Revisiting the analogue video decoder: Brushing up on your comb filters
The basic function of the video decoder is to accept analogue video from a wide variety of sources such as broadcast, DVD players, cameras and video cassette recorders, in either NTSC or PAL format and still occasionally SECAM, separate it into its component parts, luminance and chrominance, and output it in some digital video standard, usually BT656, a multiplexed Y/Cb/Cr video format with embedded timing signals running at a clock rate of 27MHz.Most major semiconductor companies, for example Texas Instruments, NXP (formerly Philips Semiconductors) and Analog Devices, and a large number of smaller companies manufacture an analogue video decoder. In addition many companies have integrated this function into SoC (System on Chip) integrated circuits.
With such a large number of video decoders in the market it might seem an unnecessary indulgence to spend time looking again at the design of this fundamental but apparently obsolete building block, and certainly new designs are not appearing on the market and haven't done so for three or four years.
However the block is still a requirement of most video designs, from televisions to video recorders because large swathes of the planet are yet to convert to digital-only broadcasting and because of the legacy requirements. The block is however being marginalized with most of the design effort spent on newer features such as higher definition and 3D.
The analogue television broadcast system was designed as a complete system, from the psycho-visual compromises made at the encoding end through to the persistence of the phosphor on the cathode ray tube (CRT). The advent of large flat screen displays, whilst in many ways being the element that allowed the widespread adoption of high definition television broadcasting, placed additional demands on the analogue video decoder where conventional sources were viewed, and for a large number of viewers that was still the case; that yellow RCA plug was still in widespread use.
The most obvious result of viewing analogue video sources on a large display is that any artifacts are, of course, larger and visually more apparent. For larger displays the analogue video decoder actually has a more stringent requirement. This problem is compounded because the flat screen displays require additional processing of the analogue source before it can be properly displayed, namely de-interlacing and scaling.
The de-interlacer in particular can amplify any artifacts left from the video decoder. This is because the de-interlacer is sensitive to motion in the image and residual artifacts and noise left from the analogue decoder cannot be discriminated from real motion in the image. The result is the de-interlacer may make the wrong mode decision resulting in additional artifacts.
The majority of the video decoders use a line comb decoder to separate the luminance from the chrominance component because there is no need of a large frame size memory for such an architecture. The separation relies of the repetitive line to line phase relationship of the colour subcarrier used to encode the chrominance component.
For example, for NTSC, each line in the field has a 180° phase shift so adding or subtracting the video signal results in cancellation or reinforcement of the chrominance. That statement is only true, of course, if the colours are of exactly the same hue and even if they are we are not acting upon spatially aligned pixels because not only are we looking at pixels a line apart, but two lines apart because there is the interlaced field line that we do not have access to. Effectively we are looking at pixels 2 lines apart and the situation is worse for PAL because of the additional 90° line based subcarrier phase shift: without specialized and complex processing it is necessary to have a 4 line spacing.
It is possible to detect most of the conditions under which a line comb filter based video decoder will fail, and under these circumstances a simple notch filter is usually reverted to, but Figure 1 shows the result of not detecting this condition.
Figure 1: 'Dot Crawl': The running line of dots highlighted in the left image is caused by poor comb adapatation, failure to detect the line comb failure condition. The right image shows a decoder without these artifacts.