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AD71028JSTRL Folha de dados(PDF) 10 Page - Analog Devices |
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AD71028JSTRL Folha de dados(HTML) 10 Page - Analog Devices |
10 / 20 page AD71028 Rev. 0 | Page 10 of 20 SIGNAL PROCESSING COMPRESSOR MATRIX 75 µs PRE-EMPHASIS FILTER OSCILLATOR TO DAC 2 × Fh CARRIER Fh PILOT L R L + R L– R Figure 3. Signal Processing Flow 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 (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 (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. 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. |
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