playback vs play back

playback is a noun but play back is not a noun.

Word	Noun	Adjective	Verb	Adverb
playback	Yes	No	No	No
play back	No	No	Yes	No

playback (noun)	play back (noun)
the act of reproducing recorded sound
electronic equipment comprising the part of a tape recorder that reproduces the recorded material

playback (verb)	play back (verb)
	reproduce (a recording) on a recorder