CAP 3001 A
After buffering (ABUF) and switching (AMUX) in the AF-
mode the signals are converted into digital form by 2 A/D
converters (ADCs). Their output is 1 bit at a rate of 8.208
MHz; in each of the two channels in the CAP 3001 A
there is a cascade of 3 lowpass filters (LPF02, LPF23
and LPF34), which suppresses the high-frequency
noise produced by the ADCs. The outputs of the filters
LPF34 are 16 bits wide and are sampled with 38 kHz;
these samples are transmitted via the input buffer to the
DSP core. After processing in the DSP, the samples are
interpolated to the eightfold sampling rate and converted
into analog shape by 2 D/A converters (DACs), filtered
(ALPF) and optionally attenuated (AVOL) to feed the
power amplifiers which produce the signals for four loud-
speakers.
2.1.5. Analog Input Systems
Fig. 2–3 shows all analog inputs and functions of the
switches S0 to S3.
420 to 430 kHz
MPX0
AML
TAPEL
AUXL
1
38 kHz
0
1
0
S4
2
Main 1
S0
3
0
AMR
1
2
3
It is assumed that the process of stereo multiplexing
used in radio broadcasting is known. The main FM-mod-
ulator can be modulated by the sum signal of left and
right channel (in baseband), a pilot tone, the difference
channel (AM-modulated, suppressed carrier), an op-
tional ARI signal (AM-modulated, unsuppressed carrier)
and optionally up to 3 SCA signals (FM-modulated). The
composite signal is the so-called MPX signal. So a vari-
ety of signals ride “piggy-back” on the main carrier,
which was originally assigned only for monophonic
transmission. In the CAP 3001 A, the SCA signals are
regarded as disturbing signals while the others are re-
garded as useful.
TAPER
AUXR
S0
Main 2
0
1
S2
ARI/RDS
POT1/
MPX1
POT2
POT3
POT4
POT5/
AVC
2
3
4
1
0
S3
5
0
Pot
In the MPX-mode there is an analog mixer AMIX in front
of the ADCs. It mixes the difference band down to base-
band. The sum channel and the difference channel are
then treated like the other baseband signals. Digital
quadrature mixers ARIMX and PILMX extract the ARI-
information and the information of the pilot signal, re-
spectively.
FMLEVEL
MPLEVEL
AMLEVEL
1
2
S1
Level
Fig. 2–3:
Analog input systems
By means of digital mixers the pilot tone and the ARI sig-
nal are mixed down to zero intermediate frequency in
quadrature representation, where their information is
sampled and sent to the DSP core. The demodulation of
the ARI signal is done by the DSP software. The band-
width of this ARI channel is sufficient to allow demodula-
tion of an RDS (Radio Data System) signal by DSP soft-
ware as well. Beside these main blocks, there are other
systems. The analog field strength information FS deliv-
ered by AM and FM tuners is A/D-converted; after low-
pass-filtering (LPF06) the samples are sent to the DSP
core, where the information could be used to control
some parameters of the entire system. Other input sig-
nals, such as signals from an external microphone and
from external potentiometers are selected by an analog
multiplexer, A/D-converted, lowpass-filtered and sent to
the DSP or to the controller via the IM-bus interface
(IMIF). The IM-bus interface is also able to receive data
from the external microcontroller and to control the sys-
tems on the CAP 3001 A.
MICRONAS INTERMETALL
2.1.6. Buffers ABUF
The analog input buffers have to adjust the individual de-
sired input levels in order to cover the entire volume
range of the A/D-converters.
The inputs can be divided into two groups: those which
have to be connected via external capacitors, and those
that are DC-coupled. One of these inputs, the POT5/
AVC-input, uses the same pin, but is DC-coupled if used
as POT5, and AC-coupled if used as AVC.
Note: Input pins POT2 to POT5 are switchable to digital
outputs via the IM-bus interface. This feature is made
possible by open drain transistors and external pull-up
resistors down to 1 kΩ.
7
MICRONAS [ MICRONAS ]
