including "non-musical" sounds such as a typewriter clacking or a dog barking. A reference center pitch indicates the actual frequency of the recorded note. Samples may also be "looped" by defining points at which a repeated section of the sample starts and ends, allowing a relatively short sample to play endlessly. In some cases, a "loop crossfade" is indicated, allowing less obvious transitions at the loop point by fading the end of the loop out while fading its beginning in. Keymaps are arranged into instruments. At this level parameters may be added to define how the keymaps are played. Filters can be applied to change the sound-color while low frequency oscillators and envelope generators can shape the amplitude, pitch, filter or other parameters of the sound. Instruments may have multiple layers of keymaps to play more than one sample at the same time and each keymap may have a different set of parameters so that the incoming note-events affect each layer differently. For example, two layers may have a different sensitivity to the velocity of the incoming note, altering the resulting timbre according to how hard the note is played. At this level, there are two basic approaches to sampler organization. In a bank approach, each instrument is assigned to a different MIDI channel and multiple banks can be stored to reconfigure the sampler. A different and more powerful approach is to associate each instrument with a patch number or ID so that each MIDI channel can be configured separately by sending controller information on the individual channel. Types Many samplers work as described above: the keymapping system "spread out" a sample over a certain range of keys. This has side-effects that may be desirable in some contexts, such as speeding up or slowing down drum loops. However, the higher and lower-pitched parts of such a keymap may sound unnatural. For example, if a harpsichord is sampled in its lower register and then the samples are moved up to very high pitches, the high notes may not sound natural and authentic. When arranging a pitched instrument over several keymaps, the transition from one to another may be too noticeable for realistic imitation of the instrument – the art is to make transitions as smooth as possible. Some phrase samplers are more optimised for triggering single "one-shot" sounds such as drum hits. Each keymap spans only a single key, requiring a