The UCLID Speech Processor
The speech processor developed at UCL has been designed to take
maximum advantage of the transparent electrical connection to
the auditory nerve that the percutaneous connector allows. It
is based on a the Texas Instruments TMS320C50 digital signal processing
(DSP) chip clocked at 69.12MHz which is able to carry out operations
such as filtering and compressing signals at very high speeds.
As well as being completely programmable the device also contains
over 250 Kbytes of reprogrammable 'flash' memory for the storage
of programs and patient-specific data tables. Sound is received
by an ear-level or processor mounted Knowles EK series electret
microphone and sampled to 14-bit resolution at up to 24kHz. Filtering
operations can be carried out at 20-30 million taps per second,
depending on the pulse parameters. Eight fully independent programmable
constant current source generators deliver rise and fall-times
of about 1µs. Pulse timings are controllable to within 1µs,
giving considerable flexibility in stimulus design.
The UCLID processor at present can use three processing strategies,
namely: a two-channel algorithm which presents 'speech pattern
elements'; a multi-channel 'compressed analogue' (CA) algorithm;
and a multi-channel implementation of 'continuous interleaved
sampling' (CIS).
The UCLID-II processor is a new design under development which
uses the TMS320VC549 DSP which can be clocked at up to 100MHz
under the control of a programmable phase-locked loop and operates
from a power supply as low as 2.3V for the processor core. A
new, highly parallel architecture, allows filtering at up to 200
millions taps per second. The relatively bulky digital logic
will be integrated into a singlememory-mapped field programmable
logic array which will also control the timing of the output pulses.
The Fitting and Programming System
The fitting and progamming sotware has been developed to fully utilise the flexibility of the speech processor. It is a pc based, mouse operated system, and is opto-isolated from the patient for safety. It offers highly flexible control over the pulse widths on individual channels and compression. For instance, when CIS is implemented the phase of a pulse can be as low as 10µs (although 35µs is probably the lowest practible level) and the interval between successive pulses can be as low as 10µs, with both parameters adjustable in 1µs steps. The parameters of each pulse can be adjusted independently.
Link to USET details