792 x 576 pixel mode

If you have an Xbox with the Focus FS454 video encoder and use PAL output, you can follow the procedure described here to set it up for overscanned 792x576 resolution, which looks much nicer than the default 640x480. PAL video is usually captured in the resolution 720x576, so the picture lines are not scaled at all in the vertical direction if a frame height of 576 pixels is used. And in the horizontal direction the pixels are fuzzy anyway (since the signal is analog), so the horizontal downscaling is not very noticeable. A completely unscaled 720x576 mode could have been used instead, but the software becomes less messy if the pixels are square, so 720x576 is only used when actually playing video.

To calculate the number of horizontal pixels needed for a square-pixel frame, use the fact that 702 of the 720 pixels represent 4/3 of the frame height: for square pixels this means that (720/702)*(4/3)*576 = 787.6923 horizontal pixels are needed. This is rounded up to the nearest multiple of 8 (because of hardware requirements) which is 792.

Non-square pixels are rather tricky beasts. If you want to learn more about this subject, I recommend this excellent web page: A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions.

Here is the procedure for setting up the 792x576 mode:

• Make X11 recognize the mode. Use something like this in your XF86Config-4-xbox:

  Mode    "792x576"
  	DotClock 29.750
  	HTimings 792 832 912 952
  	VTimings 576 599 601 625
  	Flags "-Hsync" "-Vsync"
  EndMode
  

  Then set it as the default mode in the "Display" subsection in the "Screen" section.

• You also need 720x576 so MPlayer can switch to this mode:

  Mode    "720x576"
  	DotClock 27.000
  	HTimings 720 760 824 864
  	VTimings 576 599 601 625
  	Flags "-Hsync" "-Vsync"
  EndMode
  

• At the end of the "append" line in your linuxboot.cfg, add

  video=xbox:hoc=0,voc=0
  

  The linuxboot.cfg file is found in the same place as where you installed Xebian, e.g. E:\debian\linuxboot.cfg.

  This change sets the horizontal and vertical overscan compensation to zero. It may work for Xboxes with Conexant chips also, I haven't tested that.

• Now the picture needs to be positioned correctly on the TV screen, so that a 792x576 image from the Xbox looks exactly like a broadcast PAL image. I don't know how this is done for Conexant-equipped Xboxes, or if there may be a better way to do this, but here is a program that tells the Focus FS454 to position the picture so it is correctly centered. Try this only on boxes with the FS454 chip:

  #include <fcntl.h>
  #include "linux/i2c-dev.h"
  
  int main(int argc, char *argv[]) {
    int fd = open("/dev/i2c/0", O_RDWR);
    ioctl(fd, I2C_SLAVE_FORCE, 0x6a);
    i2c_smbus_write_word_data(fd, 0x00, 42);
    i2c_smbus_write_word_data(fd, 0x02, 26);
    close(fd);
  }
  

  Compile with -I/usr/src/linux/include
  Run it (as root) after X11 has started, otherwise the settings will be overwritten.