An MP3 player uses the file.mp3 (a song for example) as its input.
It then decodes (decompresses) this as an MP3 file is a compressed file.
The resulting decoded data is then used as it's output, usually sent to a speaker with the help of a digital to analog converter and an amplifier.
A MP3 recorder would do the process in reverse and the sound would be its input and the resulting MP3 file would be it's output.
Out put
Because with most MP3 players you can only "output" media. As opposed to "input" meaning it can receive, such as a microphone, or a camera.
A song is an input. When you select a song to play, the MP3 file is decoded, and this data is sent to the speaker to be used as the output.
output
Every designer of MP3 Players has a USB hack output, however the input to the machine is different for every machine. There is no one standard USB jack.
It is an output device
Easily? No. To do so you setup your turntable so that the speaker output goes into your computer aux input and you record it with Audicity (that's what I used when I did mine) and then convert the output from WAV to mp3. It's a slow process but worth it depending on what music you have.
A monitor is first and foremost an Output Device because it displays information already held in the computer. Touchscreen monitors that allow you to input information or to use navigational features such as menus, icons and so on are also Input Devices. Another example of a device that is both an input and output device is a joystick with Force Feedback technology.
Output devices are anything that send signals to other devices. A portable MP3 player, for instance, outputs music to your headphones. Your headphones plug into the "input" jack of the MP3 player because they receive a "signal" from the MP3 player in the form of music. Other examples of output devices would be video cards, keyboards, mice, gaming controlers, or dvd/blu-ray players. These all send signals to be used by other devices but do not receieve signals back from those devices.
External Input, as in a mp3 player.
output if your referring to the headphones jack.. input if you downloading information through the usb/ bluetooth or wifi..
is an omr and input or output device?