‘To sing with fidelity... is to sing so that anyone of those singing together should remain in the form of those notes that were transmitted by our devout fathers, so that no one breaks them up into many or goes off in any way into a fifth above or a fourth below, or by leaping to another interval or by wandering off in the manner of a discant and deviating from them. For all such departures from the devout melodies of the holy fathers impedes the listeners more than it produces devotion, nor is it a sign of devotion in the singers, but rather seems to be evidence of a blameworthy levity. What is more, there are those in the chorus sufficiently prejudiced - because they often are or become the occasion of error in singing - that the rest taking care to remain in the true notes or prescribed melody, not only because of this straying beyond the accurate note in the manner of those [prejudiced ones], these [faithful ones] are impeded in their own purpose, but are also deceived by the assistance of those others who are straying, who, if they would remain with them [the faithful] in the true notes and faithfully assist them, then they would be preserved better reciprocally from confusion, just as no one can doubt.’
‘Devotionaliter cantere... est sic cantare, quod quilibet simul cantantium in forma maneat in eis notis, quae a devotis patribus nobis sunt traditae, ita quod nullus illas in plures frangat vel ab eis quomodolibet recedat in quintam supra vel in quartam infra aut in aliam concordantem saliendo vel ad modum discantus divagando et ab eis declinando. Omnes enim tales recessus a sanctorum patrum devota melodia plus in auditoribus impediunt quam generent devotionem, nec in cantantibus devotionis signa, sed potius reprehensibilis levitatis indicia esse videntur. Et quod amplius est, sunt choro non parum praeiudiciales, quia saepe sunt vel fiunt occasio confusionum et errorum in cantando, quia ceteri in vera nota sive melodia praescripta manere curantes non solum per huiusmodi quorundam extra veram notam divagationem impediuntur in suo proposito, sed etiam eorundem sic divagantium auxilio fraudantur, qui, si cum eis in vera nota manerent eosque fideliter iuvarent, melius se mutuo a confusionibus praeservarent, prout nemini venit in dubium.’
Conrad von Zabern, Die Musiktraktate Conrads von Zabern, ed. Karl-Werner Gümpel (München: Bayerische Akademie Der Wissenschaften, 1956). Translation from “De modo bene cantandi (1473) [published 1474]” Translated by Sion M. Honea” (n.d.), https://www.uco.edu/cfad/files/music/conrad-von-zabern.pdf.
Conrad seems to describe two basic techniques of which he disapproves, both of which are documented in other contexts:
Breaking up the notes into many i.e. the insertion of florid ornaments
Improvising another voice in ‘discant’
It’s possible that the comment about singing fourths or fifths above or below the chant could be reference to certain parallel structures documented around this time, but it’s equally possible that it’s an extension of Conrad’s comment on florid ornamentation.
Aside from the negative effect that these techniques supposedly had on the easily corruptible listeners, it’s noteworthy that Conrad emphasises that those singers who were (attempting) to sing only the notes were ‘often’ [‘saepe’] distracted by the florid contortions of these supposedly decadent virtuosos.
The image below is the introduction to Franchinus Gaffurius, Practica Musice (Milan: Gulielmum signer Rothomagensem, 1496). Notice in particular the group of young singers being guided by an adult singer in the bottom left-hand corner of the page.