By Kevin Nakano

Certified

Introduction

One of the most exciting aspects of Home Theater is the ability to produce 5.1 channels of high quality audio in your own living room. Many of us who invested in Prologic decoders are now looking at Dolby Digital. New formats like DSS, DVD-Video and High Definition Television incorporate this new technology; but what about the old laserdisc format? After all, more than 600 laserdiscs feature Dolby Digital/AC-3 audio. For those people who do not currently have the AC-3 RF output on their laserdisc players, this site will hopefully help you understand what is involved in modifying a player to support the Dolby Digital RF bitstream.

The following shows a step-by step procedure on how to modify a Pioneer CLD-D702 laserdisc player to output the Dolby Digital (AC-3) RF signal needed to drive Dolby Digital demodulators/decoders. My intention is to help those interested in modifying their own player; thereby avoiding the cost of upgrading to a new machine or paying a large sum of money to services that provide the Dolby Digital modification.

This modification is to be done AT YOUR OWN RISK and there is no guarantee that the modification will work for you. Failure to properly install this design correctly may result in damage to you and/or your laserdisc player. This web site takes no responsibility for any modification you may attempt.

	The custom board shown here holds the required AC-3 circuitry. Once installed, the card enables older, non-AC-3 laserdisc players the ability to output the much talked about AC-3 RF signal. This version of the board uses metal film surface mount resistors as well as surface mount ceramic capacitors and transistors to minimize the level of noise in the signal. Players which have the AC-3 circuitry from the factory, typically use surface mount components as well. The card was constructed using an iron-on type film which was generated on a laser printer from an inexpensive artwork program. Once the film was transfered, the board was put into etching solution to remove the unwanted copper.
	This is how the inside of the Pioneer CLD-D702 player looked prior to any modifications made to the unit. The COXB Assembly board (unrelated to the AC-3 circuitry) located in the upper right corner of this picture, will be used to mount the new AC-3 circuit board using the predrilled hole provided by Pioneer.
	The new circuit card needs two voltage forms (+5VDC, -5VDC) and a ground. Power is tapped from these three lines as indicated in the photo. Red carries the +5VDC; Green carries the -5VDC and Black is ground. These power lines are routed to the newly installed AC-3 card.
	The AC-3 RF output is sent to the back panel through an RCA connector. Make sure when you drill the hole you don't let any metal pieces fall into the unit. I used some tape and paper to cover the inside during drilling. Once the hole is drilled out, the RCA connector is installed with nylon washers to insulate the connector from the chassis. Note: The outer portion of the connector will eventually be AC coupled to the chassis through a 0.1uF capacitor.
	The wire shown on the back of the Audio Assembly card is the mute signal connected to pin 10 of CN201. The signal is difficult to tap from on the component side. Therefore, I chose to make the connection on the solder side. Connector CN201 interfaces with J10 on the Main Assembly board. This active high MUTE signal is needed to properly set the DC level of the AC-3 RF signal during the mute mode.
	The AFM RF signal that is used to carry both the left and right analog audio is tapped from the emitter of Q352 (2SC1740S). The AFM RF signal is located on the pin of Q352 closest to L353 as shown in the picture. The ground is connected to the jumper wire located next to R357 as shown. This coaxial signal is routed to the custom AC-3 card using RG-179 coaxial cable. Note: The current AC-3 RF outputs on production LD players contain both the left and right AFM signal. The demodulator/decoder unit must first band-pass filter and then demodulate the right channel (2.8 MHz carrier) to recover the AC-3 bitstream. If an additional RF bitstream is ever added to the left channel (2.3 MHz), no additional modification will be required in the LD player since it already outputs both RF channels.
	This picture shows the installed AC-3 card mounted to the COXB Assembly card. Once again, RG-179 cable was used to route the RF signal to the back of the unit. Three power wires and a mute signal run to the tap points mentioned above. The AC-3 card ground is AC coupled (with a 0.1uF capacitor) to the chassis of the CLD-D702.
	Finally, the project is complete. You should check the voltage levels using a scope and/or a voltmeter to insure your modification is working correctly. The AC portion of the waveform should be approximately 0.6 Vp-p with no load. The DC levels should be approximately 4.6 V. If the signal is muted, the DC level will go to zero. The AC portion of the waveform should remain the same, even during muting. AC-3 demodulators/decoders use a DC level detection circuit to determine if the audio should be muted or not. The muting threshold voltage received by demodulators manufactured by Pioneer and Yamaha are set to approximately 2.5VDC and is probably part of the interface specification. Therefore, it is very important to make sure this signal is well above this threshold point during non-muting operation to prevent any accidental muting or dropouts. This can be a problem for many installations if noise becomes significant on the RF signal. We'll talk more about this during the modification checkout.

The following oscilloscope pictures were taken from the AC-3 RF output of my Pioneer Elite CLD-99 LD player to illustrate the signal levels of a production unit. I've included pictures of both the loaded and unloaded RF signal to help others verify the drive capabilty of their buffer circuit. Note that the amplitude of the loaded signal is approximately half of the unloaded signal. This is to be expected since the RF drive output impedence is approximately equal to the load impedence at high frequencies. Matching the impedence at both ends will minimize reflections in the cable.

The unloaded AC-3 RF signal on the CLD-99 is shown in this oscilloscope picture. The oscilloscope (Tektronix 2245A) has been set to AC couple the input. The signal has several frequency components which account for the "poor" looking waveform. These pictures are for reference only, and may look slightly different on the machine being modified.

The loaded AC-3 RF signal shown here has a standard load found in most demodulator/decoder units. The RF Load circuit consists of a 75 ohm resistor in series with a 0.022uF capacitor connected to ground. Make sure you do NOT connect the 75 ohm resistor directly across the output of the RF signal without first putting the series capacitor in-line with it. Failure to do so may damage the output circuit during the non-muted mode.

The last two pictures show the DC level of both the muted and non-muted signals. This data should be used to compare the AC-3 RF output of the modified player. If the AC and DC levels appear similar, then there's a good chance you did the modification correctly.

The loaded AC-3 RF signal shown in this oscilloscope picture is in the mute mode. The oscilloscope input mode is set to DC, showing any DC offsets that may exist in the input. Notice the DC level is approximately 0V.

The same loaded AC-3 RF signal is shown in this oscilloscope picture with the mute off. Notice the DC level is approximately 4.6V. Adding this DC level to the 0.3Vpeak AC signal is still well above the 2.5VDC threshold voltage found in demodulator/decoder units. This threshold voltage is responsible for triggering the mute mode in the PD4606A (Pioneer RF demodulator) chip found in all AC-3 demodulators. A typical detection circuit is shown in the Mute Detection schematic.

LD Modification Forum

Here's the long awaited User's List for the Do-It-Yourselfers. This list has people from all over the world who have been successful modifying their own LD player. We would like to thank these individuals for providing useful information to those attempting this modification.

The circuit (#1) used in the CLD-D702 was verified using the Pioneer SP-99D AC-3 Processor along with Dolby Laboratories' AC-3 Demonstration and Test Laserdisc. The Dolby test disc contains some useful AC-3 test tracks and has some great demo material as well. The "5 Voices in 5 Channels" track is the same sample used during all the AC-3 demos at the 1995 Winter CES. This was prior to any official AC-3 movie releases.

Some modifications (not shown here) have an additional transistor (with internal biasing) used in the voltage divider portion of the mute circuit. This "extra part" which is not needed, decreases the DC level of the "mute off" mode by approximately 1.4VDC and can dangerously approach the DC level which trips the mute mode in the demodulator/decoder. To make matters worse, some decoder/demodulator units have a 100K resistor at the input of the RF signal, further lowering the DC level coming from the LD player during normal play mode. If the 2.5VDC threshold level is crossed, the demodulator chip (PD4606A) will mute the audio to the DSP. In addition, noise in the RF signal may cause intermittent dropouts in the audio. To prevent this from ever being a problem, I would recommend removing the "extra part" from that particular design. This will greatly decrease the chance of ever running into this problem. Greg Rogers has been investigating this issue and may be able to provide more information.

• Ralph Calabria, Associate Editor for DIY projects at Secrets of Home Theater and High Fidelity, has written an article on how to install a mod board, including installation tips such as soldering techniques. Check it out at http://www.sdinfo.com/volume_4_2/ac3mod.html.

• Espen Braathen wrote an AC-3 LD Modification article in Norsk Audio Video, a Norwegian A/V publication focusing on higher-end gear, where he describes how he modified his Pioneer CLD-2950 Laser Disc Player. This site is written in Norwegian and will soon have an English version available. You can check it out at http://www.hedning.no/~espenb/ac3/output.html.

• Blaine Young has a very informative web page on the modification he made to his Pioneer CLD-D703. You can link to his site here.

• Eric Koester also has a page on the modification he made to his Pioneer CLD-D703. You can link to his site here.

• Anyone who has been surfing the net for information on modifying their own laserdisc player has probably come across Colin Hunter's AC-3 web site. He is the first person I know of to setup a web site with technical information and a bit of history on the required AC-3 circuit. Colin's AC-3 web site contains much of the information listed in Espen Braathen's magazine article.

• Check out Matt Tucker's modification to his CLD-1090. He has setup a website to help out others at http://kahuna.sdsu.edu/~tucker/diyaudio/ac3rfmod/ac3rfmod.html
  
  

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  Any comments or questions regarding the Dolby Digital Modification Site should be forwarded to kevin@laaudiofile.com

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