TV Picture Format / Color System Combinations

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 color 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.

In the section below, I have tried to create as objective as possible a comparison of these various pros and cons. The techniques that are used to overcome these limitations are discussed elsewhere.

NTSC/525 Advantages

• Higher Frame Rate – Use of 30 frames per second (really 29.97) reduces visible flicker.
• Atomic Color 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 color sub-carrier at 3.58MHz, picture defects such as moire, cross-color, 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 color subcarrier phase cause shifts in the displayed color, requiring that the TV receivers be equipped 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 colors approximating to flesh tone into a "standard" fleshtone, thus hiding the effects of hue fluctuation. This does mean however that a certain range of color 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 color 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 opposite 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 color 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 noticeable 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 noticeable 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 Color Editing Accuracy – Due to the alternation of the phase of the color signal, the phase and the color 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 Color Saturation – Since PAL achieves accurate color through cancelling out phase differences between the two signals, the act of cancelling out errors can reduce the color 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 consistent saturation of color 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 color 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-colored objects.
• Lower monochrome Bandwidth – Due to one of the two color sub-carriers being at 4.25MHz (in the French version), a lower bandwidth 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.