AD1970 查看數據表（PDF） - Analog Devices
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AD1970 Datasheet PDF : 20 Pages
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AD1970
SIGNAL PROCESSING
BACKGROUND OF BTSC
BTSC is the name of the standard for adding stereo audio capa-
bility to the US television system. It is in many ways similar to
the algorithm used for FM stereo broadcasts, with the addition
of a sophisticated compressor circuit to improve the signal-to-
noise ratio.
To maintain compatibility with non-BTSC TV receivers, the
processing of mono (L = R) signals is unchanged from the
original pre-BTSC system. The L + R signal is applied to a 75 µs
pre-emphasis filter, and is then applied to a 4.5 MHz FM mod-
ulator, which is later added to the video signal to create a
composite video signal.
Stereo capability is added by taking the L − R signal, applying it
to a 2-band dynamic compressor, and then multiplying this
signal by a carrier signal at twice the horizontal scanning rate
(Fh), or about 2 × 15.734 kHz. This multiplication is known as
double sideband, suppressed-carrier modulation, and it
effectively translates the compressed L − R spectrum up in
frequency so that it sits above the audio band (see Figure 3).
For the receiver to recover this L − R signal, a pilot tone at the
horizontal rate is added to the signal. The receiver has a PLL
that locks to this pilot and generates a signal at the carrier
frequency. This signal is then used to multiply the composite
BTSC-encoded signal, which translates this component back
down to baseband. Once the L + R and L − R signals are
recovered, a simple addition/subtraction circuit (sometimes
referred to as the matrix) can be used to recover the right signal.
Since the pilot tone is added at 15.734 kHz, it is necessary to
reduce the bandwidth of the signal so that audio signals cannot
interfere with the pilot tone. In the AD1970, the bandwidth is
limited to 14 kHz; above this frequency, the response decays
very rapidly.
PERFORMANCE FACTORS
To maintain good separation between the left and right
channels, it is necessary to closely match the filtering and
companding standards set forth in the standard (FCC OET60).
Even small errors can result in poor performance. The AD1970
has been programmed to match these standards as accurately as
possible. Typical separation numbers range from 30 dB at
frequencies below 500 Hz to 15 dB at 13.5 kHz. Measuring these
numbers can be difficult, since significant differences exist
between many units sold as reference decoders, which are all
implemented with analog components.
L–R
COMPRESSOR
L
MATRIX
R
75µs
PRE-EMPH
L+R
FILTER
2× Fh CARRIER
OSCILLATOR
Fh PILOT
GAIN
1/X
RMS
DETECT
BANDPASS
SECOND
ORDER
L-R IN
PRE-EMPH
SECOND
ORDER
NONLINEAR
FORMULA
RMS
DETECT
GAIN
BANDPASS
FOURTH
ORDER
SPECTRAL TILT
FILTER
LPF
EIGHT
ORDER
Figure 3. Signal Processing Flow
TO MAIN
DAC ALGORITHM
FLOW
Rev. 0 | Page 10 of 20
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