[See NOMENCLATURE (below) for acronym definitions]

An AC-3 RF demodulator is a circuit that extracts a digital audio bitstream
from an RF carrier that usually is sourced from the analog right channel of
an AC-3/Dolby_Digital-encoded laserdisc; the process is akin to digital data
extraction from an analog carrier by a modem connected to telephone lines.

The AC-3 RF signal is stored on a LD's right analog track which still allows
a digital stereo or surround-sound signal on the PCM tracks and a commentary
or mono track on analog left which was a choice permitting compatibility with
the largest set of extant (and modern) LD players.

Older (pre-1995/1996) and "basic" model LD players do not have an AC-3 RF OUT
but can be retrofitted with kits frequently advertised in alt.video.laserdisc

The first Dolby Digital decoders were external units also featuring an AC-3
RF input (and a built-in demodulator), but these disappeared from manufacturer
product lines around 1997 as built-in (to receiver or pre-amp) decoders become
common.

Built-in (to receiver or pre-amp) AC-3 RF demodulators are today (A.D. 2000)
deprecated on all but high-end receivers/pre-amps.

Fortunately, several manufacturers have produced external AC-3 RF demodulator
units which extract the Dolby Digital audio bitstream from an AC-3 RF carrier
and provide it as either/both a coax and/or optical SP/DIF signal for input to
one's DD decoder (either external or built-in to one's receiver/pre-amp).

Unfortunately, external AC-3 RF demodulators are now (A.D. 2000) becoming
difficult to locate as most manufacturers have deleted them from their product
lines.

The LD player must have an output labelled "AC-3 RF OUT" and an RF coax
cable will be connected between that and an "AC-3 RF IN" on the demodulator.

The known (to me) external AC-3 RF demodulators include those in the following
list gleaned from AVL articles (often containing conflicting information about
INs/OUTs/autoswitching); the only unit I personally know is the Pioneer RFD-1.

  Manufacturer   Model    Comments
  ------------   -----    --------
  Pioneer        RFD-1    IN:  AC-3 RF, both coax/optical SP/DIF
                          OUT: both coax/optical SP/DIF

                          Auto-switches between the AC-3 RF signal and an
                          SP/DIF digital signal (such as from a DVD player)
                          allowing both players shared access to a single
                          digital input on a receiver/pre-amp; autoswitch
                          priority given to AC-3 RF IN.

  Yamaha         APD-1    IN:  AC-3 RF, both coax/optical SP/DIF
                          OUT: both coax/optical SP/DIF

                          Allegedly autoswitches between AC-3 RF IN and SP/DIF

  Kenwood      DEM-999D   IN:  AC-3 RF
                          OUT: coax SP/DIF

  Kenwood      DEM-9991D  IN:  AC-3 RF, both coax/optical SP/DIF
                          OUT: coax SP/DIF

                          Auto-switches between the AC-3 RF signal and an
                          SP/DIF digital signal (such as from a DVD player)
                          allowing both players shared access to a single
                          digital input on a receiver/pre-amp; autoswitch
                          priority given to AC-3 RF IN.

  SONY           MOD-RF1  IN:  AC-3 RF
                          OUT: both coax/optical SP/DIF

                          Switching between LD/DVD done at receiver/pre-amp.
                          Comes with one optical cable.

======================================================================
Following is a block diagram of Pioneer's RFD-1 AC-3 RF demodulator:

                       + - - - - - - - - - - - - - - +
                       :                             :
                       :       +-------------+       :
          AC-3 RF IN ()--------| Demodulator |       :
                       :       +-------------+       :
                       :              |              :
                       :              v              :
                       :       +-------------+       :
                       :       | Auto Switch |----+---() Coax SP/DIF OUT
                       :       +-------------+    |  :
                       :              ^           |  :
                       :              |           +---[] Optical SP/DIF OUT
      Coax SP/DIF IN ()---o           |              :
                       :   \ source   |              :
                       :    o---------+              :
                       :     select                  :
   Optical SP/DIF IN []---o                          :
                       :                             :
                       + - - - - - - - - - - - - - - +

A front-panel switch on the RFD-1 permits selecting either the coax or
the optical input as the second input signal source.

======================================================================
NOMENCLATURE

AC-3     aka "Dolby Digital".  Also codec variant 3,  Visit www.dolby.com

coax     short for coaxial; a signal transmission line with a center conductor
         enveloped in dielectric insulation then surrounded by a signal return
         which is often also the "shield".  Typical A/V cable connectors used
         with coax are RCA "phono" plugs and BNC plugs.

modem    a combined modulator and demodulator.

PCM      Pulse Coded Modulation; a digital signal track.  Visit Bob Niland's
         FAQ site at http://www.frii.com/~rjn/ for details how digital signals
         are encoded on analog laserdiscs.

RF       radio frequency

SP/DIF   SONY-Philips Digital Interface Format.  Specification of how digital
         audio is transmitted over either 75 Ohm coax cable or TOSlink optical
         (fiber).