# A Biologically Inspired Sound Localisation System Using a Silicon Cochlea Pair
## 1 Introduction  
This work is inspired by the high accuracy and robustness with which the human auditory system perceives sound in cluttered acoustical environments. In the human auditory pathway, binaural signals are pre-processed in the cochlea, transformed into neural signals in the auditory nerve (AN), and transmitted through the cochlear nucleus (CN) to the superior olivary complex (SOC). The interaural time difference (ITD) cues of the binaural signal are believed to be encoded in the medial superior olive (MSO) [1, 2]. The neurons in the MSO receive excitation from large spherical bushy cells (SBCs) of the CN in both the ipsilateral and contralateral side and inhibitory from the ipsilateral lateral nucleus of the trapezoid body. The interaural level difference (ILD) cues of the binaural signal are believed to be encoded in the lateral superior olive (LSO) [2, 3]. The neurons in the LSO receive excitation from the small SBCs of the ipsilateral CN, and inhibition from the contralateral side relayed through inhibitory neurons in the nucleus of the trapezoid body. The ITD and ILD are the primary cues for sound source localisation according to the \"duplex theory of sound localisation\" proposed by [PERSON] in Reference [4]. [PERSON] the ITD cues dominate at low frequencies while the ILD cues dominate at high frequencies (in humans, above 2-3 kHz) where the wavelength is short, and the head can act as an acoustic shadow. So far, many psychophysical experiments have shown support for the duplex theory [2, 5, 6, 7, 8, 9, 10, 11], and the duplex conception is still a standard idea for how binaural hearing works. To try to mimic the human auditory system in localising a sound source, biologically inspired sound localisation systems have been proposed and developed. For example, [PERSON] [12] proposed a computational model of binaural localisation and separation, in which peaks of short-time cross-correlation functions between each channel of two cochlear models indicate the direction of a sound source; [PERSON] et al. [13] proposed a computational model including two cochlear models and a Jeffress delay line [14] to encode the ITD cues. In a study by [PERSON] et al. [15], both ITD cues from cross-correlation functions and ILD cues were used for echoic and noisy environments. Although both ITD and ILD cues are involved in human sound localisation horizontally [16], hardware development mainly uses the ITD cue since it is relatively strong and easy to obtain without any additional requirements such as a dummy head or an artificial pinna pair. The first neuromorphic model for auditory localisation was implemented by [PERSON] and [PERSON] [17]. They built the Jeffress model [18] with a cochlear pair on an analogue chip. The system created delay lines from two cochlear channels with a maximum delay value of the maximum ITD expected and a minimum delay value equal to the system resolution. After this, [PERSON] et al. [19] implemented a two-chip system: One chip for a cochlear pair and the other one for a delay line model. [PERSON] et al. [20] built a three-chip system with four microphones to detect the 3-D location of a sound source. In the system, the first chip was for the cochlear pair and ILD extraction, the second chip was for the onset detection, and the third chip was for the ITD extraction. This system showed a root mean square (RMS) error of \(5^{\circ}\) in azimuth and elevation. However, the hardware implementation of the delay lines makes those systems large. Alternatively, delay lines can be implemented by using the inherent characteristics of the cascade cochlear structure. In a cascade cochlear filter model, each stage of the filter adds a certain delay so that the cascade itself acts as a delay line. The correlations between the two cochlear channels thus encode the ITD cues of a sound source. Such an approach was implemented on an analogue chip in Reference [21]. They presented the formations of the 2-D cochlear correlograms in detail, and it forms the basis of this work.