Determining the capture window of a capture card

Arachnotron, December 2th 2003

Introduction

I am assuming the reader is familiar with the concept of video lines, resolution, aspect ratio and active picture area. If not, look at the sites mentioned in the references.

One assumption often made in video capture documentation is that capturing equipment captures exactly the theoretical active area of a video line, i.e. 52 microseconds (= µs) for PAL and 52.66 µs for NTSC. When you capture, the picture information in this area is sampled and distributed over the number of pixels chosen by you when you set the horizontal resolution in the capture program. The higher the horizontal resolution you choose, the smaller the amount of time such a pixel represents.(PLAATJE HIER ???) If you were to make captures at 480x480 and 720x480 and compare them on your computer screen, both would depict the same part of the TV picture but the 720x480 version would look more stretched horizontally.
Basically, all cards have a single sampling rate and generate all other resolutions by resizing.

Unfortunately, the capture area used by a device is programmed by the driver and often manufacturers will set active area's which are either shorter or longer then the theoretical active area. And of course, they never tell you what it is. Even when dealing with the same capture device, for instance a BT878, different drivers can use different capture area's. In principle it is even possible that a manufacturer decides to change the capture window when it releases a new driver version.

The result of this is that when you capture at the same resolution using different devices or drivers, the results might not look the same. The capture made with the device with the shortest capture window will look stretched out horizontally compared to a capture made on the device with the longer capture window.

The capture window influences the aspect ratio of the picture. If you want the aspect ratio to be correct on playback, you have to correct for this. This page aims to give simple instructions on how to determine what capture window your card uses utilizing standard DVD player as a signal source. Look in the Doom9 TV capture Guide for more information on what to do once you know what it is.

Using the DVD signal as a reference

A DVD player operates at 13.5 MHz. In other words, it plays back 13000000 pixels in a second. This means that a single pixel in your MPEG2 file is played back in exactly
1/13000000= 0.07407 µs or 74.07 ns. More information about the way a DVD player plays back a MPEG2 file can be found here.

Note that this relationship is only valid for a standard stand-alone DVD player. A multimedia-PC will do some scaling of it's own when generating a TV picture and is unsuitable as a reference for determining the capture window!

To calibrate your capture device, simply connect your DVD player to your capture device. If possible, use an Y/C or S-video cable to do this, but a composite video cable will do if neccesary.
Now play a DVD disk, capture a small clip en see how many of the original DVD pixels made it into your capture file.
divide the number of DVD pixels by 13.5 and presto, you have your capture window in µs. For example, 720 DVD pixels equal 720/13.5 = 53.33 µs.

Unfortunately, this is not as easy as it sounds. You would need to rip the DVD and try to extract a single frame which is also present in the captured file to be able to compare the two. Also, because of the way color is handled by both DVD's and capture devices ( color information is shared by two pixels, making the resolution for color half of that for black and white) colored pixels can make it extremely difficult to see exactly where the capture ends.

To help with this I created a test DVD for both PAL and NTSC. You can download them in the download area. You need RAR to unpack the files. They contain a ready made VIDEO_TS folder which can be burned to video-DVD using Nero burning Rom or any other DVD burning program. If you don't own a DVD burner, a S-VCD iso-image can be found below for use with a CD burner though the results will be slightly less accurate.

The test DVD contains a 10 s clip in which a single frame is repeated. The DVD is set to play in a loop. Below you can see the NTSC version of the test frame. Click it to see it full-size. To be able to analyze the results with one-pixel accuracy it is black and white. To further optimize it it contains only white pixels on a 50% gray background. Why this is a good idea is told here

(Uitleg wat je aan de randen ziet, horizontale lijn)

(Plaatje lijn linker 20 DVD pixels, daar onder resulterende analogue signal ; black level, up naar white, down naar grey, met sample frame er onder en daar weer onder de resulterende pixels)

Using a SVCD as a reference

Many DVD players will also play S-VCD's. A S-VCD pixel is played back at a lower rate than DVD's, 9 MHz. This means that a single pixel is played back in exactly 1/9000000 =0.1111 µs or 111.1 ns. Because of this S-VCD pixels are 50% wider than DVD pixels and the results will be slightly less accurate. Apart from that the method works just the same as for a DVD, only now you divide by 9 instead of 13.5.
For example 480 S-VCD pixels equal 480 / 9 = 53.33 µs.

Capturing and how to interpret your results

The following picture demonstrates the relation between the original pixels in line 130 of the mpeg file on the DVD and the final pixels in your capture file. The vertical lines are 74.07 ns apart.

Demo pixels on edge cap area

A. line 130 of the 720x480 mpeg file. Pixels 1-7 are white, the following ones are 50% gray
B. On playback, the DVD player crops the first 3 pixles and replaces them with horizontal blanking.
C. The analogue signal produced by the DVD player. The signal rises from blanking level to white level, stays at white level for about 0.2 µs and drops down again to 50% grey level. Note the rise time going from blanking level to white level (rise time is the time it takes to go from 10% to 90% of maximum signal). This is almost 0.06 µs which equals 0.8 DVD pixel! This is why a vertical change from black to white on a DVD will always result in a grey pixel in the capture. To increase the precision, the test DVD uses white lines on a 50% grey background, since going from white level to 50% grey takes less time.
D. Theoretical capture card with the capture window coinciding with the signal resulting from DVD pixel 7.
E. Theoretical capture card with the sampling grid 'out of phase' with the pixelgrid on the DVD. The first pixel captured contains signal from DVD pixels 6 and 7, and is white, but capture pixel 2 contains signal from the flank and results in a grey color between white and 50% grey.
F. Theoretical capture card with the capturing window beginning before the start of the active part of the DVD signal. Because of this, two black pixels are captured on the left. The third pixel is grey, since most of the analogue signal in this area is on a slope between black and white.

Determining the capture window of a capture card

Introduction

Using the DVD signal as a reference

Using a SVCD as a reference

Capturing and how to interpret your results

Some examples

A special case: BT878 based capture cards

References