soft pedal vs treadle

soft pedal and treadle both are nouns.

Word	Noun	Adjective	Verb	Adverb
soft pedal	Yes	No	No	No
treadle	Yes	No	Yes	No

As nouns, treadle is a hypernym of soft pedal; that is, treadle is a word with a broader meaning than soft pedal:

soft pedal: a pedal on a piano that moves the action closer to the strings and so soften the sound
treadle: a lever that is operated with the foot

Other hypernyms of soft pedal include foot lever, foot pedal, pedal.

soft pedal (noun)	treadle (noun)
a pedal on a piano that moves the action closer to the strings and so soften the sound	a lever that is operated with the foot

soft pedal (verb)	treadle (verb)
	operate (machinery) by a treadle
	tread over