The digital piano is, at heart, a highly specialized computer, and like all computers, its functions are dependent on its software. When we speak of the software that runs on the digital piano, we are typically talking about what is properly classified as firmware. Firmware is software that is embedded in a hardware device such as a microprocessor or associated memory chip. This can be done in two ways: the firmware can either be permanently burned into the chip, or it can be written in the chip's memory, which also means it can be rewritten if necessary. Just as computers occasionally need a software upgrade to fix a previously undetected problem — a "bug patch" — the more complex digital pianos can benefit from the ability to accept firmware upgrades. This may never be necessary for a given model, or it may fix an obscure feature interaction or update the instrument's compatibility with external devices. In addition to checking on this capability, it's worth finding out how you would be notified of an update and what the actual update procedure involves. In most cases today, it's an easy, do-it-yourself procedure.
Headphones are by far the most popular and frequently used digital-piano accessories. One of the advantages of digital pianos is the option to practice without disturbing others — or them disturbing you. Whether you're an occasional headphone user, or your instrument or situation dictates constant headphone use, selecting the right headphones will make a big difference in your playing comfort and enjoyment.
When I select headphones, I evaluate them using four criteria: fit, sound, isolation, and budget. Although it may seem that starting with sound is the obvious choice, my first priority is fit — it doesn't matter how great they sound if you can't stand to wear them for more than a few minutes. There are three basic styles of headphones: those that fit around the ear (circumaural) with the cushions resting on your head, those that rest directly on the ear, and those that fit in the ear. The style of headphone you choose will also determine the level of isolation. If isolation is critical for your situation, it should dictate the style of headphones.
There are a couple of variations on the circumaural and in-ear styles. Circumaural headphones can be open or sealed. Open designs don't cut you off from the outside world, and their output can be heard — very softly — by anyone nearby. Sealed designs offer more isolation but introduce some acoustic design problems that are difficult to get around until you get into the higher price ranges. In-ear headphones are available in the earbud style that sits in the outer ear, and the ear-canal type that fits inside the ear canal itself. The latter offers, by far, the best isolation in both directions, even when compared to headphones with active noise canceling.
Sound is very much a matter of personal preference and perception. One thing that can make the selection process easier is to bring a familiar CD with you when you audition headphones. While you may initially favor headphones that color the sound in some attractive way, this can become sonically tiring with extended listening. If you aim for a neutral sound, you'll end up with headphones that won't tire your ears over extended periods, and that will most accurately represent the sound produced by your digital piano or by the models you're considering.