| 摘要 |
<p>This invention relates to a subscription TV system having means for decoding a generally constant frequency sine wave scrambled television signal which uses an automatic gain control circuit in a closed loop. The scrambling, applied at the head end of the television system, applies a greater scrabling level to the audio carrier than it does to the video carrier. The unscrambling signal, the detected audio carrier scrambling signal, is combined with the scrambled signal in an amplifier circuit, the output of which is a basic television signal. There is a residual of the scrambling sine wave remaining in the audio carrier output, after unscrambling, but little or no residual scrambling remaining on the video carrier. The residual audio carrier scrambling signal is detected and amplified for application as the unscrambling signal.</p> |
