英语翻译
The digital interpolation filter upsamples the output of the digital audio processing block by the required oversampling ratio before data is provided to the digital delta-sigma modulator and analog reconstruction filter stages.The filter provides a linear phase output with a group delay of 21/fS.In addition,programmable digital interpolation filtering is included to provide enhanced image filtering and reduce signal images caused by the upsampling process that are below 20 kHz.For example,upsampling an 8-kHz signal produces signal images at multiples of 8-kHz (i.e.,8 kHz,16 kHz,24 kHz,etc.).The images at 8 kHz and 16 kHz are below 20 kHz and still audible to the listener; therefore,they must be filtered heavily to maintain a good quality output.The interpolation filter is designed to maintain at least 65-dB rejection of images which are below 7.455 fS.In order to use the programmable interpolation capability,fS(ref) should be programmed to a higher rate (restricted to be in the range of 39 kHz to 53 kHz when the PLL is in use),and the actual fS is set using the NCODEC divider,where NCODEC = NDAC = NADC.For example,if fS = 8 kHz is required,then fS(ref) can be set to 48 kHz,and the DAC fS set to fS(ref)/6.This ensures that all images of the 8-kHz data are sufficiently attenuated well beyond a 20-kHz audible frequency range.