21. c) But if for some reason a choir cannot sing one or another liturgical text according to the music printed in the liturgical books, the only permissible substitution is this: that it be sung either recto tono, i.e., on a straight tone, or set to one of the psalm tones. Organ accompaniment may be used. Typical reasons for permitting such a change are an insufficient number of singers, or their lack of musical training, or even, at times, the length of a particular rite or chant.
This instruction on sacred music, and the sacred liturgy was submitted to His Holiness Pope Pius XII by the undersigned Cardinal Prefect of the Sacred Congregation of Rites. His Holiness deigned to give his special approval and authority to all its prescriptions. He also commanded that it be promulgated, and be conscientiously observed by all to whom it applies.
7. Modern sacred music is likewise sung in many voice-parts, but at times with instrumental accompaniment. Its composition is of more recent date, and in a more advanced style, developed from the previous centuries. When this music is composed specifically for liturgical use it must be animated by a spirit of devotion, and piety; only on this condition can it be admitted as suitable accompaniment for these services.
21. c) But if for some reason a choir cannot sing one or another liturgical text according to the music printed in the liturgical books, the only permissible substitution is this: that it be sung either recto tono, i.e., on a straight tone, or set to one of the psalm tones. Organ accompaniment may be used. Typical reasons for permitting such a change are an insufficient number of singers, or their lack of musical training, or even, at times, the length of a particular rite or chant.
If you review the post you are quoting, you will find that I said "The only allowed options at the EF are the chants in the book, Psalm tones, recto tono, or polyphony".
21. c) But if for some reason a choir cannot sing one or another liturgical text according to the music printed in the liturgical books, the only permissible substitution is this: that it be sung either recto tono, i.e., on a straight tone, or set to one of the psalm tones. Organ accompaniment may be used. Typical reasons for permitting such a change are an insufficient number of singers, or their lack of musical training, or even, at times, the length of a particular rite or chant.
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