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Firmware

	What is difference between DAC and PWM output in speech coding?
	It is necessary to add head and tail waveform to speech data whenever users use DAC as speech output. The purpose of head and tail waveform is making output waveform changing slowly from zero to DC level and from DC level to zero. Users must make sure that the head and tail waveform are long enough to eliminate pulse noise that happen at the beginning and end of speech. On the contrary, PWM output can not have the head and tail waveform. Otherwise, users will hear the pulse noise at the beginning and end of speech. Just using the original speech data and no more head and tail is necessary if users adopt PWM speech output.

	How to add a head and tail speech data?
	The DC value of speech is usually 40h. The head data is from 00h to 40h and tail data from 40h to 00h. Users can add 1 to data or subtract 1 from data every t. t is larger if users play the speech data faster, and t can smaller if users play slower.