Quadrophonia is one of the first multi-channel (more than 2 - stereo) audio system. As it name says, this system has four audio channels, four input audio information which are process trough the pre-amplifier and power amplifier, and send to the four outputs - speakers. The interesting thing about this system is that the "quadrophonia" is born during the recording, or in other words, the audio information is created on the stage where orchestra plays.
For example, the orchestra is playing on the stage where we need to create the quadrophonic audio record. For this purpose, we will use four microphones for recording that will be put on the specific positions on the stage, which will produce our four audio information signals. This four signals are the inputs for our audio system. The microphones located on the stage for recording purposes, need to be placed in the following order:
>> The first microphone is placed on the left side of the orchestra, therefore, the recorded signal from this mic we call the Left audio information - L.
>> The second microphone is placed on the right side of the orchestra, so the recorded signal from this mic is the Right audio information - R.
>> The third microphone is placed on the stage in front of the orchestra and the signal that it records is the Front audio information - F.
>> The fourth microphone is placed on the stage behind the orchestra, and the recorded signal from this mic is the Back audio information - B.
We have defined the recording procedure for this 4-channel audio system which produce the four audio signals that we send as inputs into the quad pre-amplifier, so now we can go further on the audio path and let see how this four audio signals are processed in the quad pre-amp. The block scheme in principle of the quad pre-amp is shown on Picture 1.
Picture 1: The principle block scheme of the quad pre-amplifier
The schematic of the pre-amp is quite simple. The four recorded audio signals L, R, F and B are send in their inputs. First stage of amplification is classic operational amplifier, one for each signal. After that, the amplified signals are combined in the way that we get two signals on the output of the pre-amp. The first signal is L + B + F which is sum of the left, back and front recorded information. The second output of the pre-amp is the R + F - B signal which is sum of the right and front signal and subtract the back signal from this sum. So, the quad pre-amp have four input signals which process it and provide two output signals. These two signals are inputs for the quad power amp. The Principle scheme of the power amp is shown on the Picture 2.
Picture 2: The principle block scheme of the quad power amplifier
The power amp at a glance looks like stereo amplifier. However, if we look this amp structure closer, we can see that this is some kind of bridge connection between two amplifiers. And as it is shown on the Picture 2, the four speakers of this audio system are connected in a way that we get these output signals:
>> The Left speaker produce the amplified L + B + F audio signal.
>> The Right speaker produce the amplified R - B + F audio signal.
>> The Back speaker produce the amplified L - D + 2B audio signal.
>> The Front speaker produce the amplified L + R + 2F audio signal.
Of course, for the best effect of this audio processing it is very important how we will placed the four speakers in the room where we will listen the recorded music. The speaker positioning in the room is shown on the Picture 3. The sound reproduced with this audio system is very natural, and the listener get the feeling that he is surrounded by the sound of the orchestra.
Picture 3: The positions of the four speakers in the room for listening
The one and the only, music band Pink Floyd have recorded in quadrophonia. Can you imagine this feeling? Pink Floyd started live quad sound right back in 1967. So, the quadrophonia is the first multi-channel audio system. Today there are a lot of modern digital audio multi-channel systems like surround 5.1, 6.1, 7.1 etc. They are six, seven, eight channel home theater audio systems, but in mine opinion, quadrophonia is the pure natural multi-channel system because it is made on natural way right on stage where music is played by musicians.
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