FFmpeg is a complete solution to record, convert and stream audio and video. It includes libavcodec, the leading audio/video codec library.
ffmpeg is a command line tool to convert one video file format to another. It also supports grabbing and encoding in real time from a TV card.

Install

sudo apt-get install ffmpeg


convert .flv to .mpg using ffmpeg

First you need to download your .flv file to a folder and you need to Open a terminal window and go in to the .flv file folder and type the following command

ffmpeg -i yourfile.flv -ab 56 -ar 22050 -b 500 -s 320×240 yourfile.mpg

yourfile.flv is the file you want to convert, so the name must be the same as the source file.You can name yourfile.mpg whatever you want as long as it has the .mpg extension.

-b bitrate: set the video bitrate in kbit/s (default = 200 kb/s)

-ab bitrate: set the audio bitrate in kbit/s (default = 64)

-ar sample rate: set the audio samplerate in Hz (default = 44100 Hz)

-s size: set frame size. The format is WxH (default 160×128 )