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For proper operation, a digital demodulator requires the power of its input signal to be in the middle of its analog to digital converter’s range. Analog demodulators also require their inputs to fall within a small power range.

V95234B1-24A28P

For proper operation, a digital demodulator requires the power of its input signal to be in the middle of its analog to digital converter’s range. Analog demodulators also require their inputs to fall within a small power range.

• Put the DSP in standby mode–this is a critical part of power optimization. To achieve this, a developer can activate the idle configurations, which include idling CPU, all peripherals, clock gen and on-chip oscillator.

• Terminate unused input pins by pull-up/down and make sure that unused output” pins are not connected to any resistive loads Enabling the bus keepers on the EMIF bus will also save power.

V95234B1-24A28P

• The real-time clock (RTC) is the only module that is completely isolated from the rest of the chip. It can be powered while the rest of the chip is not powered, and vice versa. Idling of other peripherals and modules on the DSP will not affect the RTC. To minimize power dissipation, the RTC module should be powered up when it is not being used.

• Program the on-chip analog-to-digital converter clock to the lowest possible clock rate of 4 MHz, so the maximum possible conversion clock frequency is 2 MHz. This will minimize power consumption of the A/D converter state machine. If an on-chip A/D converter module is not used, then the A/D converter connect supply and ground should be connected to the supply and ground suggested by the data sheet.

• Idle the USB module on the DSP by setting the individual bits in the USBIDLECTL register. To disable the USB port, write 1 to the idle enable (IDLEEN) bit in USBIDLECTL, and then write 1 to the PERI bit in ICR. Make sure the USBCLK is set to 48 MHz. Make sure to leave DP and DN pins floating. Finally, pull up the USB pin PU high with 10 kohms, so the USB does not reset and wake up the on-chip oscillator.

V95234B1-24A28P

Don't

• Attach the emulator or use debugging functions with the Code Composer Studio integrated development environment to put the DSP in sleep or Idle3 mode. Don't have any pending interrupts, and make sure unused input pins are pulled high. Don't do any DMA transfer or have RTDX enabled. All of these actions will reactivate the Idle3 mode. (Example code that pus the DSP in Idle3 power-down mode is available from TI.)

V95234B1-24A28P

• Have an external interrupt or an interrupt generated by the on-chip real-time clock. Don't have USB host asserts resume or system hardware reset, as these events will wake up the internal oscillator from idle or sleep mode.

• Terminate the data or address bus pins, since these have holder features to reduce the static power dissipation caused by floating unused pins. The bus holders will eliminate the need for external resistors on unused pins.

Early attempts to apply noise reduction software techniques to enhance speech recognition performance had limited success since, in most cases, these noise reduction technologies had been developed to improve human-to-human communication systems. With such technology, there is always some misidentification of 'noise' and 'speech'. Noise that is misidentified as speech will be transmitted leading to speech-like artefacts that can sound like a babbling brook, very disturbing for a human listener. On the other hand, speech that is mis-identified as noise will be removed, potentially causing the speech to sound distorted.

Achieving the optimal performance from a noise reduction technology involves a trade-off between introducing watery artefacts and causing speech distortion. In human-to-human communication, watery artefacts are usually more unacceptable than losing small parts of speech, particularly since the brain, to some extent, tends to fill in the missing bits of speech to make sense of the output. On the other hand, in a speech recognition system, even a small amount of speech distortion can result in words being unrecognisable, while watery artefacts are often ignored. Consequently it is usually necessary to design noise reduction technology specifically for enhancing the performance of speech recognition systems.

Another interesting aspect is the fact that in normal human-to-human conversation, say using a hands-free in-car phone, we talk over each other only about 6% of the time. During this time echo cancellation technology removes one of the voices to avoid echo and enhance clarity. For a speech recognition system, there is often competing background noise and 'echo' for 100% of the time, whether it is from the radio, the speech recognition system itself or even a nearby passenger.

Due to these different operational requirements, noise and echo cancellation solutions for enhancing speech recognition need to be optimised differently and solutions aimed at human listeners are often non-ideal. The quality of such solutions is dependent on how cleverly they minimise the distortion to the speech while reducing background noise and echo.

In the Real World If we were to consider an in car telematics or navigation system that requires voice instructions, it may be represented by the diagram in Fig1.

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