Overall, most keyboards are adequate for Etterna play. There are a few exceptions that perform so poorly they will affect an average player, but these keyboards are uncommon and their faults are immediately noticeable to experienced players. Unless you are gunning for very high accuracy content, or are incredibly fast and need lighter actuation in your keys to get an edge, buying a better keyboard is unlikely to make you a better player.
The Etterna community maintains a Keyboard Spreadsheet which ranks keyboards according to their Etterna performance (Credit: Foxfire). If you are looking for a new keyboard for rhythm gaming, you will want to select any keyboard with a “Fair” rating from this list. If your keyboard is not on this list and you would like to understand its performance, the following sections show how to identify common keyboard issues that limit performance. If you have analyzed the performance of your keyboard and would like to have it added to the keyboard spreadsheet, please ask for assistance in the Etterna Discord.
Most keyboards poll for inputs at a frequency of 125hz (every 8ms) or 1000hz (every 1ms) while a few especially poor performing keyboards poll as low as 62.5hz (every 16ms). The polling rate of a keyboard means that there is a delay from when one presses a key to when the key event is registered. If the polling rate is 125hz, then a keypress could take up to 8ms before the keyboard relays the event to Etterna to judge it. The Marvelous judgment window is 45ms wide (±22.5ms), so 125hz keyboards will only have 5 to 6 polling “slots” inside of the Marvelous window spaced 8ms apart. Hits that are early may end up registered by the earliest slot in the Marvelous window instead of the latest slot in the Perfect window; this would yield an incorrectly judged Marvelous. Hits that are late may end up registered by the earliest slot in the Perfect window instead of the latest slot in the Marvelous window; this would yield an incorrectly judged Perfect.
A 125hz polling rate isn't large enough to affect players that are attempting to get Marvelouses instead of Perfects, but it is large enough to affect players that care about where inside of the Marvelous window a hit registers. Accuracy-focused players that target AAAAs need 1000hz polling rate keyboards to perform well, and they will improve significantly by switching from a 125hz to 1000hz keyboard. Players that push for AAs or AAAs will not be greatly limited by a 125hz polling rate. However, players that feel limited by poor feedback from 125hz keyboards may still improve by switching to a 1000hz keyboard.
One can determine the polling rate of their keyboard by looking for polling lines on the accuracy graph at the score results screen. Keyboards with lower polling rates create line-like patterns throughout charts that have a BPM close to a multiple of the polling rate. For example, 125hz keyboards create very visible lines on 125 or 250 BPM charts and visible lines on 130 or 260 BPM charts. Keyboards with higher polling rates create noisier accuracy graphs and have no discernible lines. Other rhythm games will often create similar patterns in their accuracy graphs for different reasons (e.g. input processing being limited relative to frame rate), so this evaluation must be made on Etterna to ensure that keyboard polling rate is the only cause.
250 BPM chart Egao No Mira E from Tachyon Delta played on a 125hz keyboard (left) and 1000hz keyboard (right). (Credit: senya)
The above accuracy graphs show what sort of polling lines 125hz and 1000hz keyboards create on a song with a BPM that is a multiple of the polling rate. The left image has visible patterns of horizontal lines indicating that the keyboard polls at 125hz. The right image has no visible lines indicating that the keyboard polls at 1000hz or another frequency that is high enough to not be detectable.
Some keyboards will not register chords at the exact time they are pressed. Instead, they will split the keypresses out over multiple milliseconds or polling cycles. Especially poor keyboards can exhibit chord splitting of more than 8ms:
Chord Splitting example occurring on jump and hand keypresses. Keyboard: Dell KB216 (Credit: senya)
Chord splits on the order of 1ms are too small to be visible on accuracy graphs. For these cases, one needs to use an external program to time inputs and check for keypresses. This is an example of a chord split of around 1ms occurring while being tested.
Refer to Key Rollover on Wikipedia for a review of this keyboard mechanic. Keyboards with rollover problems will not be able to simultaneously press all keys used in the given keymode, e.g. 4key, 6key.
To test for a rollover issue, open a text editor and press and hold all of the keys used for your Etterna keymode at once. If any of the keys do not immediately show up or do not appear until you begin releasing your keypresses, then your keyboard has rollover problems for your given key combination on your given keymode. It may be possible to find a different key combination that works, or there may be no such combination altogether. Key rollover problems are uncommon on 4key for modern keyboards, but players may run into rollover problems on higher keymodes.
There are too many types of switches and player preferences to clearly classify switches as good or bad. However, the majority of players use Red (linear) or Brown (tactile) switches manufactured either by Cherry or Gateron. Players that use other switches and feel limited by their performance may see skill improvement by switching to a keyboard with these switches.