| LaserDiscs and AC-3 |
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Introduction
The LaserDisc medium, which has always been at the forefront of consumer audio/video playback quality has recently added a new (and, to date, unique) ability to its list of features: playback of Dolby AC-3 sound. I do not intend to go into great detail regarding AC-3 sound here, especially as it is comprehensively covered at Dolby's own site. Very briefly, AC-3 is a new way of digitally encoding and compressing 5 discrete, full bandwidth audio channels and 1 subwoofer effects channel into a very small space compared to the requirments for 5.1 uncompressed audio channels. In NTSC laserdiscs, the carrier for the right analog channel is substituted by the carrier for a digital Dolby AC-3 encoded bitstream using Quadrature Phase Shift Keying (QPSK) modulation. The AC-3/RF signal occupies a bandwith of only 250 kHz, which is approximately the same space as the old carrier for one analog audio channel (200 kHz). It is very important to realize that AC-3 completely replaces the right analog audio signal waveform - it does not simply recode the right analog audio to carry the AC-3 data. Thus you cannot use the LD player's existing right audio output to feed an external AC-3 decoder, but must instead extract the AC-3 signal before it reaches the player's audio processing stages, and use this signal for external decoding.
Most new LD players incorporate an AC-3 output connector for connection to an external AC-3 demodulator/decoder, but owners of older players either have to pay a fortune to get their equipment professionally modified, or buy a new player. Fortunately, if you have a modicum of skill with electronic construction there is a third option: modify the player for AC-3 output yourself.
History of this AC-3 LDP Upgrade Circuit
The construction details and circuit diagram were initially part of an article by Espen Braathen and were published in the August/September 1995 issue of Audio/Video Magazine (Norway). The circuit design was based on the AC-3 output of a Pioneer CLD704 player and from a description obtained from Pioneer Japan through Dolby Labs. The original article, which included details on assembling the circuit on multiprint board and installing it in a Pioneer CLD2950 is available from Nordisk Fagpresse, Norway, (telephone: 22 59 77 00). The circuit digram was digitized and the text translated to English by R Larsson who subsequently posted it to alt.binaries.pictures in late September, 1995.Technical questions may be sent to Espen Braathen (home cinema editor, Audio/Video magazine).
Since people on alt.video.laserdisc are still posting requests for this information, and it has long since expired from a.b.p, I decided to make this information permanently available via the WWW. Obviously, any modifications that you may decide to make to your LDP will invalidate the warranty and are completely at your own risk, and I take no responsibility if anything goes wrong. Having said that, hopefully, others with access to AC-3 decoding equipment will give the modification a try and will let me know how they get on. I already know of at least two people who have successfully used this circuit to upgrade their players (a Pioneer LX-1000U and a Pioneer CLD-2950) to AC-3 output and am keen to learn of others who are are successful so I can compile a list of players for which the circuit is known to work. I would also like people to tell me their player model number and where on the circuit board they tapped the various signals in order that I can produce a database for others with the same model LD player.
Colin Hunter
November 16th, 1995
Acknowledgements
Thanks to Roger Dressler at Dolby Labs (technical details), Bill Cruce (basic circuit diagram) and Greg Rogers (technical support) from "THE PERFECT VISION", and Kevin Nakano (Pioneer 704 AC-3/RF output diagram) for help during development of the original Audio/Video article.
| The circuit and construction details |
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