Surround Codecs:
The Dolby TrueHD and DTS-HD Master Audio codecs have been explored at length in the audio press and are now standard features even in entry-level products. The hot topic now is the trio of new object oriented codecs like Dolby Atmos, Auro-3D and DTS-X. Pixar’s Brave was the first movie to benefit from this technology when it was released in June of 2012 with a Dolby Atmos soundtrack. Today, several popular Blu-rays like American Sniper, The Hunger Games: Mockingjay Part 1 and Transformers: Age of Extinction are available with Dolby Atmos. In time, there will be many more. In a nutshell, these new technologies provide a highly immersive 3D-sound experience by adding a height dimension to the traditional 7.2 channel configuration. This is done both by adding speakers (or virtual speakers) above the listener and mixing soundtracks in a new way.
Object-oriented codecs allow studios to mix their soundtracks by moving virtual objects in a three dimensional space. An airplane for example, can be moved through space by simply showing its flight path to the computer. The lower level logic in the system then takes care of figuring out how to distribute sound across all the speakers. The object oriented concept is a very powerful one and promises considerable improvements in a surround system’s ability to precisely locate sounds in space, even above the listener.
A new way to describe channel configurations arose from the creation of Dolby Atmos. The term 7.2.4, for example, represents the 7.2 configuration we are familiar with but adds “.4” which refers to 4 height speakers. This is currently the most elaborate configuration available from mainstream products. In older nomenclature, this would have been referred to as an 11.2 channel system. Many still use this terminology.
Dolby Atmos is already widely available in many products across multiple manufacturers. In contrast, Auro-3D is usually available only via a paid firmware update and in higher end models. DTS:X was announced very recently and isn’t available quite yet but should be fairly soon. Like Auro-3D, DTS:X will likely be available for premium products via a paid firmware update. Eventually, Auro-3D and DTS:X may become standard features but until then, these restrictions apply. If finances permit, consider a receiver that can accommodate the DTS:X upgrade. There’s a good chance that DTS:X will be widely deployed in the future.
To achieve the best results with these new codecs, speakers in the ceiling must be installed. This is clearly not for everyone. An easier alternative is to equip a home theatre with Atmos enabled speakers. These function as normal left / right / surround speakers but add a secondary driver on the top of the speaker cabinet. This secondary speaker fires at an upward angle, allowing sound to bounce off the ceiling and back down to the listener. This should give the illusion of sound coming from above, thus simulating a ceiling speaker. Some manufactures, like Definitive Technologies, offer an Atmos add-on speaker that neatly clips on top of existing speakers. Another potential solution is to mount the height speakers as high as possible on the walls.
Be careful though, there is at least one 11.2 channel Atmos receiver on the market that only has 9 internal amplifiers. If the full 11.2 channel capabilities are to be utilized, a 2 channel external amplifier is required. In this situation, one should choose a good quality stereo amp for the mains and the receiver’s internal amps should be used for the center and surround channels. Once the setup is complete, throw in the special edition Gravity Blu-ray and get ready for a wild ride!
Room correction:
There are many different room correction systems on the market these days. Audyssey, ARC, Dirac and MCACC are common examples. Some of these are proprietary and others can be found across multiple manufacturers. Essentially, their purpose is to automate the process of calibrating the surround speaker system.
If a sound is simultaneously fed into all the amplifiers of a surround sound system, the wavefront emitted from every speaker should arrive at the listener’s position at the same time. However, for various reasons, this is rarely the case. Speakers are not usually all at the same distance from the listening position and certain components like active subwoofers can introduce a delay in the signal. One of the things that room correction systems do is adjust the channel delays such that all the wavefronts from each speaker arrive at the same time. This distance issue coupled with the fact that various speakers may have different efficiencies can also lead to volume discrepancies. The room calibration system will adjust the volume of each speaker so that they are all equal. The system will also verify that all speakers are in phase. Speakers are “out of phase” when the polarities are reversed.
In an anechoic chamber, a theoretically perfect speaker would have a ruler flat frequency response. In a typical room, the same speaker’s response would be all over the map. That’s because rooms have a huge influence on frequency response. Some frequencies will be amplified while others are attenuated. Furthermore, the response differs from one location to the next within the same room. The correction system will plot a frequency response graph for each individual speaker. It will then equalize each channel separately in an attempt to flatten out the frequency response. This way no frequencies are favoured over others.
These are the highlights of what room correction systems do, but don’t be fooled. In the real world, the interactions of speakers and rooms are horrifically complex. Sophisticated correction systems go much further than the basics to optimize the listening experience. One major complication is that the frequency response of a given speaker in a given location changes with different volumes. Some correction systems also attempt to deal with unwanted sound reflections and standing waves.
Not all correction systems are created equally. Within the Audyssey line, for example, the MultEQ XT32 system uses a filter resolution of 512 points for both the satellites and the subwoofers. In contrast, the MultEQ XT uses only 16 for the satellites and 128 for the subwoofers. Obviously the XT32 version is superior. When all is said and done, even the most sophisticated room correction systems aren’t perfect but one usually gets a much better result with, rather than without.
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