CXA1784AS Datasheet, PDF (22/38 Pages) Sony Corporation – US Audio Multiplexing Decoder

CXA1784AS
(TVOUT-L) (TVOUT-R)
35 34
SW1
(AUXIN-L)
38
37
(AUXIN-R)
SW2
PREVOL
SURROUND
BASS
TREBLE
-
+
(Lch) (Rch)
from MATRIX
Fig. 4. Sound processor block
(LSOUT-L)
VOL-L 6
(LSOUT-R)
VOL-R 5
(SURROUT)
VOL-S 4
(1) L + R (MAIN)
After the audio multiplexing signal input from COMPIN (Pin 17) passes through MVCA, the SAP signal
and telemetry signal are suppressed by STEREO LPF. Next, the pilot signals are canceled. Finally, the
L - R signal and SAP signal are removed by MAIN LPF, and frequency characteristics are flattened
(de-emphasized) and input to the matrix.
(2) L - R (SUB)
The L - R signal follows the same course as L + R before the pilot signal is canceled. L - R has no
carrier signal, as it is a suppressed-carrier double-sideband amplitude modulated signal (DSB-AM
modulated). For this reason, the pilot signal is used to regenerate the carrier signal (quasi-sine wave)
to be used for the demodulation of the L - R signal. In the last stage, the residual high frequency
components are removed by SUB LPF and the L - R signal is input to the dbx-TV block via the NRSW
circuit after passing through SUBVCA.
(3) SAP
SAP is an FM signal using 5fH as a carrier as shown in the Fig. 1. First, the SAP signal only is
extracted using SAP BPF. Then, this is subjected to FM detection. Finally, residual high frequency
components are removed and frequency characteristics flattened using SAP LPF, and the SAP signal
is input to the dbx-TV block via the NRSW circuit. When there is no SAP signal, the Pin 24 output is
soft muted.
(4) Mode discrimination
Stereo discrimination is performed by detecting the pilot signal amplitude. SAP discrimination is
performed by detecting the 5fH carrier amplitude. NOISE discrimination is performed by detecting the
noise near 25 kHz after FM detection of SAP signal.
(5) dbx-TV block
Either the SAP signal or L - R signal input respectively from ST IN (Pin 20) or SAP IN (Pin 25) is
selected by the mode control and input to the dbx-TV block.
The input signal then passes through the fixed de-emphasis circuit and is applied to the variable de-
emphasis circuit. The signal output from the variable de-emphasis circuit passes through an external
capacitor and is applied to VCA (voltage control amplifier). Finally, the VCA output is converted from a
current to a voltage using an operational amplifier and then input to the matrix.
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