Determine your requirements for MIDI control of your electronic music instrument. USB does not satisfy all digital instrument tasks. If you are connecting synthesizers with each other, this requires standard MIDI throughout the process. Many older computers do not support USB, and thus MIDI conversion would not be effective. As an alternative, computers usually support an internal sound card. It is possible to connect MIDI to a computer directly through a sound card's joystick interface. If USB is still the only answer, a USB MIDI converter is the only option.
Identify the number of MIDI interfaces you need for your electronic music setup. A simple single MIDI connection is possible in a converter device under $10 (see References). Other more robust devices can cost hundreds of dollars and provide multiple MIDI outputs for controlling a suite of digital equipment. Keep in mind that MIDI supports chaining, so multiple instruments may be strung together using just a single cable. If you need more than 16 channels, consider a converter that provides the required number of MIDI outputs.
Buy a simple inexpensive USB MIDI cable converter if you need a single MIDI connection through your USB port. These devices offer a standard USB cable on one end and two MIDI connectors on the other. Each MIDI connector handles opposite transfers. One connector operates as the MIDI output and the other as the MIDI input. Once connected, the cable will automatically register your MIDI equipment in any software program that support MIDI activity.
Consider more-complex hardware for MIDI applications that require more than just a single connection. MIDI equipment is often joined with audio interfaces for combined recording control. Additionally, some USB MIDI converters offer additional effects and features tailored for certain musical tasks.
USB to MIDI conversion always requires additional hardware. The extent of this hardware runs the spectrum of complexity to accommodate different recording environments and even music genres.