play back vs playback

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

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

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

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