Re: wireless keyboard interaction

[Vojtech Pavlik <vojtech@suse.cz>]

On Sat, Jan 06, 2007 at 07:34:14AM -0800, juanslayton@dslextreme.com wrote:

> Good morning and thanks for your interest.
> 
> So far as I know, all the boards I have used are 27 MHz.  That would be
> Logitech, Mozart, and, I think, Interlink.  I certainly have considered
> radio interference.  But with that, I would expect some scrambling of the
> keycodes, and I haven't observed that; the key I press seems always to be
> the key that is printed, even when I get multiple bursts.  I've tried a
> number of things and I'm going from memory here, but as I recall, the
> evdev value for those bursts was '2' (the normal value for autorepeat) and
> I was able to suppress them by writing the application to reject an event
> with value 2.  However, this still left the opposite interference, in
> which keypress on one board inhibits a read on another.  It's not obvious
> to me how signal interference would have either of these effects, or for
> that matter how there could be signal interference at all, given that usb
> devices are not peer to peer and should not be transmitting until they are
> interrogated by a USB 'host' routine.  (I'm likely using the wrong term
> here, but I think you will understand what I mean.)

It's not USB that goes over the wireless link. (*) It's a custom, much
simpler protocol, where the keyboard initates the transaction on a
keypress. This is also needed because USB doesn't like transmission
errors.

This means that two keyboards will be attempting to transmit when keys
on them are depressed at the same time, resulting in interference,
failed receives and retransmits until the situation resolves. Since the
keyboards will not be implementing random backoff like ethernet does,
this can take a while.

If a key release transmit is delayed because of this, the input core
will see a long keypress, and will generate software autorepeat (value 2
events) at a default rate of 30 per second.

So, it's really most likely interference.

(*) It would not be practical to try to push a 1.5 Mbit/sec
(bidirectional!) link over a 27 MHz radio channel. WUSB (wireless USB)
uses UWB (ultrawideband) to work.

-- 
Vojtech Pavlik
Director SuSE Labs

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