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 following shows a step-by step
description on how I modified my Pioneer CLD-1950 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
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. The circuit
I used came from Kevin's web page. Instead of the 2SA1037K and the
UN2112 I used the BC808, which is available in Europe. This made it
necessary to add a pullup from the base of Q2 to +5VDC, and a series
resistor from the same base to the mute input of the board. I used 22k
for each resistor, the value is not critical though. 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 a printout from the circuit board program I have written myself.
||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; Yellow carries the -5VDC. Ground will be fetched from the
signal input. The power lines are routed to the newly installed AC-3
||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.
Be sure to use a shielded 75 Ohm video 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.
The left of the picture shows the most important bit: this is where the
AFM RF signal that feeds the AC3-card comes from. The purple wire is
connected to the AFM signal, and the black wire is ground. I installed a
connector and soldered the wires to this connector.
signal is picked up from a wire near the A/D converter (the IC marked
"Pulseflow") on the main board. It is the green wire shown on
the picture. This active high MUTE signal is needed to properly set the
DC level of the AC-3 RF signal during the mute mode.
||This is what the AC-3 output looks
like installed on the CLD-1950. Can you tell where it is?
I am using this circuit with this Yamaha APD-1 RF
demodulator, and it has worked
great for over a year now.
Now that your 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. 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.
Checking the modification
The following oscilloscope pictures were
taken from Kevin's AC-3 RF output of his 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
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.
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
Summary of the measured data.
|~ 0.7 Vpeak
|~ 0.3 Vpeak
||~ 0.6 Vpeak
|~ 0.3 Vpeak
The 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 #2 for the CLD-1950, 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
#2 or CIRCUIT