Section Headings
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
|
EIA
| NTSC
| 3.58MHz
|
PAL-M
| 29.97/59.94
| 4:3
| 525
|
EIA
| PAL
| 3.58MHz
|
SECAM-M
| 29.97/59.94
| 4:3
| 525
|
EIA
| SECAM
| ?.??MHz
|
PAL
| 25/50
| 4:3
| 625
|
CCIR
| PAL
| 4.43MHz
|
SECAM
| 25/50
| 4:3
| 625
|
CCIR
| SECAM
| 4.25/4.40MHz
|
D-MAC
| 25/50
| 4:3 or 16:9
| 625
|
D-MAC
| D-MAC
| N/A
|
PALplus
| 25/50
| 16:9
| 625
|
CCIR
| PAL
| 4.43MHz
|
HiVision
| 60/120
| 16:9
| 1125
|
HiVision
| MUSE
| Unknown
|
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.
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.
- 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.
- 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.
- 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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