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Skriven 2009-05-04 15:41:00 av JOE NICHOLSON (1:123/140)
Ärende: TV Systems 1/3
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The Relative Merits of TV Systems

The differences between each of the main TV systems are not quite as clear cut
as one might at first imagine. While NTSC has a reputation for poor colour
accuracy, this is only really true of broadcast television and as a video
format it has some distinct advantages over the other systems. All these
systems are a compromise and many efforts have been made over the years to
address the shortcomings in each of the systems.


NTSC/525 Advantages

    * Higher Frame Rate - Use of 30 frames per second (really 29.97) reduces
visible flicker.
    * Atomic Colour Edits - With NTSC it is possible to edit at any 4 field
boundary point without disturbing the colour signal.
    * Less inherent picture noise - Almost all pieces of video equipment
achieve better signal to noise characteristics in their NTSC/525 form than in
their PAL/625. 

NTSC/525 Disadvantages

    * Lower Number of Scan Lines - Reduced clarity on large screen TVs, line
structure more visible.
    * Smaller Luminance Signal Bandwidth - Due to the placing of the colour
sub-carrier at 3.58MHz, picture defects such as moire, cross-colour, and dot
interference become more pronounced. This is because of the greater likelihood
of interaction with the monochrome picture signal at the lower sub-carrier
frequency.
    * Susceptablity to Hue Fluctuation - Variations in the colour subcarrier
phase cause shifts in the displayed colour, requiring that the TV receivers be
equiped with a Hue adjustment to compensate.
    * Lower Gamma Ratio - The gamma value for NTSC/525 is set at 2.2 as opposed
to the slightly higher 2.8 defined for PAL/625. This means that PAL/625 can
produce pictures of greater contrast.
    * Undesirable Automatic Features - Many NTSC TV receivers feature an
Auto-Tint circuit to make hue fluctuations less visible to uncritical viewers.
This circuit changes all colours approximating to flesh tone into a "standard"
fleshtone, thus hiding the effects of hue fluctuation. This does mean however
that a certain range of colour shades cannot be displayed correctly by these
sets. Up-market models often have this (mis)feature switchable, cheaper sets do
not. 



PAL/625 Advantages

    * Greater Number of Scan Lines - more picture detail.
    * Wider Luminance Signal Bandwidth - The placing of the colour Sub-Carrier
at 4.43MHz allows a larger bandwidth of monochrome information to be reproduced
than with NTSC/525.
    * Stable Hues - Due to reversal of sub-carrier phase on alternate lines,
any phase error will be corrected by an equal and oposite error on the next
line, correcting the original error. In early PAL implementations it was left
to the low resolution of the human eye's colour abilities to provide the
averaging effect; it is now done with a delay line.
    * Higher Gamma Ratio - The gamma value for PAL/625 is set at 2.8 as opposed
to the lower 2.2 figure of NTSC/525. This permits a higher level of contrast
than on NTSC/525 signals. This is particularly noticable when using
multi-standard equipment as the contrast and brightness settings need to be
changed to give a similar look to signals of the two formats. 

PAL/625 Disadvantages

    * More Flicker - Due to the lower frame rate, flicker is more noticable on
PAL/625 transmissions; particularly so for people used to viewing NTSC/525
signals.
    * Lower Signal to Noise Ratio - The higher bandwidth requirements cause
PAL/625 equipment to have slightly worse signal to noise performance than it's
equivalent NTSC/525 version.
    * Loss of Colour Editing Accuracy - Due to the alternation of the phase of
the colour signal, the phase and the colour signal only reach a common point
once every 8 fields/4 frames. This means that edits can only be performed to an
accuracy of +/- 4 frames (8 fields).
    * Variable Colour Saturation - Since PAL achieves accurate colour through
cancelling out phase differences between the two signals, the act of cancelling
out errors can reduce the colour saturation while holding the hue stable.
Fortunately, the human eye is far less sensitive to saturation variations than
to hue variations, so this is very much the lesser of two evils. 



SECAM/625 Advantages

    * Stable Hues and Constant Saturation - SECAM shares with PAL the ability
to render images with the correct hue, and goes a step further in ensuring
consistant saturation of colour as well.
    * Higher Number of Scan Lines - SECAM shares with PAL/625, the higher
number of scan lines than NTSC/525. 

SECAM/625 Disadvantages

    * Greater Flicker - (See PAL/625)
    * Mixing of two synchronous SECAM colour signals is not possible - Most TV
studios in SECAM countries originate in PAL and transcode prior to
broadcasting. More advanced home systems such as SuperVHS, Hi-8, and LaserDisc
work internally in PAL and transcode on replay in SECAM market models.
    * Patterning Effects - The FM subcarrier causes patterning effects even on
non-coloured objects.
    * Lower monochrome Bandwidth - Due to one of the two colour sub-carriers
being at 4.25MHz (in the French Version), a lower bandwith of monochrome signal
can be carried.
    * Incompatibility between different versions of SECAM - SECAM being at
least partially politically inspired, has a wide range of variants, many of
which are incompatible with each other. For example between French SECAM with
uses FM subcarrier, and MESECAM which uses an AM subcarrier. 
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