TDA7513T Datasheet, PDF (22/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
IFS0 - IFS2.
5.4 AUDIO PROCESSOR
5.4.1 Input Multiplexer
CD quasi differential 1
CD quasi differential 2
Stereodecoder input (for both FM and AM signals).
5.4.2 Input stages
The quasi-differential input stages (see figure 4) have been designed to cope with some CD players in the
market having a significant high source impedance which affects strongly the common-mode rejection of
"normal" differential input stages. The additional buffer of the CD input avoids this drawback and offers the
full common-mode rejection even with those CD players. The quasi-differential input can also be used with
normal stereo single-ended output signal sources such as TAPEOUT.
5.4.3 AutoZero
In order to reduce the number of pins there is no AC coupling between the In-Gain and the following stage,
so that in theory any offset generated by or before the In-Gain stage would be transferred or even ampli-
fied to the output. To avoid this undesired situation a special offset cancellation stage called AutoZero is
implemented. This stage is located before the Volume block to eliminate all offsets generated by the Ste-
reodecoder, the Input Stage and the In-Gain stage (please note that externally generated offsets, e.g.
those generated because of leakage current into the coupling capacitors, are not cancelled).
The auto-zeroing is started every time the APSD data byte 0 is selected and takes a maximum time of
0.6ms. The rationale behind this choice is that the APSD byte encodes the signal source selection, and
auto-zero ought to be performed every time a new source is selected. To avoid audible clicks the audio-
processor is muted before the volume stage during this time.
5.4.4 AutoZero Remain
In some cases, for example if the uP is executing a refresh cycle of the I²C bus programming, it is not
necessary to start a new AutoZero action because no new source is selected and an undesired mute
would appear at the outputs. For such applications the device can be switched in the "AutoZero Remain
mode" (Bit 6 of the APSD subaddress byte). If this bit is set to high, the APSD data byte 0 can be loaded
without invoking the AutoZero and the old adjustment value remains.
5.4.5 Softmute
The digitally controlled softmute stage allows signal muting and unmuting with a I²C bus programmable
slope. The mute process can either be activated by pin #AUDIOMUTE or I²C bus. The slope is realized
in a special S-shaped curve so as to slowly mute in the critical regions (see figure 5). For timing purposes
the Bit 3 of the I²C bus output register is set to 1 from the start of muting until the end of unmuting.
5.4.6 BASS
There are four parameters programmable in the bass filter stage: (see figs 6, 7, 8, 9):
5.4.7 Attenuation
Figure 6 shows the attenuation as a function of frequency at a center frequency at a center frequency of
80Hz.
5.4.8 Center Frequency
Figure 7 shows the four possible center frequencies: 60,70,80 and 100Hz.
5.4.9 Quality Factors
Figure 8 shows the four possible quality factors: 1, 1.25, 1.5 and 2.
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