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CAP3001A Datasheet(PDF) 12 Page - Micronas |
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CAP3001A Datasheet(HTML) 12 Page - Micronas |
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12 / 37 page  CAP 3001 A MICRONAS INTERMETALL 12 2.1.14. FM Noise Canceller (ASU) The FM Noise Canceller removes peak noise from the audio signal. No external circuitry is required. All filters, delays and the control section are implemented digitally. The function is split into two sections: – The noise detection searches for energy in the non- audio range by means of a highpass filter. The output of this filter is compared with a DSP-controlled thresh- old. If this threshold is exceeded the interpolation unit is triggered. The 19 kHz pilot tone is removed before the audio signal enters the detection highpass. Pro- grammable delay adjustment makes sure of the cor- rect timing between peak detection and peak inter- polation. – The interpolation circuit substitutes a peak-corrupted sample by the mean value of the non-corrupted adja- cent samples. Once a trigger comes from the detec- tion circuit, a programmable number (0 to 15) of successive samples is interpolated. All functions work on a 228 kHz sampling rate. At this rate the peaks are still small enough (not widened by the final decimation filters) to be removed effectively. 2.1.15. Analog Output Systems 2.1.16. D/A-Converters DAC The D/A-converters used are of the oversampling type. The samples to be converted at their sampling rate f_s are first interpolated to 8 x the sampling rate and then oversampled to a higher rate f_NS where noise shaping is performed. The output of the noise shaper is then con- verted using a highly linear D/A-converter. Its noise pow- er density increases with increasing frequency, the re- sidual noise in the baseband is very low. Within this application the DAC has to be adapted to the different modes. The digital sources (e.g. CD-player) must supply the proper clock rate in order to drive the DAC with a stable clock rate locked to the sampling rate. The clock is derived from the clock line SCLK of the PDAI bus. 2.1.17. Lowpass-Filters ALPF The analog lowpass-filters behind the DACs eliminate the high-frequent noise in order to avoid any distortions in the AM frequency range. 2.1.18. Volume Control AVOL The analog volume control together with the digital vol- ume control implemented in the digital signal proces- sor’s software provide a large volume control range. The analog volume control itself covers a range of 45 dB in 1.5 dB steps and includes an additional mute position. A sensible splitting of the total gain v_tot between the digital gain v_dig and the analog gain v_anlg is: v_tot v_anlg v_dig v_tot w0 dB 0 dB v_tot *45 dBtv_tott0 dB v_tot 0 dB v_tot t*45 dB *45 dB v_tot )45 dB All control bits for the hardware section are first ad- dressed to the DSP core program. In case of hardware read-registers the bits are transmitted to the DSP core, stored in the DSP RAM and so they are available for the controller via the DSP’s IM-bus interface. |
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