Hey Cyril,
Our inhouse composer - CK - who visited his orchestration and instrumentation lessons (Prof. Sevsay) constantly (unlike me) during his studies of composition just informed me about the following details on tremoli:
Actually there are two different terms and signs for tremoli
measured tremoli
http://members.chello.at/tozo2003/best_trm1.png">
and
unmeasured tremoli
Unmeasured tremolo means for the musician that he should play the same note as fast as possible (as repetition).
Measured tremolo means that the musician should play twice as fast as written. 1/8 will become a 1/16.
-> this sign for measured tremolo means the musician should play four times as fast (and is sometimes interpreted as an unmeasured tremolo).
A measured tremolo is "in tempo" unlike an unmeasured one.
CK suggests to use "perf-reps_spi" for Strings and "perf-rep_fast" patches for timpani in order to get a measured tremolo. In case that you like to accentuate certain parts of a measure he suggests to use our recorded rolls and to a add single note.
(see also http://www.music.vt.edu/musicdictionary/)
M a y a