A traditional Vedic yagna (fire-ritual) would be conducted by three main priests - a Hotar who would chant hymns from the Rig Veda Samhita, an Adhvaryu who would chant hymns from the Yajur Veda Samhita, and an Udgatri who would chant (or rather sing) hymns from the Sama Veda Samhita. (The Samhitas are the core part of the Vedas, consisting of verses heard directly from the gods.). Now the reason the Sama Veda priest was called an Udgatri is because the Udgitha is the name for a certain portion of the Sama Veda Samhita.
My question is, what part of the Sama Veda is the Udgitha? You can read the Sama Veda here, as you can see, it's divided into two Parts, each of which has a bunch of Books, each of which consists of a bunch of Chapters, each of which consists of a bunch of Hymns with ten verses or less. So which hymns are included in the Udgitha?
Note that if you search for the term Udgitha, you'll often come across assertions that "Om is the Uditha." Such assertions originate in the Chandogya Upanishad, but the reason the Chandogya Upanishad refers to the sound Om as the Udgitha is because the Udgitha portion of the Sama Veda is sung beginning with Om, as described in the beginning of the Chandogya Upanishad:
Let a man meditate on the syllable Om, called the udgîtha; for the udgîtha (a portion of the Sâma-veda) is sung, beginning with Om.
So let me be clear that this question is not about Om.