AD724is a low-cost RGB-to-NTSC/PAL encoder that converts red, green, and blue component signals into corresponding luminance (baseband amplitude) and chromaticity (subcarrier amplitude and phase) signals that comply with NTSC or PAL standards. At the same time, the two outputs are combined to provide a composite video output. All three outputs can simultaneously drive 75 Ω, reverse-terminated cables. All logic inputs are CMOS compatible. The chip operates from a single +5 V supply; No external delay lines or filters are required. The device can be shut down when not in use.
The AD724 accepts either FSC or 4FSC clocks. When no clock is available, a low-cost parallel resonant crystal (3.58 MHz (NTSC) or 4.43 MHz (PAL)) together with the device's on-chip oscillator generates the necessary subcarrier clock. The AD724 also accepts a subcarrier clock from an external video source.
The interface to the graphics controller is simple: the on-chip logic "XNOR" accepts the available VSYNC and HSYNC signals and generates the composite sync (CSYNC) signals on-chip. If available, the AD724 can also accept standard CSYNC signals by connecting VSYNC to +5 V and applying CSYNC to the HSYNC pin. The device has built-in decoding logic that identifies valid horizontal sync pulses for proper insertion of bursts.
The delay of the you and V chroma filters is matched by the on-chip sampled data delay line in the Y signal path. To prevent aliasing, the delay line is preceded by a 5 MHz prefilter followed by a 5 MHz postfilter to suppress harmonics in the output. These low-pass filters are optimized for minimal pulse overshoot. The overall luminance delay is timing-consistent with respect to chrominance and can be fed directly into the TV baseband. The AD724 is available in a space-saving SOIC package and is specified over the 0°C to +70°C commercial temperature range.