Dot crawl

Hailing from the English term cross luminance [ kɹɒs lu ː mɪnəns ], colloquially in English and dot crawl, describes fine, moving, not present in the original scene structures in a color television image, color information is mistakenly interpreted as brightness. These occur in highly colored areas, and particularly on vertical color edges where two strongly colored surfaces, but are contiguous with opposite colors - as when a red -clad boxer is shown in a blue boxing ring, the effects are quite clearly at the edges of his clothes. The complementary case (brightness mistakenly referred to as color ) are called cross color.

Technical cause

Cause of cross luminance is that the receiver parts of the color carrier signal ( Color) as a brightness signal ( Luminance ) be misinterpreted, especially due to poor insulation in their design simple composite connections and resulting crosstalk of the signal from one channel to the other.

PAL vs. NTSC

The effect is in favor of the better color stability in PAL than in NTSC significantly more pronounced, since NTSC displacement by a full half-line of the color sub- carrier is formed, but for PAL only a quarter of the line. For the deflection of the more common for NTSC color errors in the less conspicuous saturation namely a continuous switching of the phase of V ( = red signal of the image) for PAL takes place, which acts as a re- modulation at half the line frequency ( therefore, in PAL quarter lines instead of half-line ). Unlike NTSC thus formed in the same space of a half-line is not a beam of I - (Cyan Orange ratio) and Q ( magenta-green ratio ) component, but two different for U ' ( blue) and V' ( red ) of each a quarter line.

If you would now choose as a NTSC entire half line offset for the color subcarrier, always would be one of the two components exactly on the luminance spectrum. Therefore, in all the PAL color carrier modulation is shifted only by a quarter of a line. Lay in the gap in the luminance spectrum, in the previously another carrier ( for the transmission of the color signal IQ), are now two (V- and U - red blue). The result is that the carriers are spatially closer to each other and therefore speak about more often.

Amplification by the connection type

Existing cross- luminance artifacts in a PAL signal are each used analog connector from the VCR, DVD player, satellite receiver, video camera or the ( terrestrial ) antenna or (with cable TV) antenna socket towards the TV or the VCR in varying degrees strengthened.

  • With a component video cable cross luminance is of course not reinforced, which can be received with a SCART connector, many televisions, as it is prescribed in the SCART norm.
  • Relatively low, the gain of cross luminance by an S- video cable, which nevertheless still color and brightness are transmitted on separate channels. Also SCART cables carry an S-video signal to a TV that can understand such (which is not at all devices with SCART socket is the case entirely, since S - video has not been officially added to the SCART standard).
  • A large reinforcement of cross luminance occurs at a composite connection, where color and brightness are only transmitted on a single channel. The most common type of this connection is the so-called Belling -Lee connector, better known as the antenna cable and SCART also carries such a signal to your TV, just understand this.
  • Due to technical defects worst, although the signal itself is composite, is the connection with only appropriate suitable for audio applications RCA connectors; the strengthening of cross- luminance artifacts here is the greatest.

For digital connections (eg Firewire and USB cameras ) arises naturally no reinforcement of cross luminance.

Distance

Cross luminance can be nowadays with built- in video recorders many comb filters ( Comb) significantly reduce, especially the newer digital comb filter are quite successful in this regard. Simple but implementations can lead to artifacts, eg strings of pearls.

2D comb filter

  • Taking into account only the current frame (horizontal and vertical dimension, therefore 2D)
  • No interference when above and below lines are similar
  • Remaining faults on horizontal edges

3D comb filter

  • Taking into account the current and one or two previous images ( horizontal, vertical and temporary dimension, therefore 3D)
  • Disorders in addition to horizontal edges are reduced in static images clippings

In digital files

The reduction of cross- luminance artifacts and its complementary errors Cross Color after digitization alone at the software level is almost impossible already, if you ( denoising English) not by simply denoising will make the image blurry. That alone really for NTSC material intended Freeware plugin for VirtualDub Dotcrawl is to reduce these artifacts suitable for the AviSynth script language there is also the slightly better performing filter Guavacomb and DeDot.

Composite Video Compression artifact
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