Laptops General - Read Only

Last reply by 08-14-2016 Unsolved
Start a Discussion
3 Silver

Keyboard debouncing utility: Solution / workaround for key-bounce issues on certain Dell notebooks

I recently purchased a Dell 1470 notebook and immediately noticed that the keys were "bouncing" (multiple keystrokes registered for a single keypress).  The problem is not isolated to just my laptop or model; as you will find here, it is showing up on others as well.  If you are experiencing this problem, I feel for you. 

The problem was sporadic, but occurred often enough to make my typing look like that of a third-grader (no offense intended to any third-graders out there).  The problem persisted, even after three keyboard change-outs along with an OS wipe/ factory restore and also a motherboard swap-out.  A thorny problem, indeed.  At that point, it was determined that the problem was driver-related.  The keyboard driver is supplied by Microsoft, thus placing me at the complete mercy of the driver development team(s). Searching revealed no resolutions to this issue, no driver updates or patches, neither at Microsoft, nor at Dell, nor on the WorldWideWeb.

Now, being a firmware/software developer myself, I am familiar with keyboard bounce... it exists in all interfaces that have an electro-mechanical switch involved.  The objective is to turn the "chatter" at the switch closure into a clean change of state... "off" to "on", with no chatter at the boundary.   (If you are curious about the topic, you can read about it in detail here.)  Computer keyboards certainly fall into that category.  I realized that the problem existed either in the keyboard driver, the keyboard controller, or the design of the keyboard itself.  I also realized that this issue is not glaringly common enough to garner much priority from developers.  Hence, at least for the forseeable future, this was an "orphan" issue.

Not being a driver developer myself, I took this issue as an excuse to learn AutoHotkey.  I knew about it enough to figure that it could be used to build a keybounce-filtering utility.  Sure enough, I have written an AHK script that does just that: it detects the bounce pattern in the key stream and filters them out on their way to Windows.  You can get it here:

KeybounceFilter.ahk   <-- AutoHotkey script (requires AHK installed)

KeybounceFilter.exe  <-- Compiled script (runs all by itself, no AHK install needed)

AHK forum post

If you are already familiar with AHK and have it installed (it's a great tool BTW), then just download and run the AHK script.  If not, then you might prefer the compiled script (it's the EXE above).  With the EXE, you won't need to install AHK at all.  Just download the EXE into a safe place, then create a shortcut to it, and place that shortcut into your Startup folder (Start > All Programs > Startup).  The script will then run automatically on the next startup.

The filter has a "window time" where it looks for bounces (repeats).  Keybounces usually last in the 10's of milliseconds, being as low as 1 msec for a clean switch closure and as high as 100 msec or more for a particularly dirty one.  I set the default gating window to 40 msec, as a working ballpark figure.  I didn't want to make it too high, because this gating time also sets the upper limit on the auto-repeat rate.  In fact, if you find while running the script that your auto-repeat is choppy or doesn't work at all, you'll want to lower the auto-repeat rate until it works again (don't worry, you'll get it back).  Just go to Control Panel > Keyboard > Repeat Rate and adjust the slider down until it works reliably again.  (note: that has changed: see updates/edits below...)  If you do find that you still get double keys (or triples... I feel for you), then you should increase the gating time.  To do it, right click on the shortcut to the script that you created above, then select "Properties".  Click inside the "Target" box, and at the end of the line, after the "KeybounceFilter.exe", add a space, and then a number for the gate time in milliseconds, then click "Apply".  So for example, if you want the gate time to be  50 msec rather than the default of 40 msec, then the Target Line should look like this:

C:\Users\<path to the script>\KeybounceFilter.exe 50

You will then need to re-run the script (restart, or just right-click the little "H" icon in the tray, select "Exit", then re-run the shortcut).  Also remember to re-adjust the keyboard repeat rate afterward.  The optimal choice of gating time is therefore striking a balance between the quality of the filtering achieved and the maximum repeat rate that will be available to you.  (note: that has changed: see updates/edits below...)  The filtering probably won't be 100% perfect, but it should go a very long way to helping maintain your sanity (no guarantees, of course).

That's it.  I hope this helps a lot of you out there to keep from pulling all of your hair out!

Edit (2-17-2011): Due to popular request, the script has been updated to now support cAPSLOCK functionality (boo!! hiss!!). The dreaded cAPSLOCK is now enabled by default (no, say it isn't so!!).  Not to be left stuck in this horrible state, cAPSLOCK may still be readily disabled (yay!) by passing a second non-zero numerical argument to the command line. So for example, in the Properties>Target line of the Startup Shortcut, disabling cAPSLOCK with the same 50 msec gate time as in the above example would look like this:

C:\Users\<path to the script>\KeybounceFilter.exe 50 1

Note that for practical reasons, Shift+cAPSLOCK will also be disabled in this mode, but both Ctrl+cAPSLOCK and Alt+cAPSLOCK will still perform the dreaded cAPSLOCK function (just in case you really need it).  Note also that passing zero (0) for the second argument acts the same as passing no second argument, i.e. it leaves cAPSLOCK still enabled (boo!).  Enjoy and God Bless!

Edit (3-13-2011): Due to popular request (and because it was really bugging me, too), the script has been updated to allow the keyboard auto-repeat feature to work (much more) properly.  Before this change, the gate time for the bounce filter would set an upper limit on the repeat rate, with longer gate times resulting in lower allowable repeat rates.  Now, you can set the auto-repeat rate as high as you like, and it will fly as intended. (In fact, the faster that you set it, the better that it will work!)  The feature works by setting an upper limit on how many bounces are filtered in a row.  In practice, real key bounces should result in fewer than three or four strikes in a row, so the default switchover is set to pass key repeats with more than 3 in a row.  This does add a slight delay to the auto-repeat before it really "kicks in".  You can lower the delay by either increasing the auto-repeat rate (yay!) or lowering the switchover threshold by passing a third numerical argument to the command line.  So for example, setting the Properties>Target line of the Startup Shortcut described above as follows:

C:\Users\<path to the script>\KeybounceFilter.exe 50 1 2

would set the switchover threshold to 2, while also disabling cAPSLOCK and setting the 50 msec gate time as before.  The danger of lowering the switchover threshold is that really bad bounces (undesired repeats of three or more) would begin to get through.  So use caution, and season to taste.  This update should now remove the one large remaining peeve that existed from the start of this utility.  As always, enjoy, and God Bless!

Replies (62)
3 Silver

Hey, I'm delighted to know that this little utility has helped someone besides just myself!  (I was starting to wonder if everybody's double-keystrikes had somehow magically all disappeared!)

Before firing off suggestions, I should ask... by "slow", do you mean that the auto-repeat on the Backspace is slow?  I ask this because that's the usual way people use the Backspace key, me included.  (For the uninitiated reader, the "auto-repeat" is what happens when you hold the key down.  It fires-off the key like a machine gun, saving you from having to strike the key over and over.)  If this is the case, then please read through my top post where I talk about adjusting the auto-repeat rate down (around the third paragraph from the bottom).  This is important, because auto-repeat affects all keys, not just Backspace.  (Think how often you auto-repeat the Space bar, or dashes, or the cursor/arrow keys!)

Otherwise, you have a couple of options.  You could shorten the gating time to less than 40 by modifying the shortcut like I describe in the last two paragraphs of my top post.  Finally, you can modify the script (AHK) file to remove the Backspace (or other special keys).  If you really need it, I can tell you which lines to remove.  But you will want to install AHK so that you can run the modified script.  (I hope you understand... I'd rather not get into the business of creating custom scripts... that could turn into a full-time activity!!)

Hope that helps!

2 Bronze
2 Bronze

I now have your exe on my Inspiron 1521 which has given me double rr since new.  Talking to Dell gave no solutions.  Your exe seems to have solved the problem - hurray!!! Thank you!!!  One small thing; the backspace is now slow.  Can that key be rmoved from a key list?

Thanks again!!!

Latest Solutions
Top Contributor