CAP 3001 A
2.1.14. FM Noise Canceller (ASU)
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.
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:
2.1.18. Volume Control AVOL
– 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 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:
– 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.
v_tot
v_anlg
0 dB
v_dig
v_totw0 dB
v_tot
*45 dBtv_tott0 dB
v_tott*45 dB
v_tot
0 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.
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-
vertedusingahighlylinearD/A-converter. Itsnoisepow-
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.
TheclockisderivedfromtheclocklineSCLKofthePDAI
bus.
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MICRONAS INTERMETALL
MICRONAS [ MICRONAS ]
