Dhrupad is the oldest surviving form of Hindustani classical music.
It is the oldest form of compositions in indian classical music and a form that is also found in the South Indian as Carnatic music.
The Dagar dynasty is the main stream for 19 generations to maintain the tradition of this music. They all belonged to a prestigious family of dhrupad singers whose founder was Baba Gopal Das, an Hindu who is said to have converted to Islam at the time of Emperor Muhammad Shah. His younger son Behram Khan established himself in Jaipur, where he became a court musician of great repute. He taught his brother's grandsons Allabande and Zakiruddin Khan who used to sing together, quickly asserting themselves as the foremost dhrupad vocalists of their time.
Ustad Nasir Faiyazuddin Dagar (1934 - 1989) and Ustad Nasir Zahiruddin Dagar (1933 - 1994) are the younger brothers of the family. They trained many students, including Ustad Faiyaz Wasifuddin Dagar, the son of Nasir Faiyazuddin Dagar, who is keeping the family tradition alive along with other dagarvani exponents.
You can listen to their talent starting at 13' of this movie. Impressive!
and totaly amazing around 33'...
You can also look to the 1982's movie titled "Dhrupad" directed by Mani Kaul.
(unfortunately, no subtitles)
after years, my personal focus to this music was renewied recently by "The Hum", the famous blog runned by Bradford Bailey whose ecclectic interrests cross miscellaneous musical fields as Fluxus, electroacoustic music, improvisation areas, figures as Henry Flynt, La Monte Young, Harry Partch, John Fahey, Steve Reich, James Tenney, Glenn Branca and many many others...
"The Hum": highly recommended!