The Keybed: Pro keyboards come in a number of sizes with various numbers of keys. It's common to kind keyboards with 88, 76, 61, 49, 36, and even 25 keys. You will also hear the words 'hammer action', 'semi-weighted', and 'synth action' in regard to keys. You must determine which combination of keybed action type and number of keys will provide the best combination of feel and functionality for you.
Sound Quality/Engine: In a professional setting, the only issue of sound quality that can make a difference is the sample rate and bit depth of the samples, such as 24-bit/96kHz, which is rapidly becoming the standard for professional recording. With many synthesizers offering sounds that were recorded in world-class studios with rare vintage gear, as opposed to the lo-fi, bit-crushing and sound mangling that has become popular in some genres of music; the question of sound quality becomes an issue of whether or not the sounds a particular synth produces inspire you.
RAM/ROM - Upgradeable: This is an important issue in choosing a keyboard. ROM expansion allows you add sound cards thus extending the sonic and useful life of your keyboard. For sampling keyboards, the amount of RAM determines how much sampling time you have.
Sampling/Sample Player: To determine which to choose, a sampler, sample player, or workstation, consider the type of work you will be doing. Will you be performing, sequencing, or composing, or any combination therein? Do you prefer to create your own sounds, or would you rather spend time composing?
Type of Synthesis: This depends on the type of music you want to make and also on what other sound sources you have. If you want one keyboard, a computer and nothing else, then consider one with options of different kinds of synthesis, such as the Kurzweil VAST system. Do you want authentic analog modeling or clean sounding sample playback? If you want realism, plan to do orchestral, mainstream, or pop music; you will need convincing piano, string section, brass, and percussion sounds, in which case, a sample playback synth will work. If you are doing dance, trance, D 'N B, techno, then an analog modeling synth would be more appropriate.
Workstation/Sequencing: In the workstation you can record MIDI sequences using the preset sounds and combination programs, drum loops, high quality samples from the best sample CD-ROMs, even your own voice or guitar as audio. If this excites you, but you're not interested in the steep learning curve of a computer system, then a workstation is for you.
Effects Processing: Virtually all synthesizers come with some form of effects processing. If you are considering a workstation for all-in-one concept to CD production, then along with the usual types of dynamics processing, look for mastering effects as well.
Knobs and Controllers: The bare necessities are a functioning pitch wheel and mod wheel. Don't leave home without them. Make sure that the knobs and sliders transmit MIDI continuous controller messages. These become very useful as you find your way around the MIDI universe. You can use them to sweep filters and fade FX, not only with the keyboard's sound engine but in your sequencer as well.
Removable Media: This is important for archiving or saving your patch information. Its purpose is twofold. If your keyboard needs repair, you won't lose your settings if the keyboard is reinitialized. Also, in time, the number of patches you create may exceed the memory capabilities of the keyboard. Removable media allows you to organize your patches for use as your performance needs require.
PC Interface: USB or mLAN: Many keyboards offer a USB or mLAN (a version of FireWire) interface for data transfer, offering even greater possibilities for sample transfer and data archival. If you are working with a computer-based DAW, a keyboard with a built-in audio interface can be a very cost effective way to get high-quality audio in and out of the computer without having to invest in additional sound cards.
Roadworthiness: If you plan on gigging, then the most important factor to look for is durability. Not all keyboards are meant to leave the studio.
