I have a mac with an editing program that can do all sorts of funky things with a pre-recorded song, but I don't call it an instrument; I call it an editing-tool. You may call a turntable a real-time editing tool perhaps.
Also, there are musical groups that use those editing tools in real time on stage just like some people use turntables. For example, they would be changing settings on a phaser, flanger, EQ, turning off channels to create solos, sequencing another instrument's music on the fly to create harmonies, etc.
Anyway, saying that "It needs a record on it" is not a logical rebuttal. Doesn't a guitar need strings on it? Don't those strings make a different sound if you put different ones on? Doesn't a guitar need pickups? Don't they make a different sound depending on which you use?
Loading samples or using samples in music has been done for a very, very long time. Two turntables and a mixer allow you to manipulate those sounds in interesting ways, creating loops, repetition, modifying the arrangement, or editing the lyrics.
Take this for example: http://www.soundclick.com/player/single_player.cfm?songid=4554800&q=hi&newref=1
^ This is someone looping some shit on top of an old financial success record. (Give it some time, it kicks in at about the 1 minute mark. I guess 0:53)
Throughout the song the turntable becomes the melody, the lead instrument, even around 1:27 sounding as if the speaker is saying "I realize...I rule, I rule."
It's the kind of thing that is one lame loop on top of an old record without the turntablism, but a pretty interesting song with it.
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In my opinion, if you have a tool that can assist in rearranging a song, it should not be undervalued. Turntables, like the human voice, can be used for other things can creating music. However, when a skilled performance artist uses it as an instrument, I think I'd say it's an instrument. People bang on pots, trash cans, and whatever they can find.
http://www.youtube.com/watch?v=83TYDtYwir0
^ I guess that a shopping cart was not designed to be a musical instrument, but it can be, depending on how it's used. And that's my point. Turntables can be used as musical instruments. While it is common to use them for playback, shopping carts are commonly used to carry groceries. A guitar can be used as a weapon, but it is commonly used as an instrument.
It is whatever you decide it to be, is all I'm trying to say.