tda8295 NXP Semiconductors, tda8295 Datasheet - Page 11

• Manufacturer Part Number: tda8295
• Description: Tda8295 Digital Global Standard Low If Demodulator For Analog Tv And Fm Radio
• Manufacturer: NXP Semiconductors

NXP Semiconductors
TDA8295_1
Product data sheet
8.5 Upsampler and video DAC
8.6 SSIF/mono sound processing
8.7 Tuner IF AGC
Next comes the equalizer circuit to remove the transmitter group delay predistortion.
A video leveling stage follows, which brings the output within the SCART specification
( 3dB overall), despite heavy overmodulation. The response time is made very slow.
Finally, a video equalizer allows to compensate the perhaps non-flat frequency response
from the tuner or to change the overall video response according to customer wish (i.e.
peaking or early roll-off).
The filtered and compensated CVBS signal is connected to the oversampled 10-bit video
DAC (f
complicated LCR postfiltering by a simple first-order RC low-pass filter to remove the DAC
image frequencies sufficiently. This holds also for the sound DAC, described in
Section
The complex signal is routed via a band-pass and interpolation filter to the 10-bit sound
DAC for the recovery of the second sound carriers (SSIF). A very sharp band-pass filter at
5.5 MHz is added in the FM Radio mode to remove neighbor channels. This also eases
the dynamic burden on the following ADC in the demodulator/decoder chip. The
afore-mentioned high-selectivity band-pass, which replaces the former ceramic filter, is
located behind a frequency shifter. In there, the incoming wanted FM radio channel from
the Silicon Tuner is changed from 1.25 MHz to 5.5 MHz.
Moreover, the complex signal is demodulated in a linear CORDIC detector and low-pass
filtered to attenuate the video spectrum and the second sound carrier, respectively other
disturbers above the intercarrier. The output of the linear CORDIC (phase information) is
differentiated for getting the demodulated FM audio. The AM demodulation is executed in
a synchronous fashion by using a narrow-band PLL demodulator.
A de-emphasis filter is implemented for FM standards, before the audio is interpolated to
108 MHz as in the CVBS case.
The mono audio is made available in the sound DAC via an I
case the intercarrier path is not used for driving an external stereo demodulator.
However, if the mono audio output has to meet the SCART specification, an external
cheap operational amplifier with 12 dB gain becomes necessary, because the low supply
voltage for the TDA8295 doesn’t allow such high levels like 2 V (RMS) maximum.
This AGC controls the tuner IF AGC amplifier in the TDA8275A or TDA1827x in such a
way, that the IF ADC is always running with a permanent headroom of 3 dB for the sum of
all signals present at the ADC input. This ensures an always optimal exploitation of the
dynamic range in the IF ADC.
The detection is done in peak Search mode during a field period. The attack time is made
much faster than the decay time in order to avoid transient clipping effects in the IF ADC.
This can happen during channel change or airplane flutter conditions.
s
= 108 MHz) via an interpolation stage. The strong oversampling replaces a former
8.6.
Digital global standard low IF demodulator for analog TV and FM radio
Rev. 01 — 4 February 2008
2
C-bus controlled selector in
TDA8295
© NXP B.V. 2008. All rights reserved.
11 of 77