Try signal conditioning the audio video for an RF modulator


Posted on Feb 5, 2014

Despite the need for an IC, this ubiquitous interface between the modulator and audio-video signals has not been reduced to an integrated circuit. The major reasons for that deficiency are the difficulty of such a design, the variations required for different standards, and the variable levels required by the modulator itself. The alternative to a


Try signal conditioning the audio video for an RF modulator
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n IC interface is a discrete design. The signal-conditioning requirements include lowpass and notch filtering of the video, group-delay compensation for the video, pre-emphasis for the audio, and (to adjust the modulation level) level controls for both audio and video. Because many cable and satellite receivers, VCRs, DVDs, and TVs don`t fully comply with these signal-conditioning requirements, the modulated signals of channels 3-4 have poorer quality than that of the baseband composite (Cvbs). The discussion below explains the interface requirements and how to meet them using standard op amps and discrete components. The resulting low-cost circuitry can also provide rear-panel outputs for most audio/video (A/V) appliances. For driving RF modulators in NTSC and PAL systems, the allowable video group-delay variation and required audio pre-emphasis are clearly specified by ITU Recommendation BT. 470-6. Most of the other specifications, not as obvious, are summarized and extrapolated to include other, unstated design specifications (Tables 1-2). Those table entries are based on typical back-panel outputs for a TV, DVD, or set-top box having both baseband and RF-modulated A/V outputs (Figure 1). Some of these requirements depend on the source. If the signals to the modulator come from a DAC, for example, they obviously need reconstruction filtering to prevent out-of-band modulation and to remove artifacts, noise, and aliased...




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