Philips Semiconductors
Preliminary specification
Satellite sound receiver with I2C-bus control
TDA8745
With the input selection every input pin of the left and/or
right channel can be independently selected. Input
selection for the left channel is controlled by the IL signal
and for the right channel by the IR signal.
To adjust for different (main channel) modulation depths
used at some satellites the audio level is made adjustable,
the signal can be controlled in steps of 1 dB from
−12 dB to +3 dB by the LEV signal.
From the inputs, the signals are coupled to the
limiter/amplifier and to the PLL demodulator of each
channel. The output signal from the PLL is routed to both
the presence detector and audio level control.
Noise Reduction (NR)
To improve the quality of the secondary channels, the
audio signal is processed at the transmitter side before
modulation. For an overall flat audio response the inverse
processing must take place after demodulation. This is
achieved in the NR.
The inputs of pins 7 and 8 can be changed into digital
outputs for external switching purposes, set by the so
called Port Extension bit (PE). Not used inputs should be
connected to ground. Note that the inputs of pins 7 and 8
are also floating when not in Port Extension mode.
The NR can be regarded as an input level dependent
Low-Pass Filter (LPF) [adaptive de-emphasis system]
followed by a fixed de-emphasis. Figure 3 shows the
transfer characteristics as function of the input level
(normalized to input level, and without the fixed
de-emphasis).
Presence detector
The presence detector is used to determine if a carrier is
present on the channel of interest. It does so by measuring
the amount of high frequency noise (>20 kHz) in the audio
signal, which is directly related to the C/N (carrier-to-noise
ratio) at the IF input. If a carrier is present, these high
frequencies are fairly moderate, if no carrier is present,
strong noise components are present.
At maximum input level (50 kHz frequency deviation,
referred to as 0 dB) the frequency response of the first part
(i.e. without fixed de-emphasis) is nearly flat (note the
small dip around 3 kHz in Fig.3; this is a system attribute).
As the input level is X dB lowered, the higher output
frequencies will be reduced an extra X dB with respect to
the lower frequencies (1 : 2 expansion).
The audio signal, first high-pass filtered and then rectified,
is filtered by the components at pins 41 and 39
(PRES DET L and PRES DET R). The DC level at this pin
is then compared with an internal reference voltage. If the
level at pins 41 and 39 exceeds this voltage level, the
presence detector output goes HIGH (no carrier).
If a main carrier signal is received, the NR can be
bypassed at which the signal is fed directly to the
de-emphasis circuit. The noise reduction is active when
the NR signal (via I2C-bus) is logic 1.
This output signal can be used to drive the output mute (if
bit PDM = 1; see Section “Output selection”) and can be
monitored by reading bits PDL and PDR. The detection
level can be modified by changing the leakage resistor at
pins 41 and 39, a higher resistor value gives a ‘no carrier’
response ant C/N levels detected as ‘carrier present’ with
a lower resistor value.
De-emphasis
De-emphasis is realized by means of several internal
resistors and an external capacitor to ground. Via the
I2C-bus, the DEM signal can be switched between 50 µs,
75 µs, J17 and no de-emphasis. Figure 4 shows these
four different possibilities.
Audio level control
Output selection
Each demodulator output signal is amplified in a buffer
amplifier and DC decoupled by means of electrolytic
capacitors connected to pin 42 (left) and pin 1 (right).
With the output selector the output pins 23 and 22 can be
switched to the left and right satellite channels
(pins 33 and 31) or to the external inputs (pins 25 and 24)
for an other signal source or for connection of a decoder
box. the OS1 and OS0 bits determine this selection.
The output level of all channels is −6 dBV typical at a
frequency deviation of the FM signal of 54% of the
maximum deviation (i.e 0.54 × 85 kHz = 46 kHz for the
main channel and 0.54 × 50 kHz = 27 kHz for the
secondary channels) at 1 kHz modulation frequency
(reference level).
Pin 21 is a separate output which delivers the mono
channel. The mono signal is the sum of pin 23 (left) and
pin 22 (right) output signal [1⁄2(l + r)].
1996 Mar 11
9
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