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EETE FEB 2014

AUDI O & VIDEO PROCESING Technologies for high quality audio soundbars By Brewster LaMacchia As TVs have gotten flatter their cabinet volumes have reduced; the laws of physics dictate that their sound quality has gotten worse. At the same time audio content has gone from stereo to high resolution multichannel formats. In parallel with the growing complexity of the audio sources and content are the consumer expectations that nobody will read a manual, fiddle with multiple setup menus, or run wires around existing rooms in a home or apartment to achieve surround sound for a theaterlike experience. Active soundbars, which combine the features of a traditional A/V Receiver for multichannel sound with a dedicated set of speakers, have become popular as a way to provide a surround sound experience with TV and movie viewing. They use a digital signal processor (DSP) device to decode multichannel audio, optimize the sound for the particular transducers/ drivers, and apply psycho-acoustic processing to create a wider soundstage than the bar itself – see figure 1. A critical feature in a high quality soundbar is bass management. Due to the small cabinet volume and limited driver size, reproduction of frequencies below 150 Hz starts to become a difficult design problem. Listening to music or movies that are missing frequencies below 150 Hz would give the audio the sound quality of an old table radio and not the high impact realism of a “you are there” experience. To avoid the need for all speakers to reproduce down to the typical lower audio limit of 20 Hz, surround systems (and some stereo systems) redirect bass energy from each channel to a dedicated subwoofer. This works because human hearing is non-directional at these lower frequencies. Correctly creating the crossover filter to preserve both time and frequency domain performance when attached to real world drivers is a difficult problem with many advocates for differing methods. While soundbars can generally be wide (even a small LCD TV is over one meter in width) they are often limited in height and depth to match the visual aesthetics of a flat screen TV. The resultant limited cabinet volume of a soundbar typically results in low acoustic sensitivities, limiting the acoustic output for a given power input. In the same way that class D amplifiers have made small volume subwoofers possible (by throwing lots of amplifier power at it with limited heat generation compared to traditional class AB amplifiers) soundbars too can benefit. However this approach to solving the output level problem does require physically small drivers that can handle high (25W – 50W) power levels. Here the DSP can be used to perform intelligent dynamic range compression to achieve the desired loudness with minimal distortion and protect the driver from long term over-heating. For example, in a recently completed prototype made here at MDS an existing retail passive soundbar was used as stand-in for the final cabinet and drivers so that software developers would be able to listen to the results with actual audio content and not just view test tones on an oscilloscope. The passive crossovers were removed from the purchased soundbar and Fig. 1: The components in a typical active soundbar. electronic ones created in the amplifier ICs (TAS5727 I2S input class D amplifier ICs from Texas Instruments in this case). The SoC’s DSP core performs content decoding, bass management and other post processing. Initial listening to the bar produced a sound quality that was lacking for vocals and bass that sounded out of balance with the rest of the system. Modifying bass management, and tweaking the electronic crossover settings between the mid-woofer and tweeter, as well as a small correction in the 1.2 kHz and 3kHz area produced a sound quality that was preferred in A/B testing - see figure 2. The use of a DSP to enhance the sound quality can further be extended to accommodate listener preferences without the need to switch in different physical components like in a traditional passive speaker crossover. The DSP also offers audio companies the ability to add proprietary processing to create a unique product in the audio market. For example the trend towards more sophisticated room correction, which uses prodigious amounts of DSP computational power, will no doubt find its way into soundbars. In parallel with the technology advances in the soundbar electronics, consumers now expect to source audio content from their portable electronic devices or stream from online cloud services. Combining an ARM based host processor with a high performance DSP allows creation of a soundbar that offers the functions consumers look for. For example, the TMSBrewster LaMacchia is responsible for development of DSPbased audio and video systems, including consumer audio applications at Momentum Data Systems - www.mds.com 34 Electronic Engineering Times Europe February 2014 www.electronics-eetimes.com


EETE FEB 2014
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