Spatial audio array IC generates “virtual headphones”

EDN Europe, 14 Nov 2011

Texas Instruments, as a result of its recent acquisition of National Semiconductor, has launched a chip that it says simplifies your task if you are designing portable audio/video devices for which you require an “immersive audio” feature set. You might use it, TI says, for configuring spatially-enhanced audio for multi-speaker portable products, including laptops, tablets, sound bars and sound docking stations. The National LM48901 quad Class D spatial array handles four speakers per IC; you can cascade the chips to handle linear arrays of multiple speakers, up to 16. The concept is intended for any speaker set-up in which the speakers are arranged linearly in front of the listener (that is, several speakers as opposed to the two of stereo), and in this version, creates an optimised sound field for just one listener.

The chip accepts I2S serial digital audio feeds, or analogue stereo, and drives the speaker array to product a combination of effects. It uses beamforming of audio to place sounds for the left and right ears of the listener; additive and subtractive interference effects in the same zones to broaden the apparent sound stage: and a technique that National/TI calls HRTF (head related transfer function) set up binaural cues to persuade the listener’s hearing that sounds are coming from specific directions and to fix the apparent positions of sound sources in space.
The chip contains analogue (via an 18-bit ADC) and digital spatial processing blocks and class-D, 2-W output, audio amplifiers: you program it over an I2C interface and configure it with a software tool in which you enter details of the number and positioning of the available speakers, and the expected position of the listener. The tool then sets up all the necessary coefficients for the chip(s). There is an evaluation board, and Android drivers, as part of the design support package. The LM48901 comes in either 36-pin, 3.2-mm x 3.4-mm micro SMD, or a 32-pin 5-mm x 5-mm LLPpackage and costs $2.50 (1000). TI says that the chip is the first in a family dedicated to “an immersive audio experience for space-constrained applications”. You can view explanatory videos here and here.