Several DAWS offer the ability to mix in Atmos using expanded width tracks of 7.1.4, 9.1.4 or 9.1.6 that go beyond the maximum width of a bed. These work by either combining a 7.1 bed with static objects panned to speaker locations or by grouping up to 16 static objects that are panned to speaker locations.

 

These expanded tracks, referred to as “object bed tracks” have legitimate use cases. There are two primary applications for object bed tracks that go beyond the maximum bed width of 7.1.2. Object bed tracks are useful if you have source material from a live capture that was captured with multiple microphone arrays or decoded ambisonics capture. Object bed tracks are also very useful when applying dynamics processing to a specific bus or stem; drums for example.

 

But naturally, some mixers are tempted to lean on the convenience of mixing in Dolby Atmos only using object bed tracks and question the need for dynamic object tracks. This article explains why mixing in Dolby Atmos with a combination of beds and dynamics objects yields better results.

 

While it may be convenient to pan within the layout of object bed tracks only, there are compromises that result when dynamic objects are not used. These apply to both the binaural render and to rendering to speakers.

 

To understand the advantages of using dynamic object tracks in binaural rendering it is important to quickly revisit how binaural rendering works. Binaural rendering applies HRTF (headphone related transfer functions) filters to audio elements to create localization of the audio over headphones.

 

In Dolby Atmos binaural rendering the position/size metadata, as well as the binaural render mode distance metadata of dynamic objects, is applied to the HRTF filter of each unique object. When dynamic objects are used, the angle/distance to the listener makes the perceived directionality and localization of audio sound very natural. Dynamic objects can be moved “off the wall” which affect the direct to reverberant sound mix which further enhances the realism of a Dolby Atmos binaural render. With object bed tracks, the audio is summed to the static locations and the number of HRTF filters used is fixed to 12 (7.1.2), 14 (9.1.4) or 16 (9.1.6) which reduces the resolution, accuracy and definition of the binaural render. Moreover, panning sources to object bed tracks causes significant loudness build-up because each static location contributes to both L and R binaural outputs.

 

All beds in an Atmos master file share binaural render mode settings (near, mid, far, off) per bed channel. So, if multiple object bed tracks are used, the precision of unique binaural render mode distance setting will be lost for all channels that are not dynamic objects. When tracks are panned to speaker locations (and phantom panned between speaker locations), the location and timber can change and stability issues can occur when headtracking is enabled.

 

The use of dynamic objects is likewise superior when rendering to speakers, while relying on object bed tracks only can result in less than ideal outcomes:

 

• Panning multiple tracks to static locations can result in audio build up due to summing which can adversely affect downmixes to 5.1 and stereo.

 

• The trims controls work much better with dynamic objects in both automatic and manual modes. Trims apply attenuation or position changes based on the object position and the target speaker layout. Without dynamic objects, the efficacy of trims is reduced.

 

• Lastly, when object bed tracks are used, the panning precision is reduced when rendering to speaker layouts different than the object bed track.