A Cantor ("psalti") is a lay person who assists the priest by chanting the responses and hymns in the services or sacraments of the Church. To some extent, the choir has replaced some of what chanters do. The chanters in the Orthodox Church represent the congregation. The Byzantine chanting is the dialogue between the people and the clergy. In other words, the chanting speaks for the people.
Our chanters are responsible for the correct rendition of the Church hymns in the Orthodox Church. There is a cycle of eight different tones which are chanted in the Orthros Services, the Divine Liturgy, and other Church services. Our Church calendar is set up so that we may daily remember and honor saints and events in the life of the Church. Every day of the year has designated people and events to commemorate and aliturgy celebrated on any day has a whole set of hymns relating to those people and events, as well as a Vespers service and an Orthros service. In addition, there are the hymns sung at special services such as the compline, baptism, wedding, and memorial services. During the Divine Liturgy, hymns are integral from beginning to end.
Our Liturgy, however, is very structured, and while we may use different melodic lines, the text of the service and the hymns we sing are the same in every Orthodox Church in the world on any given Sunday. We cannot arbitrarily decide which hymns we want to include in a service, or which Scripture readings to follow.
There is a variety in Orthodox hymnology; for example, the 'kontakion' is a hymn containing a small sermon about the event or person being recognized on a particular day. If a Sunday is not a major feast day, the kontakion is "Prostasia ton Christianon."
In addition to these hymns sung during the Liturgy, there are whole sets of hymns sung during the Vespers and Orthros services. There are a series of humns after "Kyrie ekekraxa" during Vespers. there are a series of hymns following the Praises in the Orthros.
Cantors at Saint Sophia | George Papadopoulos & Aleksander Filip