Design Of Multi-functional Sound Stimulator For Auditory Evoked Potential Recording Device

Auditory evoked potentials (AEP) recording is an objective examination method, which can be used to examine the pathology of auditory nerve or central nerve within the auditory nerve pathway. In clinical, AEP recordings are performed by a system which is conventionally consisted of a stimulus generator and a brain signal acquisition devices. The working process is:the acoustic signal produced by sound signal generator is transmitted to the subject or patient through a transducer (earphone), and a trigger signal indicating the occurrence of sound onset is sent to data acquisition device and usually attached with the file stored in the system, then AEP is extracted from the recording data by various means, such as signal amplification, epoching (segmentation), filtering and averaging. It can be seen that the stimulus generator is a key part in AEP recorder. There are two types of AEP acoustic stimulus generator. The first type is using computer soundcard as its analog sound signal output, and the digital ports (such as serial or parallel port) are used to output the trigger signal to mark the stimulus onset. This type of generator is considered as "soft" stimulator, which the stimulation parameters are mainly adjusted by software. However, due to the lack of precise synchronization of the analog and digital output channels under current computer operating system, the trigger time accuracy can not be guaranteed, and the variation of trigger time scope spans generally from several milliseconds to a dozen milliseconds. As a result, this type is not suitable for early latency AEP recordings, such as the auditory brainstem response (ABR) recording. The second type of generator is called "hard" stimulator, which uses special hardware to achieve the precise synchronization of sound analog channels and digital channels. The "hard" stimulator is generally equipped with stimulus software, and it can only provide preset stimulus plans and some customized output with limited parameter adjustability. At present, the AEP recording devices used in domestic hospitals and research institutes are dominated by foreign brands, such as the Smart EP (U.S.A) and Nicolet Spirit EP recorder (U.S.A). Besides, with the advance of AEP research, the needs of special stimulus and sounds are increasing, such as the FM chirp sound, pseudo-random stimulus of maximum length sequence (MLS) and so on. It is thus necessary to address this problem by developing a customized stimulus system in some research areas. The aim of this thesis is to present a multi-functional sound stimulator that can produce various types of typical stimulus sounds with flexible parameter adjustment and more importantly, the high-precision synchronous capability.This sound stimulator system is designed by applying virtual instrument technology, and a user-defined testing system is created based on this open architecture. The stimulator adopts PCI6221 acquiring card (NI, Inc) as its hardware system. The analog outputs AO0 and AO1 of the acquiring card are designed as left and right sound channels respectively, and the 8 bits digital port PO is used to output trigger signal. To synchronize the analog output and digital output (called multifunctional synchronization), all port of the PCI6221 must share the same start trigger and the same sample clock to ensure that the operations began at the same time and the signal output at the same rate. NI's LabVIEW (laboratory virtual instrument engineering workbench) is used to accomplish the software system design (including interface design and sound editing design). To meet the needs of different users, especially the needs of users who are not familiar with LabVIEW, the software system allows users to use other software (such as MATLAB, the text editor etc.) to edit stimulus files. It also supports self-recorded sound files. Each sound stimulus parameters can be designed according to the actual needs in the software platform, the users can even make some special processing to each sample point of the stimulus data, which ensure the most flexible parameter control.Experimental results show that the system is able to provide not only the stimulus sound with arbitrary waveform, but also high-precision synchronization trigger signal of acoustic stimuli. The synchronization time difference between the stimulus and trigger signal is less than 1μs which is precise enough for any AEP recording demand. By comparing with the computer soundcard, the present system has various advantages:Its sampling digits can reach 16bits, sampling rate can reach more than 48K, frequency response curve are relatively flat, and its sound distortion is smaller than soundcard. Besides, it also can apply multi-device synchronous technology to eliminate crosstalk problem between left and right channels under the high sampling rate condition. Therefore, the system can be used as a high-performance sound stimulus generator.In summary, this thesis presents a LabVIEW based sound stimulator for AEP recording system. The architecture and implementation approach ensure the stability and reliability of the performance in various applications. The stimulator has been tested to be able to work well with the Neuroscan amplifier (synAmp2), and clinical used insert earphone (ERTone 3A). The output sound intensity, though not yet calibrated, is big enough by human sensation for practical applications. The trigger output can also be adjusted to make it easy to fix in other systems.