Only get part of blob

Hi, I've used your code to create a streaming desktop system using the following ffmpeg child process in node server:
var ffmpeg = require('child_process').spawn("ffmpeg", [
"-f",
"avfoundation",
"-i",
"1:0",
"-q:v",
"31",
"-r",
"10",
"-f",
"mjpeg",
"pipe:1"
]);

and on the stdout.on('data') function I broadcast it to my web socket as follows:
var frame = new Buffer(data).toString('base64');
ws.broadcast(frame, {binary:false});

when the canvas updates on receiving of the web socket data, it only draws the first top 100 pixels, any clues ? It looks like the frame it not complete ?