Saturday, May 12, 2007

Various Television Broadcast Standards

TV Picture Format/Colour System Combinations

The Main Video Signal Standards
Name Frame/Field rate Aspect Ratio Scan Lines
TV standard Colour System Subcarrier Freq
NTSC 29.97/59.94 4:3 525
PAL-M 29.97/59.94 4:3 525
SECAM-M 29.97/59.94 4:3 525
PAL 25/50 4:3 625
SECAM 25/50 4:3 625
CCIR SECAM 4.25/4.40MHz
D-MAC 25/50 4:3 or 16:9 625
PALplus 25/50 16:9 625
HiVision 60/120 16:9 1125
HiVision MUSE Unknown

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.


Life "16-20" said...

nice thought for blog...keep goin..
great posts on ur blog

Life "16-20" said...

nice thought for blog...keep goin..
great posts on ur blog