This piece is also known as "Ricercar pro tempore adventus". The text of the choral venerates Maria as the mother of Christ. Fischer creates a great fugue on the first segment of the melody of this choral. He writes music of such natural flowing beauty, that you have to look at the score to see how intricate he weaves his counterpoint.
The recording was done with the Hauptwerk software and the sample set of the Silbermann organ of the Stadtkirche Zöblitz by Prospectum (http://www.prospectum.com/index.php?language=english&id1=2&id2=8).
Score available here: http://partitura.org/index.php/johann-caspar-ferdinand-fischer-ave-maria-klare