A few key points to note: Philips produced a booklet entitled
"Remote control system RC5 including command tables" in Dec. 1992.
This is the  horses' mouth': the source of sources.
RC5 was a good standard at the time but is becoming inadequate.
There are 128 key codes - plenty
- but only a subset of these have an assigned function.
There are 32 device codes - plenty in 1992
- but new devices have appeared since.
The deficiencies of RC5 have been recognised.
For example you can toggle the state of some parameters
(mute on/off, door open/close) but not set them to a known state
- a problem if you have several amps working in tandem!

Marantz have tacked on extra bits at the end of the RC5 code.

We have talked to Philips about assigning new code functions
but it appears they can't be bothered.
We've not seen anything more recent than the 1992 booklet.
Philips have produced a new standard called RC6,
but they won't let anyone see the tables unless they pay for support.

I've programmed one of our latest products so that
special button-combos switch on an RC5 diagnostic screen.

This shows the RC5 codes being received.
This is a useful feature for checking the codes from prototype handsets,
without having to visually decode signals on a storage scope.
Also lets us see if other manufacturer's handsets are using RC5 codes
that might cause problems.

RC5 often appears in European equipment and mags such as Elektor,
hence it is popular with hobbyists.
But it is not a dominant standard worldwide.
Most items seem to use the NEC system, some variant,
or a completely unique system.
In my house, only the Philips music centre used RC5.

The Decca TV uses some kind of 11-bit time-between-pulse modulation
(7.5 or 5ms).

The Sanyo VCR uses a 24-bit device code plus 24-bit NEC style protocol.

The Scientific Atlanta CATV uses something NEC-like.

If anyone has any info on these, I'd like to hear it!