January Meeting

What! Hanging Dots?
Fortel DEC-312S Decoder/Synchronizer

Basic Comb Filters
A line-by-line half-cycle inversion was added to the NTSC black and white signal when color was introduced to cancel chroma artifacts. The two line comb filter takes advantage of this by adding the two adjacent lines to cancel the chroma and subtracting the same lines to cancel the luma. The three line comb filter averages the lines above and below a center line and adds or subtracts to achieve better vertical transitions. PAL comb filters add and subtract different lines but the principle is basically the same.

Adaptive Comb Filters
Line to line color transitions leave smearing and Òhanging dotsÓ as a leftover of the basic comb filter subtractive process. Adaptive comb filters have a detector that switches to a band split filter to separate the low frequency luma from the chroma and the high frequency luma when it detects a vertical transition. This eliminates the artifacts when it works right and is referred to as Òcomb failÓ mode because the comb filter is effectively taken out of the process.

Still not perfect, Fortel comb filters were developed around 1983 that could choose between subtractive and additive lines depending whether there was a horizontal or a vertical color transition. The solution, while a great improvement, added yet another artifact. Sometimes the luma edge was doubled. It would take low cost programmable logic chips to solve the problem. Raytheon introduced a comb filter decoder chip that used this technique. They demodulated the chroma and filtered it to have better information to adapt the comb. The chip works fairly well but the tradeoffs still apply. Set too high a threshold and cause Òhanging dotÓ luma doubling. Set too low a threshold and cross color and softness still surfaces.

On most composite TVs this is not a problem but it is highly visible on digital monitors and large screen TVs. Digital viewers will be able to see the difference between digitally originated programs and those decoded from composite recordings where the luma/chroma separation is not very good.

James Griffin of FortelDTV

Fortel DTV is introducing a technique of Dynamic Threshold Modification (DTM) to solve the problem. A pixel-by-pixel measurement is made on the low pass luma edge. This allows the comb to be sensitive to very small transitions. More comb filters are added so when the failure of the three line is about to add an artifact, a field comb filter or a frame comb filter minimizes the time the filters have to adapt to band split mode. An averaging detector kicks in on especially tricky diagonals. In addition Dual Band Processing, a technique developed by Fortel DTV, Improves the transitions at the fail point.

Im sure that James Griffin (james.griffin@forteldtv.com) would be more than happy to tell you more about the DEC-312S Decoder/Synchronizer and how it eliminates quantizing errors and artifacts from your pre-MPEG processing compression and HD up-conversion. It looked to me like it took care of almost any trace of artificts. Judge for yourself. Fortel DTV will be exhibiting their comb filter technology at NAB 2001, Booth L1261, LVCC North Hall N3. Introductions include a new companion audio synchronizer.