Generating spatial audio from portable products - Part 1: Spatial audio basics
[Part 2 of this article introduces two beamforming techniques and takes a close look at TI’s LM48901 spatial array audio amplifier.]
The soundstage generated by a stereophonic audio system is typically restricted by the physical locations of the speakers, while the sound events perceived by the listener are limited within the span of the two speakers. In small-size stereo speaker systems, such as those found in portable devices, the perceived stereophonic soundstage becomes very limited, almost monophonic.
Portable stereo sound systems have a limited soundstage because their design is limited by the number of speakers and the spacing between them. To overcome this size limitation, spatial audio sound generation techniques can be used to expand the stereo soundstage, achieve better crosstalk cancellation, and enhance certain spatial cues. Texas Instruments (TI) has applied these sound processing techniques in a new family of spatial audio integrated circuits for portable products with two to 16 speakers. These include smartphones, tablets, laptops, sound docking stations, and sound bars.
This two-part article provides a spatial audio tutorial and highlights the audio design techniques employed in the LM48901 quad Class D spatial array IC. The first part defines spatial audio and discusses key techniques such as head related transfer function, crosstalk cancellation and audio beamforming. Part two provides an overview of the LM48901's beamforming technique and its web-based software tool that in a few simple steps allows you to create audio effect programming to convert your product's small soundstage into an immersive cinematic experience.
Naturally occurring sound in the space around us is inherently spatial. Sound sources are located at a point in a small region of the total space around us, although some sources such as earthquakes and landslides emanate from a broader area.
The sound scatters off various objects in the environment around us, then we hear the direct and scattered sound binaurally with our ears and after some human auditory processing, the sound is finally recognized. We make decisions about the processed sound, characterizing the sound(s) with labels such as direction, location, loudness, ambience and quality, near, far, tone, fat and thin, among others. In the case of two or more sound sources, the characteristics mix of each sound source (relative to the total sound being received) is also determined.
Spatial audio is simply a term applied to the reproduction of sound by electronic/mechanical methods that attempts to artificially recreate the real world listening experience of sound(s), or alternatively attempts to artificially alter reproduced sound(s) in order to create a perceived spatial environment that may not have originally existed.
The principle of spatial audio reproduction is simple:
If the reproduced sound waves arriving binaurally at your eardrums are identical to those of the real audio source at a particular position, you will perceive the reproduced sound as coming from a source at that particular position. It does not matter that originally the sound source may have been at some other position. The sound 'data' arriving at your ears is what is processed by the brain to ultimately characterize all aspects of the sound.