Lav on harpist might work; although wonder about varying distance of harp strings; strings on far side of ‘arp perhaps 20X farther away from mic than closest strings. If improvising with what’s on hand, good position for quality lav might be hanging it from rafters.Â
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Routine solution would be single condenser mic on mic stand, with cable feed to cam through a basic mixer. Try out different positions for mic to find optimum audio.
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For a single moderate fee gig, probably not worth the time and $1000 +++ to build a studio edifice !!
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Use the rehearsal hall or whatever environment harpist customarily employs.
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Seems likely that audition school would be asking for CDs as part of application package. DVD might be proof that applicant/harpist is the one actually playing those chops. If this is the scenario, might not be critical that DVD audio is not 10,000 bit sound.
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(Note. If shooting HD, a problem for many consumer level HD cams is dramatically inferior sound compared to SD cams. Remedy in this case might be one of the many handsized digital audio recorders that are everywhere in musicianland. Did recent gig with condenser mic feeding though mixer directly (single cable, r+l) into my SonyPD170; plus right and left cables going from mixer directly into my Microtrack II audio recorder. The condenser mic was about a foot away from the cam, with cam and condenser mic about 12-feet in front of band on elevated stage.  Both the PD170 and the mini audio recorder captured superb sound from the condenser mic. The PD170 on camera mic also picked up very good audio, not as good as the condenser mic source, but quite usable for a stereo mix.)
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REGARDS … TOMSCRATCH