STV8247 Datasheet, PDF (22/149 Pages) STMicroelectronics – Digital Audio Decoder/Processor for A2 and NICAM Television/Video Recorders

Digital Demodulator
STV82x7
3.2 Demodulation
The demodulation system operates by default in Automatic mode. In this mode, the STV82x7 is able
to identify and demodulate any TV sound standard including NICAM and A2 systems (see
Table 4) without any external control via the I²C interface. It consists of the two demodulation
channels (Channel 1 = Mono Left and Channel 2 = Mono Right/Stereo) to simultaneously process
two sound carriers in order to handle all transmission modes (stereo and up to three mono
languages). The built-in Automatic Standard Recognition System (Autostandard) automatically
programs the appropriate bits in the I²C registers which are forced to Read-only mode for users (see
Section 12.1). The programming is optimized for each standard to be identified and demodulated.
Each mono and stereo standard can be removed (or added) from the List of Standards to be
recognized by programming registers AUTOSTD_STANDARD_DETECT and
AUTOSTD_STEREO_DETECT, respectively. The identified standard is displayed in register
AUTOSTD_STATUS and any change to standard is flagged to the host system via pin IRQ. This flag
must be reset by re-programming the MSBs of register AUTOSTD_CTRL while checking the
detected standard status by reading registers AUTOSTD_STATUS, NICAM_STAT and ZWT_STAT.
Moreover, the detection of Stereo mode during demodulation is also flagged in register
AUTOSTD_STATUS.
Important: L/L’ and D/K standards cannot be automatically processed because the same frequency
is used for the MONO carrier. An exclusive L/DK selection must programmed in register
AUTOSTD_CTRL. This may be externally controlled by detecting the RF modulation sign, which is
negative for all TV standards except L/L’.
To recover out-of standard FM deviations or the Sound Carrier Frequency Offset, additional I²C
controls are provided without interfering with the Automatic Standard Recognition System
(Autostandard).
DK-NICAM Overmodulation Recovery: Four different FM deviation ranges can be selected (via
register AUTOSTD_CTRL) for the DK standard while the Autostandard system remains active. The
maximum FM deviation is 500 kHz in DK Mono mode and 350 kHz in DK NICAM mode (limited by
overlapping FM and NICAM spectrum values). The demodulated signal peak level (proportional to
the FM deviation) is detected by the Peak Detector and written to registers PEAK_DET_L and
PEAK_DET_R. This value is used to implement Automatic Overmodulation Detection via an
external I²C control.
Important: Only the selection of the 50 kHz FM deviation standard is compatible with the other DK-
A2* standards (DK1, DK2 or DK3). These standards must be removed from the list of standards
(registers AUTOSTD_STANDARD_DETECT and AUTOSTD_STEREO_DETECT) when
programming larger FM deviations reserved only for DK-NICAM standards.
System Sound Type
B/G
D/K
D/K1
FM Mono
FM/NICAM
FM 2-Carrier
FM Mono
FM/NICAM
FM 2-Carrier
22/149
Table 4: Recognized Standards
Type
Name
Carrier 1
(MHz)
Carrier 2
(MHz)
FM Deviation
Nom. Max. Over
De-
emphasis
Roll
-off
(%)
Pilot
Frequency
(kHz)
5.5
5.5
A2
5.5
6.5
6.5
A2*
6.5
5.850
5.742
27 50 80
27 50 80
J17
50 µs
40
54.6875
5.850
6.258
27 50 80
J17
50 µs
40
54.6875