All-Format Analog Front Ends Handle Video And PC Formats

Incorrect luma/chroma separation brings undesirable artifacts to the picture. Luma frequencies within the bandwidth of chroma are demodulated and appear as false color in the picture. Test patterns, like vertical black and white lines, plus concentric circles, help in evaluating the comb filter's cross-color suppression.

Chroma that is output in the luma channel produces crawling dot artifacts in the picture. Test patterns containing horizontal-color bar boundaries are used to evaluate the suppression of such cross-luma by the comb filter. Noise and clock jitter also affect the performance of the comb filter.

Figure 2 illustrates false color and artifacts that occur in an NTSC test pattern when the comb filter is switched off. The effect of false color (cross color) is clearly visible in the area of the high-frequency vertical lines. Because the high-frequency luma is present around the color subcarrier frequency, it's erroneously decoded as chroma by the decoder when only a simple notch filter (also called a chroma trap) separates both.

Figure 3 shows the result of a digital comb filter on TI's TVP5145 video decoder. There's much better separation of luma and chroma (no false color), along with reduced false luma that would come from color information incorrectly demodulated as luma.

To simplify the design of a universal video/graphics input circuit, an "all-in-one" video decoder/AFE would integrate several of the discrete ICs in Figure 1. The video decoder module for NTSC/PAL/SECAM composite and S-video would be combined with the triple ADC channels for component video and PC graphics.

Such a universal front end needs to handle color spaces for both component video or graphics inputs (YCbCr and RGB) and convert between both to provide a consistent output interface to the next device. Furthermore, an embedded-controller function implements many system features, like format detection, reliable sync separation, and timing generation.

An example of such a universal front end is the TVP5200 "all-format" decoder. On this device, the exact functionality of an embedded RISC CPU can be configured at power-up via microcode downloaded through a host-port interface. With 165-MHz high-speed ADC channels, the device supports all graphics/video formats in the table, up to UXGA at 60 Hz. A separate 10-bit ADC channel performs high-quality, composite-video decoding of standard and nonstandard video signals, including Macrovision encoded video.

The authors want to thank Li Zhang for his assistance in providing the captured video images.