Not that many people that I know of go for fretless bass. I am listening closely for interval tuning differences. In the early 60's Bruce used to play Jazz, on an upright acoustic bass.
I really like concert videos. I like to watch them play. I learn from it. I like concert videos much more than the MTV type.
On MSN In Concert, the motion is too choppy to really be able to watch people play.
In these Cream Reunion pieces at the Royal Albert Hall, Jack Bruce switches back and forth between fretted and fretless basses. The long horn bodied one is fretless. The violin bodied one has frets.
Never seen anyone do that. I am not able to hear the difference, not yet. If there is a logic to it, it would be the role that the bass plays in the composition, and whether or not you want fixed tuning.
As I know, the first electric bases were always fretted. It was in the 70's that the fretless ones started to gain popularity. But that does seem to have faded.
I a Santana concert video, his bass player plays fretted most of the time, but for one piece he changes to Ampeg like stand up fretless. Again, I am not able to hear a difference. I need to learn more about this.
Wikepedia
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Fretless Bass
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There are also fretless guitars. On the web page I listen to some of the music and it is different. It is more harmonious, less disonant. It is like the difference between a violin section and a pipe organ. It is rather like the flexible tuning music of William/Wendy Carlos Williams
I cannot imagine how one could play cords on a fretless guitar.
I am interested to learn that Jaco Pastorius, the bass player added to Weather Report, was playing fretless. From the videos I could not tell.
Obviously Jack Bruce at the Cream 05 reunion is the thing to listen to to try and understand.
Fretless:
Freted
Here is 22 min from the same event, but they don't show live video.
Here is the same trio playing White Room in the 1960's, likely fretted bass.
I might be able to hear a difference, but I am not clear why he sometimes goes back to fretted.
Notice how much nicer the drum sound is in 2005, compared to the 60's.
Cream was 3 virtuosos, with very different styles. Mostly it came down to Jazz Rock.
When Clapton playes with his own people, he has had different bass players, but the play fretted, though usually 5 strings.
Ginger Baker was the first to use an expanded drum kit. Two kick drums and lots of other stuff. He is a very agressive drummer.
Here is the Gibson EB-1 fretted bass that Bruce plays, just like Paul McCartney did.
Click Here
And here is all about the Warwick fretless he plays. In the studio he sometimes plays a Warwick fretless 5 string.
As I see it, Bass led music originated in Jazz, but it is now a common feature of Rock. What made it work were two things, first being able to mic up wood basses because the sound systems had gotten better and the phonograph recording process too. Second, Charles Mingus. This influence continues today in Rap.