DE2_TV problem

In the file of itu_r656_decoder ,what's the affection of " if ((h< 51) || (h > 771)) " and " if ((h< 41) || (h > 781)) "?

And " Vde_counter >31 and Vde_counter <511",why must the Vde_counter larger than 31 ?

`define sync 8'd101

module itu_r656_decoder

(

input CLOCK,

input [7:0]TD_D,

input TD_HS,

input TD_VS,

input SW0,

input SW1,

output [7:0]Y,

output [7:0]Cb,

output [7:0]Cr,

output reg HSx2,

output VSx1,

output reg Ypix_clock,

output blank

);

assign VSx1=TD_VS;

/////// FF 00 00 SAV(EAV) //////

reg[7:0]R1,R2,R3;

reg[7:0]RR1,RR2,RR3;

wire Y_check=( (R3==8'hff) && (R2==8'h00) && (R1==8'h00) )?1:0;

always @(posedge CLOCK) begin

RR1=TD_D;

RR2=R1;

RR3=R2;

end

always @(negedge CLOCK) begin

R1=RR1;

R2=RR2;

R3=RR3;

end

/////decode SAV(EAV) ////

reg START,Field;

always @(posedge CLOCK) begin

if (Y_check==1)

begin

START=~TD_D[4];

Field= TD_D[6];

end

end

//////YUV4:2:2 to YUV4:4:4/////

reg [7:0]YY;

reg [7:0]CCb,Cbb;

reg [7:0]CCr,Crr;

reg [1:0]COUNTER;

always @(posedge CLOCK) begin

if (!START)

COUNTER=0;

else COUNTER=COUNTER+1;

end

always @(posedge CLOCK) begin

case (COUNTER)

0:begin Cbb=TD_D; Ypix_clock =0;end

1:begin YY =TD_D;CCr=Crr;CCb=Cbb; Ypix_clock =1;end

2:begin Crr=TD_D; Ypix_clock =0;end

3:begin YY =TD_D;CCr=Crr;CCb=Cbb; Ypix_clock =1;end

endcase

end

///// auto-counting H length ////

reg [10:0]H_COUNTER;

reg [10:0]RH_COUNTER;

always @(posedge CLOCK) begin

if (TD_HS) H_COUNTER=0;

else H_COUNTER=H_COUNTER+1;

end

always @(posedge TD_HS) begin

RH_COUNTER=H_COUNTER;

end

/////to generate HSx2////

always @(posedge CLOCK) begin

if (

((H_COUNTER >= 0) && (H_COUNTER < `sync)) ||

((H_COUNTER >= RH_COUNTER[10:1]) && (H_COUNTER < (RH_COUNTER[10:1]+`sync+1)))

)

HSx2=0;

else

HSx2=1;

end

/////H blank for HSx2/////

reg [10:0]h;

reg h_tr;

reg h_tr_h;

always @(posedge CLOCK) begin

if(!HSx2) h=0;

else

h=h+1;

end

always @(posedge CLOCK) begin

if ((h< 51) || (h > 771))

h_tr=0;

else

h_tr=1;

end

always @(posedge CLOCK) begin

if ((h< 41) || (h > 781))

h_tr_h=0;

else

h_tr_h=1;

end

/////V blank for HSx2//////

reg[10:0]Vde_counter;

always@(posedge HSx2)begin

if (TD_VS==0)

Vde_counter=0;

else

Vde_counter=Vde_counter+1;

end

///v-h blank output//

wire vde=((Vde_counter > 31) && (Vde_counter < 511)) ? 1:0;//480

wire blank_h = h_tr & vde;

///vga blank output//

assign blank = h_tr_h & vde;

/////dual port RAM////

wire [7:0]Yw;

wire [7:0]Cwr;

wire [7:0]Cwb;

dul_port_c1024 YYYR(

.iDATA(YY[7:0]),

.iHSYNC(TD_HS),

.iHSYNCx2(blank),

.Y_CLOCK(Ypix_clock),

.Y_CLOCKx2(CLOCK),

.oDATA(Yw[7:0]),

.field(Field),

.VS(TD_VS)

);

dul_port_c1024 CBB(

.iDATA(CCb[7:0]),

.iHSYNC(TD_HS),

.iHSYNCx2(blank),

.Y_CLOCK(Ypix_clock),

.Y_CLOCKx2(CLOCK),

.oDATA(Cwb[7:0]),

.field(Field),

.VS(TD_VS)

);

dul_port_c1024 CRR(

.iDATA(CCr[7:0]),

.iHSYNC(TD_HS),

.iHSYNCx2(blank),

.Y_CLOCK(Ypix_clock),

.Y_CLOCKx2(CLOCK),

.oDATA(Cwr[7:0]),

.field(Field),

.VS(TD_VS)

);

/////YUV 4:4:4 output////

assign Y = (blank_h)?Yw :8'h10;

assign Cr = (blank_h)?Cwr :8'h80;

assign Cb = (blank_h)?Cwb :8'h80;

endmodule