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 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.
CLD-D702 Modification Procedure
||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.
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
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.
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.
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.
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.
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.
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.
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
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
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
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
Summary of the measured data
AC-3 RF Circuit Designs
following circuits are three popular AC-3 buffer designs used in most laserdisc
players currently on the market. Since all the designs are simple variations of
an emitter-follower, these circuits act as current buffers with the ability to
change the DC level for muting. I decided to use Circuit #1 for the CLD-D702,
although the others may work just fine. Be careful of Circuit #3 as the emitter-follower
has a limited range where it will operate correctly. Make sure the AFM tap point
in the player has the correct DC level. You may download CIRCUIT
#1, CIRCUIT #2 or CIRCUIT
there seems to be an overwhelming number of people wanting to ask specific questions
about their particular player, I've connected a forum to this site for those interested.
Feel free to post or answer any questions at the forum site regarding LD modifications.
Go to the LD Modification
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.
are many of you out there who would rather not tackle the installation of an AC-3
RF board. There are several companies willing to provide the upgrade for a fee.
One such company is MSB Technology Corporation.
They have a very good reputation for their AC-3 RF board and will install it into
virtually any laserdisc player.
Laboratories' Demonstration and Test Laserdisc
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.
(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.
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.
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.
Young has a very informative web page on the modification he made to his Pioneer
CLD-D703. You can link to his site here.
Koester also has a page on the modification he made to his Pioneer CLD-D703. You
can link to his site here.
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
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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