Stereophonic sound is usually only achievable when speakers are physically separated from each other by a considerable distance. When speakers are situated closer together in mobile phones it is possible to simulate stereo by using wave interference to cancel the left channel sounds in the vicinity of the listener’s right ear, and vice versa.

This technique, called transaural crosstalk cancellation, can yield an apparent separation between the speakers a factor of four or greater than what would be perceived for a given physical separation.

To better understand this phenomenon, we need to look at how the ears and brain process information to determine the location of a sound source. The human ear is sensitive to sound waves in the range of approximately 20Hz to 20kHz. As the sound wave reaches the ear, the shape of the outer ear changes the sound wave before it reaches the inner ear drum.

This reshaping of the wave changes the resonate properties of the wave depends on the direction from which the sound wave enters the ear. This spectrum allows the brain to determine the direction from which the sound wave originates.

As sound waves propagate towards a listener’s head from any specific direction, the time difference from between sound waves reaching the right and the left ear also helps to determine from which direction the sound is coming. This time delay is known as interaural time delay (IDT) and operates in combination with the ear’s spectral properties to determine the head-related transfer function (HRTF) associated with each ear.

The HRTF is the mathematical transfer function used to relate a specific sound source to the ears of a listener. The HRTF takes into account the location of the sound source to the listener’s head with regards to the delta between the two ears as well the frequency.

The basic idea for simulating a 3D effect is to produce the acoustic signals at the two ears that would occur in a normal listening situation. This is accomplished by combining each source signal with the pair of HRTFs that correspond to the direction of the source.

Most stereo multimedia products that offer 3D enhancement do not take into account all directional information needed to create a fully 3D sound. In most cases these multimedia systems use HRTFs comprising simple phase delaying circuits to produce a widening effect of the perceived sound field. This technique allows closely located speakers to have the perceived sense of being separated by a greater distance.

When listening to sounds coming from a source such as two speakers, the right ear hears a little of the left-channel and the left ear hears a little of the right-channel; the left-channel signal arrives at the left ear before it arrives at the right ear, whereas the right-channel signal arrives at the right ear before it arrives at the left. This is known as transaural
acoustic crosstalk.

As the speakers are moved closer together the time delay from ear to ear becomes less and less until the time delay is very small such that the sound from the two speakers appears as if a single speaker is used. It is the crosstalk that tells our brain that the sources are close together. In order to simulate a greater separation between two closely located sources this crosstalk
must be eliminated. 

Crosstalk cancellation is performed by adding a cancellation signal from one speaker to the other in a way such that, when the listener is directly in front of the source, the crosstalk is acoustically cancelled at the ears, giving the listener the sensation of a wider separation
of the speakers.

The technique of phase-delaying a signal between emitters is commonly used to control the beamwidth and directivity of radio antenna arrays.  Whereas a single antenna aligned along the z direction will radiate equally in all directions along the x-y plane, by lining up several antennas, it is possible to limit the spread of the radiation to several distinct lobes along the x-y plane. 

The width of the lobes decreases with the number of antennas in the array and with the frequency of the radio waves, for a given distance between the antennas.  A typical radiation pattern of a five-element array with zero phase difference between the elements (i.e. all antennas are emitting the same signal) will have narrow lobes 180 degrees apart.

Besides changing its width, you can rotate the main lobe in the x-y plane by delaying the signal to each successive element in the array by a constant phase angle.

Since sound waves also obey the superposition principle, it is possible to apply these results toward the creation of a “speaker array,” which can direct the sound from one stereo channel to one ear and sound from the other channel to the other ear. 

Networks that produce a constant phase difference or shift find considerable application in radio electronics, and techniques for their design have been around since the 1950s.  The basic topology comprises two cascades of first-order all-pass stages, which produce a non-constant phase shift relative to their common input. Over a specified frequency range, they are able to maintain an approximately constant phase shift relative to each other. 

While passive implementations exist, commonly used first-order active circuits use a phase-shift filter with ft = 10kHz for the linear signal (the proper input channel) and a ft = 1khz for the quadrature signal (the other input).

The purpose is to have a 90 degrees phase shift between the linear and quadrature signal in the audio bandwidth of 1k-10kHz. The performance of a cascaded first-order all-pass circuit manages to achieve close to 90 degrees of phase difference between the L and Q outputs through the frequency range 1kHz to 10kHz. Since most of the speakers in portable audio devices are very small and do not support ‘full spectrum’ audio (very little response below 300Hz typically), a frequency range 1kHz to 10kHz gives acceptable performance.

More stages can be cascaded (and aligned appropriately) to widen the frequency range over which the 90 degree phase shift is established, giving a better ‘3D’ effect.  A cascade of two stages is a good trade off between circuit complexity and power consumption vs. performance.

Recently emerging on to the market are audio IC’s such as Maxim’s MAX9775 that incorporates a phase delay circuit with an audio amplifier to give the user a one chip solution for achieving a wider perceived sound field.

Robert Nicoletti is an applications engineer with Maxim Integrated Products