Ok.I fixed it.the problem was the drivers of the usb dvb-t (dib0700),when you are recording a terrestrial channel,you cannot change to another bouquet(get freezed).so,i've removed the dib0700 drivers,and download the drivers of usbserial(1.0-r0),and now it works perfectly.the box is Xtrend 9200 and the usb dvb-t is hauppauge wintv nova td.Thank you all.