10/19/2020

4:1:1 more appropriate for 4:3, 4:2:0 more appropriate for 16:9

 In the last posting in February I came down on the side of 4:1:1 for regular normal (plain) VHS resolution capture.

I still think that is true, in particular for high velocity action sequences and where post capture Editing of the captured material may be a possibility.

 But also, the geometry of traditional SD capture renders the pixel Aspect ratio as 4:3 which is much closer to a "square" shape, than a 16:9 wide-screen pixel Aspect ratio.

 So for two reasons, 1. the limited TVL horizontal resolution of the VHS format, and 2. the shape of the pixel due to Aspect ratio.. choosing a 4:1:1 capture format may have advantages over a 4:2:0.

I am vaguely aware of a difference between pixel "display" Aspect ratio versus pixel "capture" or sampling Aspect ratio.. and that there is a terminology assigned to those differences. It can become quite confusing.

 It comes up when trying to display a wide screen formatted video in a viewer or display device without a proper attribute in the file header to indicate the intended shape of the pixels to be displayed.. aka the "wide screen attribute".

This leaves me with thinking (in simple terms) that reserving DV capture for VHS and MPEG2 capture for S-VHS is an appropriate choice.. but especially when capturing Wide-Screen movies or video that is in the wide screen display format.. it is literally focusing more of the horizontal color sampling on the horizontal wide screen major axis in that case.

Broadcast video is a whole other situation, but video capture from a VHS tape is vastly different from video capture from a Broadcast signal.. since there actually are more TVL resolution in the horzontal axis to be captured. In that case it becomes a question if 4:2:2 or 4:2:0 is the better choice since 4:1:1 would clearly not be preferred.

In the days when Broadcast signals were over NTSC this would be an interesting debate, but in the current era where transmissions are "digital" and mostly MPEG-TS or MPEG-PS it makes more sense to just capture the digital signal for storage, rather than converting back to Analog and then capturing to Digital.

 

2/10/2020

4:1:1 vs 4:2:0 for VHS to Digital Transfer

I'm still wrestling with the 4:1:1 color sub sampling versus 4:2:0 color sub sampling.

4:1:1 is used with DV (Digital Video) equipment, sometimes called DV25

4:2:0 is used with MPEG2 or DVD-video

DV is what was used for the PC and Mac standard camcorder equipment, and subsequently the DV or AVI files way back in the 2000-2004 time frame.

DVD-video came about for transfering film.

Basically 4:1:1 refer to an imaginary 4 x 2 sampling realm of 4 samples in two rows, for a total of eight pixels.

The second digit "1" refers to the fact that only one color sample is taken for every four pixels, but there are four pixels samples for their bright or darkness values. When reproduced for viewing that one color sample is spread out over the four pixels for that row. This saves on the amount of information that must be stored and makes the files smaller.

The third digit "1" refers to the fact that only one color sample is taken for every two rows of pixels vertically.. or in fact "Every" row gets one sample out of four pixels per row. In the vertical direction a sample is take for each row. That is there is more information sampled in the vertical direction, than there is in the horizontal row direction. This does not save on the amount of information that must be stored, but there are fewer rows (or lines) than there are virtual horizontal pixels.

This is known as a type of virtual compression.. or economy savings by "sampling" a courser grid of color pixels which are used to stretch over the higher resolution bright and dark grid of image pixels.

VHS and S-VHS (not to be confused with s-video) are two methods of recording Broadcast video to tape.

VHS has a virtual horizontal resolution of pixels along a horizontal line of about 250 pixels

S-VHS has a virtual horizontal resolution of pixels along a horizontal line of about 500 pixels

VHS has about 1/2 the horizontal tv vertical line resolution of S-VHS

Each of these is "less than" the sampling rate of 720 pixels.

When you digitize an S-VHS tape the number of sampling pixels along that horizontal line is more important for S-VHS than it is for VHS.

So

4:2:0

Indicates for the same four pixels horizontally "2" refers to the fact that two color samples will be taken.

But for the two rows vertically "0" indicates there will not be another or different sample taken for the second row, reducing the color sampling in the vertical direction.. effectively cutting in half the vertical resolution of the image color "wise".

4:2:0 may be more important when digitizing an S-VHS tape, than 4:1:1

But 4:1:1 maybe as good or "better" overall for VHS tapes, since 4:2:0 would be over sampling the same low TVL virtual horizontal resolution.. and VHS simply does not have anything more to give in the horizontal direction. Over sampling can better handle noise rejection, when filters are applied.. but it would not enhance the picture any better by itself. Color resolution in that direction would not be lost, and it would in fact be enhanced in the vertical row direction since more absolute color sample would be taken.. even when sampling at 720 x 480.

Also for VHS, capturing in 4:1:1 does not introduce long GOP frame dependencies making editing far easier, and less CPU intensive.

4:1:1 is optimal for capturing fast moving and unpredictable motion, where as 4:2:0 does not handle it well and requires multiple passes to reduce artifact errors when capturing high velocity motion changes.. reallocating bit rate at the cost of other parts of the picture.

4:1:1 is a higher bit rate and constant bit rate for a reason.

When transferring a Broadcast signal with a very high TVL (tv vertical line) resolution, or virtual horizontal resolution.. a case can be made for capturing only in 4:2:2 color sub sample space and then reducing it to 4:2:0 through a multi-pass process "after" editing to produce a smaller distribution format file.

When transferring an S-VHS signal with a very high TVL, 4:2:0 may offer benefits over 4:1:1

But when transferring a VHS signal with moderate "lesser" TVL, 4:1:1 should offer similar, if not superior picture quality due both to the equivalent or more appropriate color sub sampling in the horizontal direction, as well as the definitely superior (double) the color sub sampling in the vertical direction.. and the full frame non-GOP capture format.

In addition, after editing the 4:1:1 format can still be reduced through a multi-pass process to 4:2:0.

Therefore:

4:2:2 is without a doubt superior for Broadcast and S-VHS transfer

4:1:1 is arguably better for VHS transfer

And

4:2:0 remains a viable distribution format

4:2:2 could also be used  for VHS transfer, but as an "Oversampling" tool for further filter and noise rejection methods to extract the best picture possible.. since it requires specialized equipment and faster capture hardware.. its benefits over 4:1:1 may be marginal at best

If no noise filtering or further preparation before editing to take advantage of the "Oversampling" of a VHS tape.. 4:2:2 would be adding unnecessary strain on resources for little gain.

Recording direct to 4:2:0 has advantages when dedicated 4:2:0 compression hardware is present and the intended target includes eventually producing a DVD-video anyway.. with minimal or course editing, possibly using Chapter marks as defined by the DVD-VR specification for VOB units and menus. In fact the editing could be done away with entirely as VOB units can be referred to (without) actually editing and re-encoding the video stream.. sacrificing storage space, simply by including a series of virtual "skip" markers in the video menus, stringing them together as DVD-video Programs chains.

Which re-visits the issue and appropriateness when capturing Broadcast, S-VHS and VHS using 4:2:2 or 4:1:1 and either directly recording to 4:2:0 for simplicity and expediency sake, or Professionally capturing as 4:2:2 where the source material still retains a greater 400-500 TVL horizontal resolution. Or Consumer capturing as 4:1:1 where the source material is a lessor 240-250 TVL horizontal resolution who will also plan some level of editing and post production on a shoe string budget.

4:2:2 can be thought of as Z:X:Y

Where Z: is the size of the grid defined as 4

Where X is the number of samples along the horizontal line, as in how many samples per line

Where Y is the number of different samples vertically in a line spanning both rows, at each X sample point.

4:1:1 has 1 sample per horizontal row, but only 1 different sample between each sampled horzontal

4:2:2 has 2 sample per horizontal row, but 2 different sample between each sampled horizontal

4:2:0 has two sample per horizontal row, but 0 different sample between each sampled horizontal

Another way of describing it:

4:1:1 has 1+1 = 2 different possible color samples
4:2:2 has 2+2 = 4 different possible color samples
4:2:0 has 2+0 = 2 different possible color samples

The difference is the direction of "emphasis" when color sampling.

4:2:2 "emphasizes" both directions
4:1:1 "emphasizes" the vertical direction, and captures more fine detail in the vertical direction
4:2:0 "emphasizes" the horizontal direction, capturing more fine detail in the horizontal direction

normally greater horizontal resolution is better, especially in the monochrome bright and dark field of vision, where color is reduced in that axis.. meaning VHS color reduction is not perceived as much as the greater monochrome bright and dark resolution of an S-VHS higher resolution picture

Its appropriate to scale the color resolution with line resolution, but just as valid to descale it when the horizontal vertial line resolution (TVL) is reduced as well. Its practically a zero sum game.

Thinking along these lines:

Use of

4:2:2 for S-VHS transfer
4:1:1 for VHS transfer

would seem appropriate

Going futher

720 x 480 x 4:2:2
360 x 480 x 4:1:1

Might make some sense, reconstructed with the appropriate aspect ratio for viewing

By convention however

720 x 480 x 4:2:2
720 x 480 x 4:1:1
720 x 480 x 4:2:0

are more common

Or

720 x 480 x 4:2:2 for S-VHS
720 x 480 x 4:2:0 for S-VHS distribution

720 x 480 x 4:1:1 for VHS
720 x 480 x 4:1:0 for VHS distribution

Since VCD or smaller than DVD-video is archaic and unconventional these days. 4:1:0 probably is overkill where synthetic color sub sampling is concerned. And most programs do not contain a profile for it.

Stepping up to the lessor color sub sampling 4:2:0 post editing of 4:1:1 was (and is) a well known and practiced behavior in the edit room.

4:2:0 is also known as "co-siting" color samples, referring to the centering of the virtual coilor sample "spot" at the center of a square representing the area surrounding the sample spot that will inherit the same color information upon reproduction. For aspect ratios where the pixels are square this is optimal, but most aspect ratios are not reproduced with "square pixels" and this is sub-optimal leading to "jaggies" or anti-aliasing problems in the color field conflicting with the monochrome information.. or "color bleed" depending on how bad the difference between the co-siting and the aspect ratio pixels. 4:2:2 handles this problem "well", 4:2:0 does not without additional post capture anti aliasing methods.. and some color artifacting is unavoidable.

4:1:1 actually aligns the color information "better" because the smallest ratio is 4:3 and goes up from there preferentially reproducing horizontal rectangles.

4:2:0 was more optimal when re-sizing or re scaling a wide film format for the 4:3 aspect ratio of consumer television.. that is it was not as noticeable.  It is more noticeable when capturing and reproducing S-VHS or VHS from 4:2:2 to 4:2:0 even though the color sampling is good.. artifacts are more likely in the color space.

4:2:2 consuimer camcorders and dslrs are just beginning to capture in this format matively

4:4:4 studio cameras are regularly capturing in this format, as are some higher end prosumer equipment