Rāmānujaon Brahma Sūtra 3.3.9

Since the pranava, which is a part of the udgîtha, is introduced as the subject of meditation in the first prapâthaka of the Chândogya, and extends over the later vidyâs also, it is appropriate to assume that also in the clause 'the gods took the udgîtha'—which stands in the middle—the term udgîtha denotes the pranava. Expressions such as 'the cloth is burned' show that frequently the whole denotes the part.—The conclusion from all this is that in the Chândogya the object of meditation is constituted by the pranava—there termed udgîtha—viewed under the form of prâna; while in the Vâjasaneyaka the term udgîtha denotes the whole udgîtha, and the object of meditation is he who produces the udgîtha, i.e. the udgâtri, viewed under the form of prâna. And this proves that the two vidyâs are separate.—Here terminates the adhikarana of 'difference.'