|Württemberg Landesbibliothek Stuttgart, Cod. bibl. fol. 23: folio 163v|
The psalter shows King David playing not the typically-seen lyre—an Anglo-Saxon lyre in the English-Irish psalters and a Greco-Roman one in others—but rather he is shown playing a chordophone instrument referred to as a cithara in the Latin psalm text, from which the word guitar (gitarra in Spanish) is ultimately derived.
This instrument appears a half-dozen times in the Psalter, shown perhaps most clearly at folio 055r where it is played with a plectrum, and it appears also theorbo-sized folio 108r, suggesting that it may have been used at service because it is played in front of an altar with the gospels open on it.
In the picture we see King David and his Germanically-costumed hot combo, which includes a woman playing Moroccan clackers in the upper right, a naked dancing boy, and some guys trying to figure out how to make the bellows work the pipe-organ in the lower right, plus a cat upper left blowing a horn that looks suspiciously like a cornetto, since it's bent in shape.
I think it would be fun to reconstruct these instruments and play them together, noting that the basic requirements for a modern R&B group are satisfied: percussion, keyboard, sax, plus of course lead guitar and vocals. (Only a bass is missing, for which we have to wait till the invention of Renaissance consort instruments.) It is nonetheless clear that there may have been more going on in Paris in the 820's than we suspected!
Considering that the invention of the guitar evidently goes back to the late Bronze age as explained here, it is conceivable that King David— if he existed to begin with—might have played one in the 10th c. BCE.
The hour-glass shaped instrument is fretted, which suggests that the Hittites may have used musical scales.