We continue to receive numerous queries about why an existing coaxial digital or toslink optical connector can't be used to deliver Dolby Digital AC-3 data from an LD player to a receiver/decoder. Hopefully the following explaination will help both future and present customers.
In the earliest days of Laserdisc both video and audio were stored in an analog form on the disc. Video data is the same today as it was then with only evolutionary advances to improve it's quality. Audio data however was shortly thereafter commonly stored as digital data on the disc in almost exactly the same way that it is stored on a regular audio Compact Disc. To maintain backwards compatibility with the older analog-only players, BOTH types of data were placed on the disc. As a result, laserdiscs have specific 'tracks' for Video, Analog Audio Left Channel, Analog Audio Right Channel, Digital Audio Left Channel, and Digital Audio Right Channel. If you have a fairly modern laserdisc player and switch between Analog and Digital audio while playing back most any modern disc you're just switching between those tracks. An older analog-only player can only play back the audio stored on those two analog audio tracks.
In the early 90's Dolby Laboratories and Pioneer developed a way to place a full 5.1 channels of discrete digital audio on a laserdisc. Since the standard was in place and there were already 4 separate 'tracks' for audio they couldn't add a new one for this data since it would probably break backwards compatibility. Older analog-only players they figured were few and far between so they decided to replace the right analog audio track. A mono soundtrack for the movie could still be placed on the left analog track with the regular standard stereo and/or Dolby Prologic encoded data on the two digital tracks. In place of the right analog audio track they placed a compressed digital signal which contains ALL of the 5.1 channels of Dolby Digital (DD for short) AC-3 encoded data. If you play a DD encoded disc and switch to analog audio you'll probably hear a 'hissing' sound in the right channel. This is the digital data for the DD 5.1 audio.
What many people are thinking is "Why can't I just hook a cable from my right audio output jack to the DD input of my receiver/decoder and get the data to it in that way?". You could IF it was not filtered and processed by the player's audio circuitry. Thd LD player does not know you're sending that data to an outboard processor for decoding. That output is supposed to be delivering formatted audio for a human's ears, not for input to a processor for further decoding.
Other people are thinking "Why can't I just hook a cable from the existing Coaxial Digital or TOSLINK Optical output on my LD player to the appropriate input on my receiver/decoder and get DD 5.1 sound that way?". Remember from the description above WHERE the Dolby Digital AC-3 data is stored on the laserdisc physically. It's on the RIGHT ANALOG audio track. The Coaxial or Optical digital outputs on an LD player output the PCM digital data stored on the two digital audio tracks of the disc or of course the digital audio data stored on a regular audio CD. There is no Dolby Digital AC-3 data stored on the disc on those two digital audio tracks and therefore any existing digital outputs on the player will NOT have any usable data as far as the receiver/decoder will be concerned.
How do you store "digital" audio data on an "analog" audio track? Dolby Digital as the name implies is a purely digital audio specification. Powerful computer chips take a stream of compressed digital audio and decode it and split it into anywhere from 1 to 5.1 channels of audio. This is performed inside the receiver or Dolby Digital decoder if the receiver doesn't have the decoder built-in. DD data is stored on the laserdisc in an RF (Radio Frequency) form. Technically it's QPSK modulated and resides within the 2.8Megahertz space allocated for the right analog audio track.
An LD player that came stock with an "AC-3 RF OUT" connector OR a player that has been modified for it output the same type of signal. It's a buffered RF signal which technically is analog. That analog data however represents the digital data in much the same way a modem can move digital data across an analog telephone line.
Dolby Digital decoders and receivers with decoding built-in used to all come with an RF Demodulator built-in. They had an input labeled "AC-3 RF IN" or "LVD IN" or something similar to designate the input SPECIFICALLY DESIGNED for laserdisc players. They also had either/or or both Coaxial Digital and TOSLINK Optical inputs. Recently though many of them do not come with an RF Demodulator built-in, removing it as a cost-cutting measure. The manufacturers figure that a majority of their customers are buying the receiver/decoder for use with a DVD player and the demodulator is just an extra expense. Most manufacturers discuss this in the owner's manual and also most of them offer an external RF Demodulator that can be purchased.
An external RF Demodulator is much like a modem on a computer. It takes the analog signal output from the LD player and 'demodulates' it into a digital signal. This digital signal can then be fed into the Coaxial or Optical digital inputs of a receiver or decoder. The same input you'd hook a DVD player straight to without the need for any RF Demodulation. DVD Players and mini-dish satellite systems transmit audio data totally in the digital domain and therefore can send that data to a decoder as-is.
If your receiver and/or decoder does not SPECIFICALLY have an input for a laserdisc player then you will need an RF Demodulator, period. RF Demodulators sell for ~$100 (more expensive ones really don't buy you anything) and common ones are:
Why does it have to be so complicated and require so many 'extra' pieces of hardware? Keep in mind that Laserdisc was an established standard for many years before 5.1 channel digital sound moved from the big screen into the home. It was added to the disc specification after the fact. Had it been available from laserdisc's inception, a separate digital audio track specifically for it would have probably been set aside. This is how it was handled with DVD and even though it's outside the scope of this discussion, the same problem exists with DTS on DVD's since early ones don't know how to read the DTS audio track.
Which finally brings us to DTS. The folks at Digital Theater Systems decided to forsake backwards compatibility by replacing BOTH the digital audio tracks with DTS-encoded digital data. Since the data is stored on the digital audio tracks the existing coax or optical outputs on the LD player will fully support DTS with no modification necessary. Hook either output to a DTS decoder or suitably equipped receiver and that's it.Without having a DTS decoder you'd be forced to listen to the analog tracks if you're playing a DTS disc. DTS uses a higher bit-rate and less compression as compared to Dolby Digital and therefore could not be squeezed into one track, it replaces both digital tracks.