INTRODUCTION TO SYNTHESIZERS - hol.gr

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ecreated. The circuit reading the samples from the memory and converting them to sounds is called the "digital-to-analog converter", or D/A-converter. Just as the A/D converter, this circuit is also controlled by a clock. For each tick of the clock, a new sample point is read out and added to the waveform. The speed of the clock controlling the D/A-converter is called the "readout rate". If the readout rate is set to the same rate as the sample was recorded, the result is an exact reproduction of the original sound source - or as exact as the sampling frequency and sampling resolution used for the recording process will allow. But we can also choose to change the readout rate to achieve a pitch change, or transposition. If we increase the readout rate, it will be just like playing a tape with higher speed - the sound will have a higher pitch. If we instead read the samples at a slower rate, the pitch will drop. This method is often used to map the sampled sound over the keyboard and thus creating a playable sound. By doubling the readout rate, the pitch of the sound will be exactly one octave above its original value. Another transposition method is "skipping" some of the sample points while playing back a sampled sound, and thereby speeding up the playback rate. By skipping every other sample value, we in fact raise the sound with one octave. To lower a sample beyond its original pitch, new artificial values need to be added between two sampled values. We can lower the sound with one octave by simply retrieving each sample point twice. Multisampling Unfortunately we can't just take one sampled sound and transpose it over the whole keyboard range. As we increase the sample readout rate, we will notice that not only the pitch will rise, but the sound will also become very weird and "chipmunk-like". And at the opposite side, an artificially lowered sound will start to break up and distort just a few notes below the normal pitch. To avoid this, we can use a technique called multisampling. Instead of using just one sample over the whole keyboard range, we use several different samples with different pitches, and combine these to cover the playable range. We could instance sample a C3 note, a E3 note and an A3 note to cover one octave. Now we don't have to force one individual sample to stretch further than just a few notes. If we wish, we can use a different sample for each key on the keyboard, but quite often we can get away with as few as five or six different samples. To further enhance the playability of our sampled sounds, we can even assign different sets of samples to be played depending on how hard or soft we strike a key on the keyboard. Such a map of samples is usually called a program. Editing Once a sound has been digitized and resides inside our computer or sampler, we can manipulate and change it in an almost infinite number of ways. By using a computer, we can visualize the sample and edit it almost any way we like. Samples can be time-stretched, cut, shortened, reversed, flanged and twisted to create completely new and astonishing sounds. One of the most useful editing functions is the "loop" function. Looping a sound means that a small part of the sampled sound is played repeatedly over and over again. By using a clever loop, a short sample of e.g. a violin sound can be stretched out to last as long as we hold down the key. A benefit of the loop function is of course the saving of precious sample memory. But there is nothing to stop us from sampling whole bars of music. We can in fact rip off drum patterns, vocal chords, guitar riffs or cool grooves from our favorite album and loop it to use them in our own music. Ethically this might of course be highly doubtful and since we're most likely infringing on copyrighted material, it can even involve legal aspects. Playback modules 12
Sampling is an art - it is very hard to create musically useful samples. To create a good multisample of a string ensemble, we would first and foremost need to have a real string ensemble at our disposal - not very likely, is it? Fortunately we can let others do the job for us - we can for instance purchase third party sample-CD's and CD- ROM's. But there are also a large number of synthesizer instruments, which already come fully loaded with hundreds of crisp and clear 16-bit stereo multisamples from the factory. These instruments are usually called sample-based or PCM-based instruments, which means that their sound relies completely on the samples onboard. They can't sample new sounds, but instead they use these prefab samples as their internal waveforms. The problem with this technology is that the sound palette of these instruments is rather limited. Even if they sometimes are expandable with new sound-cards, basically we are stuck with the sounds provided by the manufacturer. While this may not be a concern for a country-band keyboard player, a highly realistic soprano sax may not be of any value to the new-age synthesist. MIDI In the early 80's, every synthesizer manufacturer had their own standards on how to control their synthesizers remotely. Two Moog synthesizers could for instance quite easily be linked together, but if you wanted to connect an Oberheim OBX-a synthesizer to an ARP 2600 synthesizer, you were in trouble: the two synthesizers used totally different control signals and different voltage ranges. To remedy this problem, some of the leading synthesizer manufacturers decided upon a standardized set of signals, that would allow a more flexible communication between different synthesizer models. This standard was named MIDI, which is an acronym for Musical Instrument Digital Interface. It is a standardized language by which electronic instruments and computers can communicate and exchange information with each other, regardless of make or model. Connecting several synthesizers to form a network can be very useful for many reasons - for instance, we may wish to control more than one synthesizer remotely by using only one keyboard. But the most useful and most common application of MIDI is the recording and editing of an entire composition with a MIDI sequencer or equivalent software. We will speak more about this in a moment, but let us first take a closer look at MIDI. What kind of information is handled by MIDI? Let's illustrate this with an example: Say that we press down a key on a MIDI synthesizer. Besides the obvious fact that the synthesizer produces a note, it will also generate a MIDI signal. This signal looks something like this: NoteOn-60-127 For a synthesizer or computer the message is plain and clear: Depress the middle C key on the keyboard with maximum velocity It's important to understand that this signal has nothing to do with the sound itself. It doesn't state anything about if the sound is supposed to be a flute or a snare drum sound. You cannot process the MIDI signal like audio signals, such as adding a reverb to it. The generated MIDI signal is now transmitted through the MIDI Output jack on the back of the synthesizer. If this output jack is connected with a cable to another synthesizer, the signal will travel through the cable and into the second synthesizer. This second synthesizer in turn will recognize the incoming signal and will respond accordingly. It will also produce a note - just like as if you had played its own keyboard. In such a setup, the first synthesizer is usually called the "master", and the second synthesizer is called the "slave". 13
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Page 5 and 6: more "rounded". The more overtones
Page 7 and 8: Of course not just the loudness of
Page 9 and 10: assign it to the new note. One soun
Page 11: The illustration above shows a simp
Page 15 and 16: accordion or the didgeridoo (even t
Page 17 and 18: By connecting a MIDI-keyboard to th
Page 19 and 20: ... and the Roland D-550 module But
Page 21 and 22: Software-based sequencers are proba
Page 23 and 24: Click on the icon to listen to a mo
Page 25 and 26: The faders The sound volume for eac
Page 27 and 28: pattern. The echoes can be set to e
Page 29 and 30: Sampled or synthetic If you are int
Page 31 and 32: the display for flickering when you

INTRODUCTION TO SYNTHESIZERS - hol.gr

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