diff --git a/public/webrtc.js b/public/webrtc.js
index c84ac3d..968f2f4 100755
--- a/public/webrtc.js
+++ b/public/webrtc.js
@@ -104,7 +104,7 @@ class Entropy {
    static local = null;
    
    static pageReady() {
-      Entropy.local = new Local("video");
+      Entropy.local = Local.build("video");
    }
    
    static start(isCaller) {
@@ -166,41 +166,54 @@ class Entropy {
 }
 
 class Local {
-   type = null;
    stream = null;
 
-   constructor(type) {
-      this.type = type;
-      switch (type) {
-         case "video":
-         case "audio":
-            var constraints = {
-               video: type == "video",
-               audio: type == "audio",
-            };
-            if(!navigator.mediaDevices.getUserMedia) {
-               View.error('Your browser does not support getUserMedia API');
-            }
-            navigator.mediaDevices.getUserMedia(constraints)
-               .then((stream) => {
-                  if (type == "video") {
-                     new Preview('preview', stream);
-                  }
-                  this.stream = stream;
-               })
-               .catch(View.error);
+   static build(type) {
+      switch(type) {
+         case "audio":  return new Audio();
+         case "video":  return new Video();
+         case "text":   return new Text();
+         default:       View.error("unknown build", type);
       }
    }
 
+   constructor() {
+      var constraints = {
+         video: this.gettype() == "video",
+         audio: this.gettype() == "audio",
+      };
+      if(!navigator.mediaDevices.getUserMedia) {
+         View.error('Your browser does not support getUserMedia API');
+      }
+      navigator.mediaDevices.getUserMedia(constraints)
+         .then((stream) => {
+            if (this.gettype() == "video") {
+               new Preview('preview', stream);
+            }
+            this.stream = stream;
+         })
+         .catch(View.error);
+   }
+
+   gettype() {
+      View.error("not impl");
+   }
+
    start() {}
 
    stop() {}
 }
 
 class Audio extends Local {
+   gettype() {
+      return "audio";
+   }
 }
 
 class Video extends Local {
+   gettype() {
+      return "video";
+   }
 }
 
 class Text extends Local {