3Compressed Analog: The First Multichannel Waveform Strategy
Compressed Analog (CA) split the speech waveform into a handful of contiguous bands and delivered the filtered analog signals simultaneously and continuously to corresponding electrodes. It outperformed single-channel devices but introduced a fatal flaw — simultaneous stimulation sums electric fields, the channel-interaction problem that CIS would solve.
TThe CA principle and chain
CA is a waveform (analog) multi-channel strategy: the broadband signal is compressed to fit the patient's dynamic range, split into a few contiguous bands, and the filtered analog waveforms are delivered simultaneously to the corresponding electrodes The chain is: mic to fast-acting broadband AGC (logarithmic compression into the hearing dynamic range) to about four contiguous band-pass filters to adjustable per-channel gain to simultaneous analog signals on electrodes EL-1 to EL-4 Band centre frequencies were about 0.5, 1, 2 and 3.4 kHz, spanning roughly 0.1-0.7 / 0.7-1.4 / 1.4-2.3 / 2.3-5.0 kHz CA preserves the within-channel temporal fine structure of each band.[1999][2008]
FThe exemplar devices
The Ineraid device, manufactured by Symbion Inc. of Utah, was a key CA implementation CA used a percutaneous (skin-piercing) connector to drive the electrodes UCSF/Storz and Ineraid systems implemented compressed analog (Eddington 1980; Merzenich 1984 lineage) CA outperformed single-channel devices in speech recognition.[2008][1999]
CThe channel-interaction flaw
Simultaneous stimulation of multiple electrodes causes uncontrolled vector summation of electric fields at sites of neural excitation, distorting spectral information The phase relationships between simultaneous channels are uncontrolled, degrading channel independence Front-end broadband compression introduces spurious spectral components not present in the input, mainly in the high-frequency channels, depending on AGC attack/release and compression ratio Many patients did not perceive within-channel frequency changes above about 300 Hz as pitch, so much delivered information was unavailable to them.[1991][2008]
CWhy CA was superseded
The dominant channel interaction (field summation) and the spurious spectral components of front-end compression motivated a nonsimultaneous, back-end-compressed approach CIS, using temporally interleaved pulses, showed marked superiority over CA in the landmark 1991 study CA is no longer in widespread use and is no longer offered by any major manufacturer The CA-to-CIS step is the canonical example of 'remove the simultaneity, remove the interaction'.[1991][1999]
TBy the numbers
Which CA-specific mechanism best explains the worsening high-frequency percept with level?
What is the principal channel-interaction mechanism in simultaneous analog strategies like CA?
Which device/manufacturer is the classic exemplar of the Compressed Analog strategy?