AD71028JST AD [Analog Devices], AD71028JST Datasheet - Page 10

• Manufacturer Part Number: AD71028JST
• Description: Dual Digital BTSC Encoder with Integrated DAC
• Manufacturer: AD [Analog Devices]

AD71028
SIGNAL PROCESSING
BACKGROUND OF BTSC
BTSC is the name of the standard for adding stereo audio
capability 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.
The processing of mono (L = R) signals is unchanged from the
original pre-BTSC system in order to maintain compatibility
with non-BTSC TV receivers. The L + R signal is applied to a
75 µs pre-emphasis filter, and is then applied to a 4.5 MHz FM
modulator, which is later added into 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
(F
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 (Figure 3).
In order 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. The L – R signal is then applied to a 2-band
expander, which is the complement to the earlier compressor
step. 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 L and R signal.
Since the pilot tone is added at 15.734 kHz, it is necessary to
reduce the signal’s bandwidth so that audio signals cannot
interfere with the pilot tone. In the AD71028, the bandwidth is
limited to 14 kHz; above this frequency, the response decays
very rapidly.
h
), or about 2 × 15.734 kHz. This multiplication is known as
R
L
MATRIX
L – R
L + R
PRE-EMPHASIS
FILTER
75µs
COMPRESSOR
Figure 3. Signal Processing Flow
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OSCILLATOR
PERFORMANCE FACTORS
In order to maintain good separation between left and right, it is
necessary to closely match the filtering and companding stan-
dards set forth in the standard (FCC OET60). Even small errors
can result in poor performance. The AD71028 has been pro-
grammed to match these standards as accurately as possible.
Separation typically ranges from 30 dB at frequencies below
1 kHz to 15 dB at 14 kHz. Measuring these numbers can be
difficult as significant differences exist between many so-called
reference decoders, which are all implemented with analog
components.
SEPARATION ALIGNMENT
The BTSC encoder outputs are all specified in terms of the
deviation of the FM 4.5 MHz carrier. For the AD71028, a digital
input level of 0 dB (mono signal) should cause a carrier
deviation of ±25 kHz without the 75 µs pre-emphasis filter. In
practice, the pre-emphasis filter may be left in for this adjust-
ment, as long as the frequency is low enough to not be affected
by the pre-emphasis filter. It is critical to maintain the proper
gain relationship between the BTSC encoder and the 4.5 MHz
FM modulator. A common mistake is to assume that changing
the gain between the BTSC encoder output and the FM
modulator input has the same effect as changing the audio
input level going in to the BTSC encoder. The presence of a
complicated 2-band nonlinear dynamics processor means that
the encoder output must be connected to the decoder input
(through the FM modulation/demodulation process) with a
known gain. If this gain is changed, the separation will
significantly suffer.
When measuring the AD71028 on the bench, it is possible to
use a BTSC reference decoder box, so that the FM modulation/
demodulation process may be skipped. These units have a
method of adjusting the input voltage sensitivity to achieve best
separation. The output level of the AD71028 can also be
adjusted over a wide range using either the SPI control port or
by adjusting the values of the components used in the external
analog low-pass filter that is between the BTSC encoder output
and the input to the FM modulator.
2× Fh CARRIER
Fh PILOT
TO
DAC