I have written on this subject many times before, but it was suggested to me that the thing to do would be to put it in it's own thread to make it easier to find for newcomers. I will try to make this explanation informative and complete, but I will not address extra features on some demodulators such as crosstalk cancellation that are not on most demodulators. So here it is:
To start with, it helps to know what a CD-4 demodulator does, and that helps you understand what you are doing when you adjust one. The CD-4 record is essentially a stereophonic record with added supersonic carriers. Each groove wall is modulated with it's own audio and carrier signal. The cartridge separates the left and right channels the same way it does a stereo record, by each channel's coils being only sensitive to movement in the plane of direction of the signal that it's trying to pick up. In actuality, a CD-4 demodulator is in fact two demodulators in one box. Each demodulator separates it's channel into signals, front and back. Now, the original signal was in fact four channels, and it was mixed into two channels during the recording process, plus two carriers mixed with the two channels. Let's look at one of these channels:
Left front and left back signals enter the CD-4 modulator and are both split into two signals, each identical to it's counterpart. Let's call them "A" asd "B" for the sake of clarity. So then, "A" front is mixed with "A" rear. This is then sent through RIAA equalization and to the output mixer. This signal is low pass filtered to remove anything above 15KHz and recorded on the groove wall as left channel total, and if played with stereo gear, will yield all of the left channel information in the left speaker. So how do you get that into the original left front and left rear channels? The secret is in the carrier. A carrier is a sine wave frequency that "carries" a signal within it. A good example for this purpose is an FM radio signal. In the FM signal, the frequency is shifted in accordance to the modulating audio But the FM radio signal is transmittable, whereas, the audio is not, for all practical purposes. But on the CD-4 record, we're not trying to transmit anything. But what we are trying to do is to include another signal on the record. This signal will be the left front and left back signals that we are discussing now. So now we mix together the front "B" signal with the rear "B" signal. But before we do this, we do something with the rear "B" signal that we didn't do with the "A" signals. We invert the signal so that the positive peaks are now negative and vice versa. then the two "B" signals are mixed together which we will call "B" total. This represents the difference signal with which we will modulate the carrier. But first, it must go through an ANRS noise reduction process. ANRS is JVC's version of Dolby B, it's not the same, but it does something similar and we won't go into that for the purpose of this discussion. It's not germaine to understanding CD-4. So, once the B total signal is ANRS processed, we send it to the modulator. This is a phase locked loop system, and actually produces three types of modulation according to audio frequency band,, but they are similar and for this discussion we will call it FM. FM is actually one of the three types. The carrier is a 30 Kilohertz (KHZ) signal and is above the range of human hearing. The modulating "B" total signal will cause this signal to swing form 15 KHZ to 45KHZ. This modulated 30KHZ carrier is then sent to the output mixer where it is mixed with the "A" total signal and on to the cutting head amplifier. There is an additional process called Neutrex, which is corrective predistortion added to correct stylus tracking distortion. But again, that is not germaine. Now we have the recorded CD-4 signal. The right channel process is identical to the left, except perhaps in the Neutrex, as it is recorded on the other wall of the groove.
Now having said all that, now we have to separate out the four channels. Like the CD-4 modulator, The demodulator is actually two demodulators one for left and one for right. For the purpose of this discussion again, I will only discuss the left channel. The left channel signal is picked off the record by the cartridge and sent to the demod's left input. It is preamplified and brought to "line" level. It then goes through a band splitter, which splits the signal into two bands, one is everything below 15KHZ and the other is everything above 15KHZ. The below 15KHZ band is the left channel total signal referred above as the "A" total. It is sent through the RIAA equalization and then to the output mixers. The above 15KHZ signal is the carrier, and is sent to the phase locked loop discriminator to be demodulated to audio. This is the "B" total signal referred to above. It is sent to the ANRS processor to undo the ANRS encoding, and then to the output mixers. In the output mixer, the "A" total is split in two identical parts which we will call "A1" and "A2". The same is done with the "B" total signal. Now the A1 signal is mixed with the B1 signal. Here's where the majic takes place. Since the A1 and the B1 signal both contains the Left Front signal, the left front signal is doubled. But the A1 signal also has the rear left signal. The B1 signal also contains this signal, but as you recall, during the encoding process, in the B1 signal, this signal was inverted. So what happens is that when A1 and B1 are mixed together, the rear left signals cancell each other out, being exact opposites. So now you have only the front left signal which is sent to the output. Now to get the rear left signal, we invert the B2 signal and mix it with the A2 signal which is left uninverted. Remember that the A2 signal is the total left channel audio. Also, the B2 signal contains the left front uninverted, and the left rear inverted. But once this signal is inverted, we now have the left front signal inverted and the left back signal is uninverted all mixed together. We then mix that with the "A2" signal, and the A2 front channel signal mixes with the B2 front channel signal and since the B2's front channel signal is inverted, it is opposite and cancels. The A2 rear channel signal mixes with the B2 rear channel signal, and because the rear signals are both uninverted, they add together, and become the left rear signal. Once again, the same process occurs in the other demodulator for the right channels.
Are y'all still with me? Now for the actual setup.
OK, so how do you adjust the demodulator? On most demodulators, you will find three adjustments. They are Left channel separation, Right channel separation, and carrier level. After you get your turntable cartridge aligned, (a whole 'nother subject) you start with putting on a CD-4 record. I assume the demod is connected properly and switched to CD-4 mode. As before we will discuss only the left channel, which mirrors the right. The carrier level control is actually a ganged pot, and controls the left and right together. So once this setting is done, it is good for both channels. We'll start with it. Start by turning down the separation pots all the way counter clockwise. Turn up the carrier level all the way. Put on a record, I would suggest Doobie Bros, because it has good separation and will make adjustment easier. What you will hear, is the difference signals for both left and right signals. It will sound hollow, this is proper and normal, but it may also sound harsh and scratchy, which is not good, and indicates that the phase locked loop circuit is having trouble tracking the signal. Back down the pot slowly until you get a hollow sounding signal that does not sound harsh or scratchy. The more natural (except for the hollowness) the sound, the better. You may have to go through the whole range of the pot to find the best spot, and I recommend doing this. When you get it close, move the pot slightly, and then listen after you move it. Once you've obtained the least possible distortion, you have it set. Note that if you have a new stylus, it has to be broken in, and you might not get all the distortion out until this occurs. The best way to break in a stylus is to play about five stereo or SQ records with it. That way, you won't have to put up with scratchy sound while the stylus is breaking in. If your demodulator has two carrier level pots, one for left, one for right, adjust them with the amplifier's balance controls set for left front or right front channel only depending on which channel you are adjusting. If your demod has crosstalk cancellation pots, turn these down all the way counter clockwise to turn it off, unless you have the adjustment record that came with the demod. (The Technics SH400 is a good example of this type demod) You cannot properly adjust these controls without the record, and if you aligned your turntable correctly, you won't need it.
Now, you have a nice hollow sound oozing from your speakers, but it's not quite right sounduing, and it's definitely not quad. So now pick the left channel. Set the amplifier controls so that only the front left speaker is heard. Now start slowly raising the left separation control. You will hear the sound get louder as you bring in the left total signal. But loudness is not what you are going for. as the level of the left total signal catches up with the level of the left difference signal, you will notice that some of the instruments fade away while some get louder. This is what you are listening for. set the pot for maximum cancellation. Now, here's where you have to make a choice. Do you want maximum separation, or would you be willing to sacrifice a little separation for better fidelity? If the answer is better separation, then you are there. If you want better fidelity, then increase the pot just a little more past the null point. The more you turn it, the louder it gets and the more separation you lose, so don't go too far. You can get all the loudness you want with the amplifier's volume control. Then set the amplifier's controls so that you hear only the right front signal and repeat the same procedure for the right separation pot. Once done, if the demod is working right and you did it right, the left front and the right front channels should be about the same levels, and ditto for the back channels. Set the amplifier balance controls to center,, pop a brewsky, and sit back and enjoy some great quad.
P.S. I invite discussion and questions on this subject.