more efficient compression, it is also designed to improve the viewer's experience through dialogue enhancement. Although many TVs include a dialogue enhancement feature the processing is done in the TV where there is limited processing power. With AC-4, the processing can be done on the encoder side with more powerful processing and algorithms available and thus superior results. The feature will be very useful in situations where dialogue is difficult to hear, such as sports commentary where the sounds of the crowd might drown out the commentator. We saw a demonstration using the commentary from a Liverpool v Man City game and the dialogue enhancement processing was genuinely effective.

    Dolby also plan to use AC-4 to provide a premium experience to viewers, even those watching in different languages or with disabilities. So the technology's highly efficient encoding means that you can have an English language track, a Spanish language track, a descriptive audio track in English and a descriptive audio track in Spanish all for the same content. There is a 5.1-channel bed with mono channels for each of the different dialogue tracks (each encoded at 32Kbps), so a Spanish-speaking viewer with impaired sight could choose the Spanish language track and the descriptive audio track in Spanish. We saw a demonstration of this feature, with the user selecting different soundtracks depending on their needs.

    This same technology can also be used to personalise your viewing experience, so again there will be a 5.1-channel bed for a sporting event, with separate mono channels for the commentary tracks. The viewer can then choose which commentary track they want to listen to, English or Spanish for example, they can also enhance the dialogue or, and this is particularly exciting, just turn the commentary off completely and simply watch the game. We saw a demonstration of a basketball match and we could simply switch between commentaries, enhance the dialogue if we liked or turn the commentary off completely.

    Finally AC-4 uses advanced processing and psychoacoustics to create a more immersive audio experience from the stereo speakers on a TV. The feature convinces the brain that sounds are come from other directions, thus producing a pseudo-Atmos effect. We were sceptical at first but watching a Dolby Atmos trailer that we were very familiar with was a revelation. The sound was definitely more open and immersive and whilst not comparable to a real Dolby Atmos mix, we could't believe that Dolby were getting such an enveloping sound from a pair of TV speakers. To prove how effective the feature was, the Dolby representative turned it off so the sound was simple stereo and then turned it on again so that it filled the room. There's no denying that Dolby's AC-4 is an impressive technology that brings their lossy audio codec into the 21st century.

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