RE: Re: USB-UIRT and xPL

["Keith Doxey" <ukha@xxxxxxxxxxx>]

Hi John

> By the way (and this is probably not the best group to post this
> question to, but I'm sure someone knows the answer), the USB-UIRT
> hardware comes in two flavors : a non-56Khz-receiver unit, and a
> 56Khz-receiver unit for $5 more.
>
> When would I need the 56KHz version ?  Is it that some components
> use that range, and if so, is there any way to determine if my
> components use that range ?
>

IR signals use a modulated carrier signal to carry the information. This
can
vary between 32-40 kHz, some 56kHz and then Bang & Olufsen who use
455kHz.
IR Receiver modules (unless they are "Wideband Receivers") are
tuned to be
most sensitive at a particular carrier frequency.

The advantages of being tuned to a specific frequency are

1. Increased sensitivity to the wanted signal giving greater range
2. Vastly improved rejection of unwanted signals.

95% of equipment operates between 36-40kHz because the three main families
of IR codes tend to use the following frequencies

Phillips IR Formats RC5/RC5X/RC6/RC6A operate around 36kHz
NEC formats (several different variants) operate around 38kHz
Sony Formats operate around 40kHz

I use a 38kHz receiver which will accept 36k and 40k signals with a
slightly
reduced range. The further you move away from the centre frequency the
sharper the drop off in sensitivity and although a 38K module may receive a
56k signal it would be prone to errors and have a very short reception
range.

The unit you mention above costs $5 more because it has two IR receiver
modules.

If you have a HomeVision or Pronto you can find out the carrier frequency
from them.

IR signals are horible things to work with as there are so many different
formats and abuses  of formats. Tonight I built an RC5/6
Transmitter/Receiver from a circuit I found on the web.

I have discovered that I have a Pacific DVD player that uses the RC5 Device
family "Lighting 1" and just random command buttons rather than
the proper
command eg Keypad 0-9 should be 0,1,2,3,4,5,6,7,8,9 whereas this has
7,18,3,2,10,etc

I also have a Beko TV that used the correct family "0" which is
"TV" but
again random assignemt of command codes.

I point this out as it has implications for the work people are doing on IR
as a keypad button "4" might not neccesarily have the same code
for
different devices using the same IR protocol.

Likewise, Sony are bloody awkward in shifting tings down by one eg

Button	Sony		RC5
0		9		0
1		0		1
2		1		2
3		2		3

etc.

Hope that helps

Keith