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Sample Libraries Since a sampler is a type of recording system, the quality of the samples depends on the quality of the recording techniques. Making high-quality samples requires good players with fine instruments, excellent microphones, and favorable recording environments. An Assessment of Samplers In any case, it is understandable that the "naturalness" or "realism" of a sampler should be held up as a criterion for judging between different brands. It is well known that a given instrument tone may sound much more realistic on one sampler than it does on another. In expressive instruments like voices, saxophones, sitars, guitars, and others, each note is created in a musical context.In addition to these contextual cues, transitional sounds like breathing, tonguing, key clicks, and sliding fingers along strings punctuate the phrasing. Constraints of style and taste determine when context-sensitive effects such as rubato, portamento, vibrato, crescendi and diminuendi, and other nuances are applied. These problems can be broken into two parts: (1) How can we model the sound microstructure of note-to-note transitions? (2) How can we interpret (analyze) scores to render a context-sensitive performance according to style-specific rules?
Pitch-shifting In an inexpensive sampler it may not be possible to store every note played by an acoustic instrument. These samplers store only every third or fourth semitone and obtain intermediate notes by shifting the pitch of a nearby stored note. If you record a sound into a sampler memory and play it back by pressing different keys, the sampler carries out the same pitch-shifting technique. A side effect of simple pitch shifting is that the sound's duration increases or decreases, depending on the key pressed. Two methods of simple pitch shifting exist. Both of these methods are called time-domain techniques, since they operate directly on the time-domain waveform. This is different from the frequency-domain pitch-shifting techniques discussed. Pitch-shifting by sample-rate conversion with a constant playback sampling frequency. (Top) If every other sample is skipped on playback,the signal is decimated and the pitch is shifted up an octave. (Bottom) If twice the number of samples are used by means of interpolation on playback, the signal is shifted down an octave. Sample-rate Conversion Without Pitch-shifting Many digital audio recorders operate at the standard sampling rates of 48 or 44.1 KHz. How can we resample a recording at one of these frequencies so as to play it back at the other frequency with no pitch shift?To convert a signal between the standard sampling rates of 44.1 and 48 KHz without a pitch change, a rather elaborate conversion process is required. These ratios can be implemented as six stages of interpolations and decimations by factors of 2, 3, 5, and 7. 1. Interpolate by 4 from 44,100 to 176,400 Hz 2. Decimate by 3 from 176,400 to 58,800 Hz 3. Interpolate by 4 from 58,800 to 235,200 Hz 4. Decimate by 7 from 235,200 to 33,600 Hz 5. Interpolate by 10 from 33,600 to 336,000 Hz 6. Decimate by 7 from 336,000 to 48,000 Hz The signal can then be played back at a sampling rate of 48 KHz with no change of pitch.
Looping Looping extends the duration of sampled sounds played by a musical keyboard. If the musician holds down a key, the sampler should scan "seamlessly" through the note until the musician releases the key. This is accomplished by specifying beginning and ending loop points in the sampled sound. After the attack of the note is finished, the sampler reads repeatedly through the looped part of the wavetable until the key is released; then it plays the note's final portion of the wavetable. Creating a seamless but "natural" loop out of a traditional instrument tone requires care. The loop should begin after the attack of the note and should end before the decay. The beginning and ending points of a loop can either be spliced together at a common sample point or crossfaded. A splice is a cut from one sound to the next. Splicing waveforms results in a click, pop, or thump at the splice point, unless the beginning and ending points are well matched. Crossfading means that the end part of each looped event gradually fades out while the beginning part slowly fades in again. The crossfade looping process repeats over and over as the note is sustained.
Sampling Synthesis The term "sampling" derives from established notions of digital samples and sampling rate. Sampling instruments, with or without musical keyboards, are widely available. All sampling instruments are designed around the basic notion of playing back prerecorded sounds, shifted to the desired pitch. Instead of scanning a small fixed wavetable containing one cycle of a waveform, a sampling system scans a large wavetable that contains thousands of individual cyclesseveral seconds of prerecorded sound. Since the sampled waveform changes over the attack, sustain, and decay portion of the event, the result is a rich and time-varying sound. The length of the sampling wavetable can be arbitrarily long, limited only by the memory capacity of the sampler. Musique Concrète and Sampling: Background After experiments with variable-speed phonographs in the late 1940s, Pierre Schaeffer founded the Studio de Musique Concrète at Paris in 1950 (see figure 4.1). He and Pierre Henry began to use tape recorders to record and manipulate concrète sounds. Musique concrète refers to the use of microphone-recorded sounds, rather than synthetically generated tones as in pure electronic music. But it also refers to the manner of working with such sounds. Composers of musique concrète work directly with sound objects (Schaeffer 1977; Chion 1982). Their compositions demand new forms of graphic notation, outside the boundaries of traditional scores for orchestra (Bayle 1993). The Fairlight Computer Music Instrument (CMI) was the first commercial keyboard sampler (1979, Australia).
