Generating spatial audio from portable products - Part 2: Acoustic beamforming using the LM48901

Texas Instruments has addressed this challenge with the LM48901, the first in a series of spatial array audio amplifiers for implementing loudspeaker arrays to produce an immersive audio experience for space-constrained applications.

The LM48901 is a four-channel Class D audio IC implementing distributed, electronic beamforming algorithms, plus sound placement using HRTF data to create beamformed loudspeaker arrays. Audio input can be either stereo digital audio (I2S format) or analog stereo audio. The LM48901 array processing system is non-complementary, meaning it does not require encoded stereo source material to produce the loudspeaker array spatial effects.

Electronic beamforming functionality is distributed across the various array filters placed within or connected to the various audio amplifiers. The filtered audio signals are coupled to one or more loudspeaker amplifiers to implement beamforming and produce desired spatial effects. The LM48901's basic block diagram is shown in Figure 5.

When using the HRTFs in the pre-filter stage, the horizontal span of the soundstage can easily be controlled by loading HRTFs that correspond to different angles. The overall effect generated by the two stage processing creates a pair of virtual speakers in front of the listener with adjustable separation between the two speakers.

As a result, a less powerful centralized DSP (and in some cases, no centralized DSP) is needed in an audio system driving loudspeaker arrays. The distributed processing also allows for more flexibility in the geometric arrangement of speaker arrays, as well as the number of speakers in the array.

The LM48901 provides four filterless Class D amplifiers capable of up to 1.7W per channel into 8-Ω speakers, or 2.8W per channel into 4-Ω speakers for a combined four-channel output power of 6.8W and 11.2W, respectively, using a 5V supply voltage. These output power levels are sufficient for many consumer audio applications. Lower output power levels are achieved with lower supply voltages down to a minimum of 2.7V. Overall efficiency exceeds 80% with 4-Ω loads, and 85% with 8-Ω loads.

Two of the channels can be connected in parallel to increase output power for applications requiring a subwoofer. In this way, a single LM48901 can implement a spatialized 2.1-channel audio system with three speakers.

In addition, several LM48901 devices can be daisy-chained together to support more loudspeakers, with a maximum of four loudspeakers supported per device. This feature greatly simplifies the design and implementation of speaker arrays of any size up to 64 loudspeakers.

The LM48901 filter coefficients can be updated dynamically in order to provide different beamforming functions. This can be useful when the configuration of a speaker array changes, such as when speakers are added to or removed from the array. The LM48901's scalable speaker array technology easily supports multi-dimensional speaker array configurations.

Figure 6 shows a web-based design tool (http://webench.national.com/sawa/#main) that generates loudspeaker array filter coefficients based on user inputs about the array geometry and listener position. Use of the web design tool requires user registration.

During the array design process, the user takes the following steps:

1. Click on "New Config" to start a new configuration.
2. If this design is a cross-over-design, set the cross-over-frequency value for the design. Non-cross-over-design has a minimum of 4 tweeter speakers. Cross-over design has a minimum of 4 tweeter speakers and 2 woofer speakers.
3. Choose the sampling frequency for your design.
4. Choose the number of speakers for your design.
5. Set the Spatial effect Beamforming Bypass Frequency range. Note: Use a lower value to achieve more spatial effect; or a larger value to achieve more solid center sound.
6. Specify the Listener Sweet Spot co-ordinate values.
7. Apply the above settings to create Speaker Table listing speaker details and a co-ordinate system.
8. Edit the speaker table entries or move the speaker images in Top View or Front View to create desired speaker array configuration.
9. Press Generate Coefficients button to create the coefficients file for your designed configuration.

The web based design tool eliminates the need for the system engineer to program specific acoustic beamforming and/or HRTF positioning algorithms, or to use general purpose DSP programming language and system design kits.

A very useful feature of the design tool allows the user to click on speaker icons in the array, or the listener icon, and drag and drop them to the design geometric configuration. During the system design process the filter coefficients are then downloaded into the coefficient memory of the LM48901. The LM48901 based system is then tested for the desired results. The design can easily be changed by specifying different array parameters and reloading the coefficients.

A separate equalization tool can be downloaded that permits further frequency equalization across the speaker array for tuning the output audio quality. While the LM48901 simplifies creating the spatial effects using loudspeaker arrays, it does not eliminate the need to do careful acoustic design in the final product. Nor will it adequately compensate for using poor performance speakers in the array.

Spatial array technology video's showing more information about the product and the web-based design tool can be found at http://www.ti.com/spatial-pr.

SUMMARY
Spatial audio techniques to improve clarity and depth are becoming more widespread in consumer audio applications spanning smartphones to large flat-screen TV's. The LM48901 spatial array IC provides a very simple and fast way to implement those techniques in loudspeaker arrays to expand the soundstage and output power for products with limited space for speakers.

A single LM48901, with its accurate sound beamforming and crosstalk cancellation, can be used in two, three or four speaker applications. Daisy-chaining several LM48901 ICs enables the design of eight, 12, and 16 speaker arrays that deliver a much wider and exciting audio experience.

About the author:
Kenneth Boyce is Audio Technologist for Texas Instruments Silicon Valley Labs. He previously served as Technology Director for National Semiconductor's Audio Products Group, and before joining National, Boyce served as director of the Audio and Communications Division at Oak Technology, which developed initial implementations of AC-97 Codecs and Digital Audio Controllers. He holds a bachelor of science degree in electronics from West Virginia University.

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