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Beginning in the mid-1980s with the Roland D-50 and the Korg M1, sample playback synthesizers became an important instrument category. For a number of years, sample playback eclipsed all competing technologies. It’s still a vital technology, found in millions of instruments — but as the speed of digital chips has increased, it has become practical to build synths that can create complex, musically useful tones using other methods. Today, sample playback is only one color in a well-rounded musical palette.

The basics of digital sampling were covered in Chapter Two. To recap, in sampling an actual sound is recorded digitally. The sound could be anything: a single note on a clarinet, a two-bar beat played on a drum kit, a guitar playing a distorted power chord, a human voice saying “doh!,” or you name it. After recording, the sample is stored in memory and assigned to the keyboard. When you play a key, the sample is played back. To be a little more technical, the oscillator reads the sample from memory and sends it on to the rest of the synth (the filter and so on).

The advantages of sample playback over traditional analog synthesis are considerable. With sampling, we can create complex and realistic sounds with the greatest of ease. Simply replace the clarinet sample with a bassoon sample (which you can do by selecting the bassoon waveform on the oscillator’s edit page), and your synth will sound like a bassoon rather than a clarinet. Sounds like an unbeatable deal — but there are limitations to sample playback technology, as we’ll see.

In order to be musically useful, a sample playback oscillator has to be able to change the frequency of the sound as you play up and down the keyboard. This is not a trivial issue, but fortunately it isn’t anything you need to worry about as a musician and synth programmer. The details of how a sample playback oscillator changes the frequency of the sample are beyond the scope of this book, but a brief detour might be useful. An analogy that’s often used (not quite correctly) is that a sample playback oscillator changes the frequency of the sample by playing the sample faster or slower. In this analogy, sample playback works much like an analog tape deck or turntable. The latter devices change the frequency of playback by passing the recording (tape or vinyl groove) across the playback device (tape head or stylus) faster or slower.

The tricky bit is, in a sample playback synth you can’t actually speed up or slow down the rate of playback. The entire synth always operates at one fixed clock speed — for example, 44.1kHz. All of the samples will always be played back at that speed. In order to create the illusion that the playback of a given sample has been sped up or slowed down, a sample playback oscillator uses interpolation. If the sample needs to be slowed down in order to sound at a lower pitch, the oscillator actually adds new sample words between the existing sample words. Conversely, if the sample needs to be sped up, the oscillator will drop some of the sample words so that it takes less time to play through the whole sample.

Interpolation, however, changes the shape of the waveform. In other words, it introduces distortion. In first-generation digital samplers, such as the Ensoniq Mirage, this distortion was quite audible. (In fact, it’s one of the reasons why these instruments are sometimes prized for their distinctive sound.) But in general, uncontrolled distortion is a bad thing. So software engineers have developed some clever mathematical operations to perform better interpolation, thereby reducing the distortion. I’m not aware of any instrument in which the interpolation algorithm is a user-programmable choice.

Interpolation can become a sonic issue when a sample is assigned to a wide keyboard range and is then played several octaves below the frequency at which it was recorded. All sounds change character radically when transposed down by several octaves, but poor interpolation can give the sample a grainy, chirpy, breathy quality. Which might be exactly what you want, of course.

Because of technical limitations in their interpolation algorithms, some instruments limit the amount of upward transposition of samples to an octave or so.

Multisampling. As you might have gathered from the foregoing, the character of a sampled sound will usually change as its pitch is raised or lowered. When the pitch is raised or lowered by more than a few half-steps, the change can become musically objectionable. This happens because some of the components of the sound will no longer be realistic.

For instance, when a sample of an acoustic piano is transposed up or down, the “thwock” of the hammer striking the strings is transposed along with the string tone itself. But in in a real piano, this percussive attack is always at pretty much the same pitch, no matter what key you strike (it’s a little higher in the upper register, because the hammers in the piano’s upper register are smaller, but the change from key to key is much less than a half-step). If you take a sample of a piano playing a single note and transpose it up by an octave, it will sound as if the strings are being struck with a tiny aluminum hammer.

Another example: If a violinist is sampled playing a note with vibrato, when the note is transpose upward more than two or three half-steps, the vibrato gets fast and twittery. When the note is transposed down, the vibrato gets slow and sea-sick — good for a special effect, but not good for playing conventional violin sounds.

In order to make sampled instrument sounds such as guitar and electric piano playable over a wide range of the keyboard, manufacturers use a technique called multisampling.

In multisampling, the manufacturer or sound designer records a number of samples from the same source, such as an acoustic instrument, at various pitches. These samples are then assigned to various zones of the keyboard. With an electric piano, for instance, there might be two or more samples in every octave, each assigned to a range of keys (a zone) spanning only four or five half-steps. In this case each sample only needs to be transposed up or down by a few half-steps, so the sonic changes caused by transposition are less apparent.

When you choose a sampled “waveform” for a synth patch, quite often what you’re really choosing is not a single waveform but a multisample — a group of related samples that are already mapped to the keyboard in an appropriate way. In some synths, such as the Kurzweil K2000 series, it’s possible to change the way the samples in a factory (ROM-based) multisample are assigned to the keyboard, but in many instruments this mapping is not user-programmable.

Multisampling can give a good approximation of the sound of an acoustic instrument across its whole range. By editing the MIDI data in a sequencer track with care, you may be able to give listeners the impression that they’re hearing a performance on the original instrument. (The tricks involved in editing the sequencer data are well beyond the scope of this book.) But not all multisamples lend themselves to realistic performance.

The biggest problem with multisampling is that quite often the individual samples in the multisample don’t match one another very well. When you play a scale up or down the keyboard, the places where one sample stops and another begins (called multisample split points) can be distressingly obvious. This can happen because the person doing the sampling is unable to find samples that match one another, or because he or she isn’t listening to the samples closely enough. But when samples are assigned to zones that are a number of keys wide, the note just below the split point plays a sample that has been transposed up by a number of half-steps, while the note just above the split point plays a sample that has been transposed down by a number of half-steps. The change in sound quality due to transposition exaggerates even tiny differences between the samples.

One solution to the latter problem would be a multisample in which each key on the keyboard plays a different sample. If the samples were reasonably well matched, such a multisample would minimize the problems of both split points and transposition artifacts. Unfortunately, such a multisample takes up a lot of memory.

Synthesizer manufacturers naturally want to provide their customers with a wide array of sampled sounds. But no instrument has an unlimited amount of memory. So compromises are necessary.

Manufacturers have to compromise: If they dedicate twice as much memory to an electric piano multisample, the instrument will have room for fewer multisamples overall. Also, developing a good multisample takes time. As memory gets cheaper, a manufacturer may elect to use an existing piano or woodwind multisample from one of their older instruments and spend their limited R&D budget on entirely new samples.

Velocity Cross-Switching. Many — indeed, most — acoustic instruments sound quite different when played softly than when played hard. Typically, a loudly played note will be brighter. In the case of plucked and percussive instruments, louder notes will also have more prominent attack transients, and the sound will last longer. This fact poses a problem for sample playback instruments.

If we sample a single note on a snare drum, for instance, by playing the drum at a medium loudness level, we can simulate some of the characteristic dynamic differences of a real drum by using a lowpass filter to filter out some of the high overtones at low MIDI velocities. (Filters are discussed in Chapter Five.) But such a snare drum preset won’t sound completely realistic at either low or high velocities.

This problem is addressed in many sample playback synths by means of velocity cross-switching.

Instead of providing one sample of a given snare drum, the manufacturer or sound designer records two or more samples of the same drum. The drum is sampled being tapped lightly, being hit very hard, and perhaps at a number of levels in between. These samples are then assigned to the multisampie in velocity zones, so that low MIDI velocities cause the light tap sample to be played and so on.

I’d love to be able to tell you that this type of multisampling guarantees that the multisample will respond to a MIDI performance in a realistic way, but real life is messier than that. Some velocity cross-switched multisamples are very good indeed, but many of them are offensively bad. Electric piano multisamples seem to be among the worst offenders. Quite often, a synthesizer’s electric piano sound will be beautiful and smooth if you play at moderate velocities, but if you spank the keys hard enough that the high-velocity samples kick in, you’ll hear a radically different timbre. Controlling your keyboard performance so as to get smooth transitions between moderate and high velocities may well be impossible.

As with other types of multisampling, the problems are due to cost and limited memory. In order to provide enough samples at different velocities to allow the instrument to be played in a musically satisfying fashion, the manufacturer would have to spend far more time developing the multisample, and dedicate more memory space to it in the synth.

Actually, the real problem is a little deeper than that. Sample playback is not, in the end, a very good technology for creating synthesizers that can respond to a musician’s performance in a satisfying way. The early promise of sampling — “Hey, you can sound exactly like any instrument!” — has proven gloriously optimistic. Sampling is very good for some musical tasks, but if you want the sound of a real acoustic instrument, the best solution (though not the cheapest or easiest one) is still to hire someone who plays the instrument well and record them playing the entire part, not single notes.

Sample Playback Modulation. The sound of sample playback is made a little more musically responsive in some instruments by the inclusion of features for modifying the sample data before or during playback. The most common parameter for this purpose, found in E-mu synths among others, is the sample start point. Normally, sample playback starts at the beginning of the sample. But when the sample start point has been set to some value other than zero, playback will start at some other point in memory — presumably at some point after the beginning of the sample.

This is useful because the sounds made by acoustic instruments usually change most rapidly during their first 50ms or so. The changes that take place during this period are called the attack transients. The attack transients give listeners important aural clues about what they’re hearing. Once the instrument has

settled down to produce a steady tone, it has less individual character.

By modulating the sample start point from velocity, you can program a sound so that the full portion of the sample containing the attack transients will be heard only at high velocities. At low velocities, some or all of the attack transients will be skipped. This will make the sound less percussive at low velocities, mimicking (to some extent) the behavior of an acoustic instrument. Used with care, this technique can produce fairly realistic results.

In some synths, you may be given a choice of waveforms with or without the attack transients. Since the sample will typically go into a loop after the attack, you may see waveforms with names like “EP1” and

”EP1(loop).” The two waveforms will most likely use exactly the same wave data, but the first will contain the attack transients followed by the sustaining loop, and the second will contain only the loop.

Another type of control over the sample start, found on some instruments, works in exactly the opposite way. Instead of starting the sample playback immediately when the note is played, the synth waits for some period of time (from a few milliseconds to a couple of seconds) and then starts the sample — from the beginning, but late. A start delay parameter is useful mainly for special effects. Usually it’s used only on multi-oscillator instruments, and one oscillator is programmed for no delay so as to define the start of the note. Small amounts of delay can smear the attacks of the notes by spreading out the attack transients.

Larger amounts can be used for harmonic effects. On a three-oscillator synth, for instance, you may be able to tune the three oscillators to a triad, delay the third of the triad by some amount, and delay the fifth by twice as much, producing an arpeggiation on each note.

Drum Kit Multisamples. Many sample playback synths provide special multisamples in which many of the instruments in a percussion setup can be played from the keyboard at the same time. In some instruments these drum kit multisamples can be edited by the user so as to use a different snare or hi-hat, or to bring together all of the samples that will be needed for a given musical project.

In many synths, drum kit editing goes far beyond selecting drum samples for individual notes on the keyboard. You may also be able to tune the samples individually, give each key its own filter settings and panning, send specific drums to specific effects buses, and so on. (Effects busing is covered in Chapter Nine.)

Hi-hat groups are an important feature in drum kit programming. Other terms used for the same feature include “exclusive channels” and “mute groups,” but the feature dates back to early drum machines, when it was used exclusively for the hi-hat voices. In a real trap set, the hi-hat (a pair of cymbals on a pedal-operated stand) can be open or closed, and can be either struck with a drumstick or brought closed by pressing the pedal. If the hi-hat is struck while open and is then closed with the pedal, the open sound, in which the cymbals ring for some period of time, is abruptly cut off. To simulate this, synthesizers and drum machines allow two or more keys playing separate hi-hat samples to be assigned to the same voice channel. (Voice channels are discussed in Chapter Three.) When the musician triggers the closed hi-hat sample, it will cut off the playback of the open hi-hat sample.

Your synth may provide several independent hi-hat groups. If it does, try assigning other types of semi-sustained percussion sounds to these groups and experiment with the kinds of rhythmic effects you can create.