DScaler -- Temporal Comb Filter

| Temporal Comb Filter

This filter removes dot crawl and some kinds of shimmering. The effect is most noticeable on computer generated images like subtitles, weather maps, and station logos using a composite or poorly separated S-Video signal. Normal video is more subtly improved.

Who Should Use It?

The comb filter is pretty slow, requiring roughly 225 MHz of processor time. So unless you’ve got a Pentium 3, Athlon, or better, it’s probably not worth the cost.

If your computer is fast enough, you should run the comb filter whenever you’re watching a composite signal. That’s what you have if you’re using an internal TV tuner or an RCA connector.

You should also use the comb filter if your S-Video source isn’t separated very well. DVD players should have great separation if you connect them with S-Video. LaserDiscs and S-VHS VCRs vary — If they have a “3D comb filter,” then dot crawl should already be taken care of. If you have any other kind of comb filter, then the Temporal Comb may or may not help — you’ll have to judge for yourself. If your S-VHS VCR or LaserDisc does not have a comb filter, then the Temporal Comb filter should be very worthwhile.

Note that the more recently supported cards — The SAA7134 and CX2388X — both have a built-in 2D comb filter. But since those filters work spatially, and this one works temporally, there can be some benifit to using both.

The comb filter helps a great deal with composite (but not S-Video) connected games for recent consoles such as the PS2 — but it doesn’t do a thing for video from older consoles like the Super Nintendo or original PlayStation. To get rid of color crosstalk from older consoles, turn off this filter and use the Old Game deinterlacer, instead.

SECAM handles color in a very different way than PAL or NTSC. Except for people in Saudi Arabia (which uses SECAM with PAL-style color encoding), SECAM users should turn this filter off.

The Settings

I’m going to describe how to configure this filter conservatively, so you can forget about it afterward. If you like to fiddle with sliders, you can instead use much more aggressive settings (lower Recall and Shimmering values) and turn them up only when you have to.

Recall of Past Shimmering

This is the really important setting. It decides just how far back into the past the filter will look in order to confirm that there’s really some shimmering going on. Set it high (above 75%) to avoid artifacts in scenes with repetitive motion. But don’t set it too high (above 95%), or it’ll take a long time for the filter to kick in.

When you change this setting, it’ll take a moment before the screen can adjust. Really high settings (above 90%) need especially long to take.

Shimmering to Activate

This setting has a much subtler effect. If you set it high, the filter will only correct when there has been uninterrupted shimmering. Set it low, and the filter will tolerate more interruptions — but will also be a little more susceptible to artifacts.

If you’re running the Gradual or Temporal noise filter, too, you can afford to set this low — 30% works well for me. That’s a good idea, since it will allow the filter to correct more subtle shimmering. Without one of those filters, you’ll need to set it pretty high (about 70%) to avoid transient spots where dot crawl has just disappeared. If you don’t care about shimmering and just want to take care of dot crawl, you can safely set this near 100%.

Maximum Color Variation

This option lets you choose just how much the color can change before the filter decides the difference must be due to motion. It can be set much higher than the thresholds in the Temporal Noise Filter, since this filter uses a stricter motion detection method. If you’ve enabled Even Luma Peaking and Odd Luma Peaking (in the Advanced Video Flags dialog), you’ll need to set this very high — to 45 or 50. With peaking disabled, you can drop this setting to around 35. You can lower it by about 10 points if you turn on High Quality.

The higher you set this, the more likely you are to see transient spots. So if you set this really high, you’ll want to increase Recall and/or Shimmering.

There isn’t an intuitive way to choose the Maximum Color Variation, so here’s a guide to help you set it:

- First, tune in the worst dot crawl or rainbows you can find. In the US, CNN Headlines is a dependable source, as are the “T” in the Country Music Station’s logo and the radar maps on the Weather Channel. If you’ve got a video reference DVD, connect it via composite and show a still with neighboring green and magenta color bars.
- Turn Recall and Shimmering all the way down. Turn Maximum Color Variation up really high (to about 80). The artifacts should disappear. (Well, bad dot crawl does still leave a residual shimmer unless you enable High Quality.)
- Move Maximum Color Variation down until the dot crawl or shimmering returns. Then raise it until the artifacts disappear, again. You’ve now got yourself the best value for it.
- Turn Recall and Shimmering back up to more reasonable values.

High Quality

This option improves the accuracy and colors — but slows things down, and uses an additional 2 MB (for NTSC) or 4 MB (for PAL) of memory (that’s on top of the ~400K it always needs).

As with Recall, you’ll need to wait a moment before this setting takes effect.

If you are using a SAA7134 based input card, then you must use the High Quality mode — otherwise this filter won’t work.

Video Mode

Usually, people in North America and Japan should set this to NTSC, while everyone else should set this to PAL. If this is set incorrectly, the comb filter won’t work at all.

Fast Memory Access

This is a compatibility setting which should normally be turned on.

Looking for Trouble

If you want to find artifacts to help you pick settings, there are a couple types of material which are especially problematic. Strobe effects in music videos may cause fits. But the very worst case is the ticker tape on CNBC (on US Cable), which goes by at the right speed to fool the filter. Worse yet, it has bad (but moving) dot crawl. You’ll need to set the Recall option pretty high (about 85%) to avoid problems with it.

What the Heck is a Comb Filter?

In technical terms, it averages out chrominance/luminance crosstalk caused by imperfect separation from a composite signal. It’s called a “comb” for a really obscure reason — because the frequency response looks a (very little) bit like a comb. Apparently, signal processing engineers are obsessed with grooming.

Here’s an example of the problem, taken from a US Cable TV broadcast with an internal tuner. There’s heavy dot crawl in the weather map and CNN logo, and more subtle color problems in and around the text. On your screen, those dots would seem to be moving slowly upward. →

(Try to ignore the JPEG artifacts.)

And here’s the effect of the Temporal Comb Filter on the same (well, a really similar) picture. This was taken with the default settings. →
The comb filter also helps more subtly with edges in cartoons and normal video. This first picture shows “Jack” without filtering. ←
...and here he is with the comb filter enabled. Note the smoother boundary between his head/neck and the background. (Ignore the stuff under his lip — it comes from the image compression.) This picture was taken with High Quality turned on. ←

But this comb filter only works temporally, not spatially. As a result, it does not help with dot crawl and shimmering in moving images.

Note that this filter is not in any way related to “comb” in the sense used by many video folks! It’s not supposed to reduce weave artifacts (lagging horizontal lines) at all. For that, you need to take a look at the deinterlacing methods.

Version History

With DScaler 4.1, the comb filter now works for PAL as well as NTSC signals. Also, the Trade Speed for Accuracy option was renamed High Quality, so I wouldn’t have to type as much.

With DScaler 4.0, reduction of chroma shimmering is improved. The settings are also more finely scaled, especially when Recall is set fairly low.