A pedal trill is produced by plucking the string once and then moving the pedal back and forth between two positions. A pedal trill will always be slower than a trill produced with the fingers. Both single (short) trills and long sustained ones can be produced with the pedals.
A pedal trill can also be produced without plucking the string. When the pedals are moved the tuning discs touch the string thus making it swing. (see also pedal notes ) In this case the trill will be softer and not defined by a certain octave.
A pedal trill and a regular trill sound quite different from one another. The pedal trill sounds more like a slide back and forth between two notes. Note that the tuning discs hit all the strings the pedal controls. Therefore, although only one string is plucked the trill will also be audible in the other octaves.
A pedal trill is notated in the same way as a regular trill with the addition of pedal indications. The pedal indications are mandatory in order to distinguish the pedal trill from a normal trill.
Pedal trills are possible on the whole range of the harp. Long trills are most effective in the lower and middle register, since the resonance of the strings lasts longer there.