Breakfast Bytes Blogs

Dolby Atmos is immersive sound. As Dolby puts it:

Dolby transports you into the story with moving audio that flows all around you with breathtaking realism

Well, there is some level of colorful marketing there, but it really does have to be heard to be believed. The sound moves around in a way that is completely beyond regular stereo. It's like listening to Dolby 5.1 sound, except that there aren't 5 speakers, there are only two. Yet somehow Atmos can make you believe the sound is coming from behind you even though there are no rear speakers.

Atmos was originally developed for movie theaters, and then moved into home theater receivers. The next step was to move it into mainstream televisions, using dual Tensilica HiFi 3 DSPs. You can read about that in my post Dolby Atmos and Tensilica. It was then moved into video game consoles, such as Xbox, and PCs, using a single Tensilica HiFi 3 DSP. I covered that in my CES18 Preview. Now the form factor has shrunk again, and it is available in the Huawei P20 and P20 Pro phones. It still gives you the same immersive sound, despite only using two comparatively low-end speakers (or headphones). In a phone, of course, the speakers are even smaller than in a PC. It also works with a wide range of Bluetooth headphones that support HWA (Hi-Res Wireless Audio).

The "trick" of Atmos uses Head Related Transfer Functions (HRTF), which is a fancy way of matching the way the sound is delivered to match the way our ears and brain function. Or, as Dolby themselves put it:

Head Related Transfer Functions, HRTFs, are measurements of how sounds are modified by the head, torso, and external ears as they make their way to our individual eardrums. Sounds from different directions are modified in distinctive ways, giving our hearing system some clues about where the sound is coming from. However, most of the directional information comes from differences in the sounds arriving at both ears, the inter-aural amplitude and time differences as functions of frequency.

This is a very different approach than multi-speaker systems. Originally we had mono, with just a single speaker. Stereo used a left and right speaker, and by varying the volume of a sound to the two speakers, could spread sounds across the soundstage. Dolby 5.1 has 5 speakers (plus the 0.1, which is a subwoofer). Three are at the front and two are at the rear, so again by varying the volume it is possible to spread sounds out, largely across the sound-stage in front, with a small amount of sound from the rear to add to the realism. Atmos works by varying the sound in the way that it would appear to our ears, not just varying the volume, but varying the timing, and shaping the frequency envelope in different ways, depending on the direction of the sound.

Obviously, this requires a lot of digital signal processing under a very tight power envelope. Which brings us to the....

Tensilica HiFi DSP

The Huawei P20 implementation is on a single Tensilica HiFi 3 DSP. By using an offload processor instead of the main application processor, the power is reduced significantly. The HiFi3 is very energy efficient. It is the most popular licensable audio/voice DSP. The Tensilica HiFi has more than 90 licensees, more than 120 software ecosystem partners, and over 1B cores ship each year.

The HiFi DSPs are designed for AVS (audio/voice/speech) processing. The HiFi DSP instruction set architecture (ISA) is highly optimized for AVS applications. By optimizing more than 200 software packages, the HiFi ISA provides the lowest energy, highest performance DSPs for audio and voice. This performance scales across the entire DSP product family, from the ultra-low-energy 24x24 dual MAC HiFi Mini, to the energy-efficient 24x24 quad MAC HiFi 3, to the high-performance 32x32 quad-MAC HiFi 4.

Offloading Audio Processing

Watch how the use of a Tensilica HiFi DSP can reduce the power by 14X and so can double the playback time for audio (this is not specifically about Dolby Atmos, although the same basic principles apply).