diff --git a/README.md b/README.md
index e25267b..c02cc79 100644
--- a/README.md
+++ b/README.md
@@ -85,7 +85,7 @@ See [UDPspeeder + openvpn config guide](https://github.com/wangyu-/UDPspeeder/wi
 ### Full Options
 ```
 UDPspeeder V2
-git version: 6f55b8a2fc    build date: Nov 19 2017 06:11:23
+git version: 3e248b414c    build date: Aug  5 2018 21:59:52
 repository: https://github.com/wangyu-/UDPspeeder
 
 usage:
@@ -110,6 +110,8 @@ advanced options:
                                           do not use if you dont know what it means.
     -i,--interval         <number>        scatter each fec group to a interval of <number> ms, to protect burst packet loss.
                                           default value: 0. do not use if you dont know what it means.
+    -f,--fec              x1:y1,x2:y2,..  similiar to -f/--fec above,fine-grained fec parameters,may help save bandwidth.
+                                          example: "-f 1:3,2:4,10:6,20:10". check repo for details
     --random-drop         <number>        simulate packet loss, unit: 0.01%. default value: 0.
     --disable-obscure     <number>        disable obscure, to save a bit bandwidth and cpu.
 developer options:
diff --git a/doc/README.zh-cn.md b/doc/README.zh-cn.md
index 623eb36..dd0f3ac 100644
--- a/doc/README.zh-cn.md
+++ b/doc/README.zh-cn.md
@@ -106,7 +106,7 @@ https://github.com/wangyu-/udp2raw-tunnel
 ### 命令选项
 ```
 UDPspeeder V2
-git version: 6f55b8a2fc    build date: Nov 19 2017 06:11:23
+git version: 3e248b414c    build date: Aug  5 2018 21:59:52
 repository: https://github.com/wangyu-/UDPspeeder
 
 usage:
@@ -131,6 +131,8 @@ advanced options:
                                           do not use if you dont know what it means.
     -i,--interval         <number>        scatter each fec group to a interval of <number> ms, to protect burst packet loss.
                                           default value: 0. do not use if you dont know what it means.
+    -f,--fec              x1:y1,x2:y2,..  similiar to -f/--fec above,fine-grained fec parameters,may help save bandwidth.
+                                          example: "-f 1:3,2:4,10:6,20:10". check repo for details
     --random-drop         <number>        simulate packet loss, unit: 0.01%. default value: 0.
     --disable-obscure     <number>        disable obscure, to save a bit bandwidth and cpu.
 developer options:
@@ -151,8 +153,6 @@ log and help options:
     --disable-color                       disable log color
     -h,--help                             print this help message
 
-
-
 ```
 ### 包发送选项，两端设置可以不同。 只影响本地包发送。
 ##### `-f` 选项
diff --git a/fec_manager.h b/fec_manager.h
index 41aaa5a..3f191e6 100644
--- a/fec_manager.h
+++ b/fec_manager.h
@@ -35,12 +35,16 @@ struct fec_parameter_t
 	{
 		unsigned char x;//AKA fec_data_num  (x should be same as <index of rs_par>+1 at the moment)
 		unsigned char y;//fec_redundant_num
-	}rs_par[255+10];
+	}rs_par[max_fec_packet_num+10];
 
 	int rs_from_str(char * s)//todo inefficient
 	{
 		vector<string> str_vec=string_to_vec(s,",");
-		if(str_vec.size()<1) return -1;
+		if(str_vec.size()<1)
+		{
+			mylog(log_warn,"failed to parse [%s]\n",s);
+			return -1;
+		}
 		vector<rs_parameter_t> par_vec;
 		for(int i=0;i<(int)str_vec.size();i++)
 		{
@@ -54,7 +58,7 @@ struct fec_parameter_t
 			}
 			if(x<1||y<0||x+y>max_fec_packet_num)
 			{
-				mylog(log_warn,"invaild value x=%d y=%d\n",x,y);
+				mylog(log_warn,"invaild value x=%d y=%d, x should >=1, y should >=0, x +y should <%d\n",x,y,max_fec_packet_num);
 				return -1;
 			}
 			tmp_par.x=x;
@@ -63,6 +67,7 @@ struct fec_parameter_t
 		}
 		assert(par_vec.size()==str_vec.size());
 
+		int found_problem=0;
 		for(int i=1;i<(int)par_vec.size();i++)
 		{
 			if(par_vec[i].x<=par_vec[i-1].x)
@@ -80,9 +85,18 @@ struct fec_parameter_t
 
 			if(pre_ratio<now_ratio)
 			{
-				mylog(log_warn,"%d/%d < %d/%d ,not suggested\n",pre_y,pre_x,now_y,now_x);
+				if(found_problem==0)
+				{
+					log_bare(log_warn,"possible problems: %d/%d < %d/%d ",pre_y,pre_x,now_y,now_x);
+					found_problem=1;
+				}
+				log_bare(log_warn,",%d/%d < %d/%d",pre_y,pre_x,now_y,now_x);
 			}
 		}
+		if(found_problem)
+		{
+			log_bare(log_warn,"\n");
+		}
 
 		{ //special treatment for first parameter
 			int x=par_vec[0].x;
@@ -103,16 +117,27 @@ struct fec_parameter_t
 			rs_par[now_x-1].x=now_x;
 			rs_par[now_x-1].y=now_y;
 
-			double k= double(now_y-pre_y)/double(now_x-pre_x);
+			double now_ratio=double(par_vec[i].y)/par_vec[i].x;
+			double pre_ratio=double(par_vec[i-1].y)/par_vec[i-1].x;
+
+			//double k= double(now_y-pre_y)/double(now_x-pre_x);
 			for(int j=pre_x+1;j<=now_x-1;j++)
 			{
 				int in_x=j;
-				int in_y= double(pre_y) + double(in_x-pre_x)*k+ 0.9999;// round to upper
+
+			////////	int in_y= double(pre_y) + double(in_x-pre_x)*k+ 0.9999;// round to upper
+
+			double distance=now_x-pre_x;
+			///////	double in_ratio=pre_ratio*(1.0-(in_x-pre_x)/distance)   +   now_ratio *(1.0- (now_x-in_x)/distance);
+			//////	int in_y= in_x*in_ratio + 0.9999;
+				int in_y= pre_y +(now_y-pre_y) *(in_x-pre_x)/distance +0.99999;
+
 				if(in_x+in_y>max_fec_packet_num)
 				{
 					in_y=max_fec_packet_num-in_x;
 					assert(in_y>=0&&in_y<=max_fec_packet_num);
 				}
+
 				rs_par[in_x-1].x=in_x;
 				rs_par[in_x-1].y=in_y;
 			}
@@ -130,7 +155,9 @@ struct fec_parameter_t
 		assert(rs_cnt>=1);
 		for(int i=0;i<rs_cnt;i++)
 		{
-			sprintf(tmp_buf,"<%d,%d> ",int(rs_par[i].x),int(rs_par[i].y));
+			sprintf(tmp_buf,"%d:%d",int(rs_par[i].x),int(rs_par[i].y));
+			if(i!=0)
+				tmp_string+=",";
 			tmp_string+=tmp_buf;
 		}
 		strcpy(res,tmp_string.c_str());
diff --git a/main.cpp b/main.cpp
index 3c97180..55315d4 100644
--- a/main.cpp
+++ b/main.cpp
@@ -44,13 +44,16 @@ static void print_help()
 	printf("                                          default value: 1250. you typically shouldnt change this value.\n");
     printf("    -q,--queue-len        <number>        fec queue len, only for mode 0, fec will be performed immediately after queue is full.\n");
 	printf("                                          default value: 200. \n");
+
 	printf("    -j,--jitter           <number>        simulated jitter. randomly delay first packet for 0~<number> ms, default value: 0.\n");
 	printf("                                          do not use if you dont know what it means.\n");
 	printf("    -i,--interval         <number>        scatter each fec group to a interval of <number> ms, to protect burst packet loss.\n");
 	printf("                                          default value: 0. do not use if you dont know what it means.\n");
+	printf("    -f,--fec              x1:y1,x2:y2,..  similiar to -f/--fec above,fine-grained fec parameters,may help save bandwidth.\n");
+	printf("                                          example: \"-f 1:3,2:4,10:6,20:10\". check repo for details\n");
 	printf("    --random-drop         <number>        simulate packet loss, unit: 0.01%%. default value: 0.\n");
 	printf("    --disable-obscure     <number>        disable obscure, to save a bit bandwidth and cpu.\n");
-//	printf("    --disable-xor         <number>        disable xor\n");
+	//printf("    --disable-xor         <number>        disable xor\n");
 
 	printf("developer options:\n");
 	printf("    --fifo                <string>        use a fifo(named pipe) for sending commands to the running program, so that you\n");