10 Determining the capture window of a capture card

10.1 Introduction

In this section, It is assumed the reader is familiar with the concept of video lines, size, aspect ratio and active picture area. If not, please read section 3 first. This is just a quick guide on how to determine which part of a video line is captured by a video capture device.

The official numbers, according to Rec. ITU-R BT.470-6,  for the active part of a video line are:
For PAL, a whole line takes 64 Ás and the active part is 52.0 Ás, but may lie between 51.7 and 52.0 Ás.
For NTSC, a whole line takes 63.555 Ás and the active part is 52.66 Ás, but may lie between 52.46 and 52.86 Ás

(Actually, for NTSC there are a number of standards which all differ slightly on the size of the active area and even on how to measure it. But the number used in Rec. ITU-R BT.470-6 seems to be the one most widely used when dealing with digital video formats.)

Unfortunately, manufacturers of capture devices have different ideas about how much of the total signal actually should be captured.

Vertically, all devices capture 480 (NTSC) or 576 (PAL) lines and resize to the required vertical size afterwards. The only exception to this is when vertical sizes below half these numbers are captured. In that case, capture devices simply drop every second field and resize the remaining field if needed. Because of this, you only need to determine the horizontal width of the capture window.

A capture device will typically sample the whole video line at a fixed high sampling rate, crop off all pixels which fall in the horizontal blanking area and resize the remainder to whatever horizontal size was requested by the user. (Actually, the whole line is resampled to a lower size and cropping horizontal blanking data is the last step, but the result is the same). The areas to be cropped are set to a fixed percentage of the whole line by the driver and mostly cannot be changed by the user. The problem lies in that different manufacturers have different ideas about how to interpret standards and how  much of the original video line can be cropped. Since it is the driver that determines the capture window, even two identical devices can give different results when used with different drivers.

The main difficulty is that the capture window influences the aspect ratio of the picture. If two devices capture at 720x480, but one of them uses a wider capture window than the other to generate those 720 pixels, the resulting pictures will have different aspect ratios.  The picture below illustrates this effect. It shows four strips with the same number of captured pixels horizontally that have been cropped from 720x576 captures made using different devices and with a test DVD as a source. The white crosses on the right originate from the center of the test picture, and all strips have been aligned relative to it.

Picture with different captures

The four strips are:

  1. Original, cropped directly from a rip of the test DVD picture. A DVD player outputs its 720 pixels in 53.33 Ás (see below).
  2. Terratec Cameo Grabster, a device with a capture window of 53.33 Ás.
  3. Hauppauge BT878a based card using the BTWincap drivers, capture window 52.00 Ás.
  4. Hauppauge CX23881 based card, Hauppauge drivers, capture window is 51.56 Ás.
As the capture window becomes smaller, the pixels are divided over a smaller area and the resulting picture is stretched out horizontally!

If you want the aspect ratio to be correct on playback, you have to correct for this effect. 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.

10.2 Using the analogue signal from a DVD player as a reference

A DVD player operates at 13.5 MHz when it plays back a DVD. In other words, it plays back 13500000 pixels in a second. This means that a single pixel in your MPEG2 file is played back in exactly 1/13500000 = 0.07407 Ás or 74.07 ns. 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 necessary. 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 you have your capture window in Ás. For example, 720 DVD pixels equals 720/13.5 = 53.33 Ás.

Many  DVD players will also play SVCD's. A SVCD pixel  is played back at a lower rate than DVD's, namely 9 MHz. This means that a single pixel is played back in exactly  1/9000000 = 0.1111 Ás 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 SVCD pixels equals 480 / 9 =  53.33 Ás.

There is a catch in using a DVD player as a reference source. Depending on the brand, not all pixels on a DVD are played back. Some players will crop pixels at the edges and output blanking signal instead. This shows up as black vertical bars on either side in your capture. Though this does not change the width of individual pixels, it can make it difficult to determine how many DVD pixels made it into the captured signal. To help with this I created a test DVD for both PAL and NTSC. You can download these in the download area. You need RAR to unpack the files. DVD players can also crop off some lines at the top or bottom and replace them with black lines, but since we are only interested in calibrating horizontally that is not a problem.

The test DVD contains a 10 s clip in which a single frame is repeated. This frame features vertical lines at a fixed known interval which can be used to calibrate your capture device using the procedure below. 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. The RAR files 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 SVCD compliant MPEG2 file can be found here also. Use your favorite SVCD authoring program to make a SVCD out of it. The SVCD MPEG file contains 20 seconds of the test image shown below (click on it to see it full size).

small testpic example

To be able to analyze the results with one-pixel accuracy it is grayscale. Both DVD's and capture devices distribute color values across two or more neighbouring pixels, which makes a color picture difficult to use accurately.  The picture contains a cross  made up of a sequence of horizontal and a sequence of vertical perpendicular lines. The lines are spaced exactly 20 pixels apart. Every 40 pixels, the pixel number is written next to a line. The DVD pictures are exactly 720x480 and 720x576  pixels in size, the SVCD pictures 480x480 and 480x576.

10.3 Capturing and how to interpret your results

  1. Connect the DVD player using a good quality S-Video lead. Composite also works, but gives less detail.
  2. Capture at a high horizontal size, preferably 720x480/576 or higher, using a lossless codec like huffyuv if possible. If your capture application won't let you do that, try VirtualVCR which can be obtained here.
  3. Load the resulting AVI in VirtualDub.
  4. Select a clean frame and copy it to the clipboard with Ctrl+1.
  5. Open Microsoft paint. (Start, Programs, accessories) and paste the picture from the clipboard Ctrl+v
  6. Zoom in to 800% and enable the grid (view, zoom, Custom ...., 800% and view, zoom, show grid.) The picture below shows sections of a PAL capture made using a BT878 based card and the BTWincap drivers v5.3.6.1 at 720x576.

    Blow up captured frame

    The picture shows sections of a PAL capture made using a BT878 based card and the BTWincap drivers v5.3.6.1 at 720x576. Click on the image to see the whole captured frame.


  7. Search for the first visible vertical line on the left. Note the DVD pixel number (the numbers written above the lines), i.e. 20, 40, 60 etc.The vertical line is probably composed out of 2 or more gray lines with varying shades of gray. look for the line with the lightest shade and note the number of pixels up to but not including it. If the vertical line contains two lines of exactly the same shade of gray, note the number of pixels to the left of the first of those two lines and add 0.5. In the illustration above the line on the leftmost position originates from DVD vertical line number 20. The vertical line with the lightest shade of gray in the capture has 12 pixels leading up to it as indicated by the red arrow.
  8. Do the same for the last vertical line that is still in your capture window on the right. In the illustration above the line on the rightmost position originates from DVD vertical line number 700. The one pixel vertical line with the lightest shade of gray in this line has 10 pixels to the right of it, as indicated by the red arrow.
  9. Calculate the number of capture pixels between the two. This number is 720 - (pixels to the left) - (pixels to the right). In the example this is 720 - 12 - 10 = 698 pixels.
  10. Calculate the number of DVD pixels between the lines from the numbers printed above/below  the lines. This is  (Right DVD pixel number - left number) +1. In the example, this is 700 - 20 + 1 = 681 DVD pixels .
  11. From this you can calculate the ratio of DVD pixels per captured pixel. This is (DVD pixels from step 10) / (capture pixels from step 9). In the example this number is 681 / 698 = 0.9756 DVD pixels per captured pixel.
  12. Now multiply this number by the horizontal size of your capture. In this example: 0.9756 * 720 = 702.4 DVD pixels in the capture window.
  13. Divide this number by 13.5 and you have the window in Ás. In the example: 702.4 / 13.5 = 52.03 Ás.
Note: you might see some black vertical bars on the left and right sides of your capture. This is normal. Some DVD players crop a few pixels from the sides. In the capture these show up as black pixels. These black pixels still count, your capture is still 720x480 or 720x576 and the receipe above still works.

The procedure when using a SVCD is identical, only at step 14 you divide by 9 instead of 13.5.

10.4 Why are the white lines in my capture so blurred?

When you capture the DVD signal from the test DVD you will probably notice that instead of a nice sharp single pixel wide vertical line you get a several pixels wide line made up out of single pixel wide lines with varying shades of grey. This has nothing to do with the quality of your capture card. This 'blurring' is caused by the way a DVD player plays back a single pixel. The picture below shows what happens when a DVD player processes a single gray line with a white pixel in it:

analogue signal DVD

The row of squares on top represent some of the pixels in the original line. Below that is a graph representing the part of the analogue composite video signal resulting from it. It was taken from an actual oscilloscope picture made from the signal of my DVD player. When the signal goes up, the color is white. Down is black; anything in between is a shade of gray. The peak resulting from one pixel is actually almost three DVD pixels wide, with oscillations at the side. Why this is so is beyond the scope of this guide.
When you capture a signal like this using for example a BT878 based card with BTWincap drivers at 720x576, the pixels resulting from this will look like the row of pixels below. So the signal resulting from a single white pixel on the DVD is smeared out over pixels with several shades of grey in your capture. However, by looking for the lightest pixel in your capture you can still locate where the top of the peak is and where the original white pixel must have been.

10.5 A special case: BT878 based capture cards

TV cards using the Brooktree/Conexant BT878 chip have been around for a long time. As a result there are a number of specialized tools for this chip floating around on the internet. One of them, BTtool, enables you to read back how the chip is being programmed by the driver and the capture application. To function, it needs a driver, BT878.sys to be installed. Both can be found in the download area. After installation of the driver, a reboot is required, even if the installer does not request it.

To understand the values you use below, a short description of how BT878 cards (and for that matter CX2388x based cards) operate is in order. The BT878 is a programmable chip, and it is programmed by its drivers. The drivers do this by setting registers, the digital equivalent of switches and nobs on the chip.

First, the card digitizes the whole scanline, 64 Ás, with a sample rate of  17.73447 MHz. This yield 64 * 17.73447 = 1135 samples per complete line (for PAL; for NTSC the number is 63.5555 * 14.31818 = 910 pixels). These 1135 samples are reduced to the required number of samples per line by interpolation. In other words, the original samples are sampled again, but at a lower rate resulting in the desired reduced number of pixels per line. The amount of reduction is set by a scaled sampling rate. The HScale register contains the ratio between the unscaled and the scaled sample ratio.
Because a whole line contains non-picture information on the left (horizontal blanking) and for a short stretch on the end (Front Porch) the resulting pixels have to be cropped to obtain the required number of pixels. The HDelay register determines how many of the scaled pixels are to be cropped on the left. The HActive register determines hor many pixels after that are used. Any pixel left after HDelay and HActive is ignored.

More on how the scaler in a capture device operates can be found here. More details on the BT878 registers can be found in the DataSheet.

You can determine the capture window of a BT878 based device as follows:

Warning! The BTtool and driver can make your system unstable while they are running. Please do not run any other applications while testing to prevent data-loss!
  1. Do not start BTtool yet!
  2. Open your capture application, set the capture size and enable the preview.
  3. Start BTtool and write down the following  hexadecimal numbers which can be found in the upper left corner of the BTtool screen:
    1. Crop
    2. Vactive Lo
    3. HDelay low
    4. HActive Lo
    5. Hscale Hi
    6. HScale Lo
  4. Select your TV system and fill in the numbers in the calculator provided below.
  5. Press the Compute button

Warning: Do not change your capture settings while BTtool is running! First close BTtool, make your changes, and restart BTtool afterwards.

Example: The picture shows the BTtool readings for the PAL 720x576 capture on a BT878 based card with BTWincap drivers from the example used in the previous section. These values have already been set as defaults in the calculator below. Press Compute to see the results.
BTtool screendump
In our example, the HScale setting results in a scaled sampling rate of 13.849 MHz. This results in 13.849 * 64 = 886 scaled pixels per line.
These lines of pixels still contain the horizontal blanking information before the active video area begins. So HDelay sets the number of pixels at the start which must be cropped, 143 in the example. The final register is HActive, which sets  the number of pixels following HDelay which hopefully contain the picture. HActive will normally be the number of horizontal pixels you chose for the capture, and in this case is 720. Once these 720 pixels are in, the final 886 - 143 - 720 = 23 pixels are skipped.

From this information, the capture window can be determined:

The capture window now is 720 / 13.849 = 51.99 Ás. This equals 51.99 * 13.5 = 701.9 DVD pixels.

TV system:
Enter BT878 registry information from BTTool:
CROP: 0x
VActive Lo: 0x
HDelay Lo: 0x
HActive Lo: 0x
HScale Hi: 0x
HScale Lo: 0x
Capture information:
Scaled sampling rate: MHz
Scaled pixels over complete line: pixels
Horizontal Delay: pixels
Scaled pixels captured: pixels
Number of captured lines: lines
PAR for this capture:
Device information:
Capture window: Ás
Capture window: DVD pixels
Capture Window IAR:

Digital Video Resolutions: A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions.
Video Formation, Perception and Representation (pdf file)
: by Y Wang.
Der Karl's Capture Karten aspect ratio fuer Dummies
: Der Karl's Capture Card Aspect Ratio for Dummies (in German).
Capture-Cards and aspect-ratio for Dummies: Der Karl's Capture Card Aspect Ratio for Dummies (translated by Arachnotron).
Introduction to digital aspect ratio conversion (White paper at Snell & Willcox)

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English version last edited on: 06/13/2004 | First release: n/a | Author: Arachnotron | Content by Doom9.org