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

[Part 1 of this article introduces spatial audio reproduction and defines head related transfer function (HRTF), crosstalk cancellation and audio beamforming.]

Acoustic Beamforming Techniques
There are generally two types of acoustic beamforming techniques used today:

• Mechanical beamformers, which rely on the physical sizes and positions of the speakers to produce desired spatial effects.
• Electronic beamformers which rely on signal processing that is performed by a digital signal processor (DSP) before audio signals are provided to the speakers.

Of course, both methods may be combined as needed depending upon the application.

Electronic beamforming was initially developed for radar applications. Its application to audio first appeared in microphone arrays designed for speech and audio capture. The abundant applications in this area have led to many years of innovations in audio beamforming algorithms.

The basic idea of microphone array beamforming is to individually adjust the phase and amplitude of the received signal of each array element so that the combined output can achieve maximum signal-to-noise ratio (SNR) in certain directions. The concept is similar to extracting the desired signal in the frequency domain by bandpass filtering, but here it's done in the spatial domain and the passband can be considered as a range of directions. Many beamforming techniques exist and are well documented, and the selection of certain technique usually depends on the requirements and constrains of a specific application.

Although audio beamforming is widely used in capturing audio signals, its application in audio playback is relatively limited. A major reason is that stereo systems have been delivering relatively good performance and it was unnecessary to use more than two speakers for many applications.

However, the ever shrinking mechanical space in portable devices now poses major challenges for stereo audio playback. For example, output volume level loss due to speaker size reduction, and diminished stereo sound image due to narrow speaker spacing are both problematic.

Flat-panel TV's are trending toward very thin enclosures that severely limit loudspeaker cone excursion, which in turn diminishes output volume levels as well as audio quality. One way to overcome these limitations is to use an array of small speakers to increase the overall volume and to render a more desirable sound field using audio beamforming techniques.

It should be pointed out that there are many other methods that can deliver spatial audio over a speaker array system, such as wave field synthesis (WFS) and ambisonics. They typically require dozens to hundreds of speakers and a large span in space. Therefore they are mostly used to design speaker systems in theaters and sound rooms, but are not suitable for small to mid-size speaker arrays.

Generally speaking, beamforming techniques for microphone array applications can be readily adapted to speaker array applications since the playback is basically a reverse of the capturing process. However, full bandwidth audio playback requires much wider bandwidth than typical speech applications and therefore requires special considerations in algorithm choice and array setup.

To deliver spatial stereo sound over a speaker array requires a very effective crosstalk cancellation algorithm. Additionally, speaker array algorithms need to minimize the distortion of audio playback as much as possible, including minimizing artifacts and coloration.

As a quick review, generating spatial audio effects utilizes three techniques as seen in Figure 4: crosstalk cancellation, mechanical and electronic beamforming, and HRTF data.

The challenge has been to develop techniques suitable for small arrays which will yield subjectively better results than using stereo alone, and apply those in a manner that use several speakers without requiring complex algorithmic programming skills.