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AD1970 Scheda tecnica(PDF) 10 Page - Analog Devices |
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AD1970 Scheda tecnica(HTML) 10 Page - Analog Devices |
10 / 20 page AD1970 Rev. 0 | Page 10 of 20 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. MATRIX COMPRESSOR 75 µs PRE-EMPH FILTER OSCILLATOR RMS DETECT PRE-EMPH SECOND ORDER LPF EIGHT ORDER GAIN BANDPASS SECOND ORDER GAIN BANDPASS FOURTH ORDER 1/X RMS DETECT NONLINEAR FORMULA L-R IN 2 × Fh CARRIER Fh PILOT L–R L+R L R MAIN ALGORITHM FLOW TO DAC SPECTRAL TILT FILTER Figure 3. Signal Processing Flow |
Codice articolo simile - AD1970_15 |
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Descrizione simile - AD1970_15 |
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