Anti-Image Filter with Zeroes at Multiples of the Sampling Frequency
Publication Date: 2003-Jul-09
The IP.com Prior Art Database
An analog anti-image filter, with zeroes at multiples of a Digital to Analog Converter (DAC) sampling frequency, processes analog data prior to the DAC. The anti-image filter may be implemented as a first, second, third, fourth, fifth, sixth, or higher order analog filter, and may be optimized to provide a flat group-delay (assuming ideal components) from DC through the cut-off frequency and beyond. The anti-image filter includes a sample-and-hold (S&H) circuit that may serve as a first stage of a Successive Approximation (SA) DAC. As with standard analog anti-image filters, the anti-image filter may derive additional benefits from over-sampling and digital decimation.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information successfully reaches the hair cells in the cochlea. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In the case of many profoundly deaf people, however, their deafness is the result of sensorineural hearing loss. The destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses, is responsible for this type of hearing loss. People who experience sensorineural hearing loss receive no benefit from conventional hearing aid systems, regardless of the volume of the acoustic stimulus, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses are unable to be generated directly from sound.
To overcome sensorineural deafness, numerous Implantable Cochlear Stimulation (ICS) systems --or cochlear prostheses-- have been developed which seek to bypass the hair cells in the cochlea (the hair cells are located in the vicinity of the radially outer wall of the cochlea) by presenting electrical stimulation directly to the auditory nerve fibers. The direct stimulation on the auditory nerve leads to the perception of sound in the brain resulting in an at least partial restoration of hearing function. Common elements in most ICS systems include an electrode array implanted into the cochlea and a suitable external source of an electrical signal for the electrode array.
The signal provided to the electrode array is generated by a speech-processing component of the ICS system. In known ICS systems, the microphone converts the received acoustic signal into an analog electrical signal. The analog electrical signal is then processed by an Digital to Analog Converter (DAC) to generate a digital signal for processing by a Digital Signal Processor (DSP). Following the digital signal processing, the digital signal is converted back into an analog signal by a Digital to Analog Converter (DAC).
Known ICS systems utilize a Successive Approximation (SA), Sigma-Delta (SD), or other DAC. Such DAC requires an anti-image filter with a steep roll-off in order to attenuate high frequency signals above half the sampling frequency of the DAC, which high frequency signals are created during the digital to analog conversion process. Such steep roll-off filters can create significant phase distortion at and around the cut-off frequency. While over-sam...