Field synchronization signal for 625/50/i systems per itu-r.

bt.470, not for system N, with h=H/2 one half line period: 
  total vertical blanking = 50h,

  odd field, fsync is line 1, image at line 23.5: 
    5h fsync + 5h equ + text,white,text,WSS + 575h image + 5h equ,

  even field, fsync is line 313.5, image at line 336:
    5h fsync + 5h equ + bk,text,white,text  + 575h image + 5h equ.

  The decoder vertical reset ends synchronous with the trailing edge 
  of the field sync.  So the image begins after a delay of 
  total_vblank-equ-fsync=50-5-5=40h or (23.5-1)-fsync/2h=20 lines.

  The code cx25840-core.c initializes vblank656=40 which works with 
  firmware 2.05.032.  

bt.656
  Each field is 576h long and the line number changes at hblank start.  
  Thus the analog line number lags the digital line number.  A word 
  stream consisting of one frame gives a timing for the odd field: 

  A--- 40h delay ---------------------------->
  A                            <--------- line 23 -------->
  S____.v.------------------..h____...--wss------image--..h
  D                         <--------- line 23 -------->
  D--- 39h delay ------------->
  D--- 38h delay >
  +----------------------------------------------------------- time ->

  and for the even field: 

  A--- 40h delay ---------------------------->
  A                                           <-------- line 336 -------->
  S------------fvh____fv.------------------f.h____f..-------image------f.h
  D            <-------- line 335 --------><-------- line 336 -------->
  D--- 39h delay ------------->
  D--- 38h delay >
  +----------------------------------------------------------- time ->

  with f=field, v=vertical blanking, h=horizontal blanking.

  odd image at line 23 up to 311 --> delay 39h --> vblank= 38 or 37,
  even image at line 336 up to 624 --> delay 40h --> vblank= 39 or 38.

  Thus vblank=38 for a frame consisting of 576 lines.  The decoder 
  requires 4h of preceding data.  Increasing the vertical active time 
  to 580h results in vblank=38-4=34. 

  The decoder autoconfig defaults to vblank=34 and vblank656=38 which 
  works well with firmware 2.06.039.  

  However, cx25840-core.c std_setup() sets vblank=36 which shifts the 
  field capture window 2h or 1 line down.  

  The resulting frame has 2 bottom lines of vsync, like in 
  http://picaros.org/xorg/v36-40.png .
  The bottom left white halfline is line 336+(575h-1h)/2h=623.  

  Hence setting ctl_crop_top/cur_val to -2 solves the problem, see
  http://picaros.org/xorg/v34-38.png .
  The encoder firmware now generates the last frame line from previous 
  lines so the white halfline 623 dissapears.