However as I started designing the layout, it occurred to me that if I planned it properly, I could add the other circuitry later on if I needed hence it turned into a three-part project. I also did not need the logic control circuitry for additional separation, hence I figured just a small decoder costing less than ten pounds would be enough to quench my curiosity. My amplifier has component 4-channel audio input sockets, with balance and volume controls so I did not need the gain control circuitry. The logic control essentially switched ON or OFF the relevant pairs of speakers to produce better separation.Īt first, my intention was to build just the MC1312P matrix decoder circuit, and feed its output to my amplifier. Looking at the documentation of this IC, I noticed that it operated with two more support chips, where the MC1314P IC provides the gain control for the channels, and the MC1315P provides the logic control to improve the separation between the back speakers and front speakers. This IC seems to be available and affordable so I figured it would make a good project for my hi-fi. Luckily, I stumbled upon a Motorola MC1312P IC, which is a four channel SQ decoder chip that reduces the component count drastically. Consequently, in this build, it is advantageous to get high-quality and precision components for a good result. The circuitry is able to produce 90° phase shift between 100 Hz to 10 kHz, and phase ripple is only ☘.5°. These networks are of the Wein bridge type, and their high-precision resistance arms are within the IC package. Internally, the MC1312P consists of all-pass networks that produce a phase shift without amplitude variation. However most of the discrete transistor circuits tend to be huge and elaborate, so that was out of the question. Vintage amplifiers that have this decoding facility are usually expensive, which leaves the possibility of making a circuit. I was fascinated to learn that it was possible to have four channels on an LP and wanted to get hold of a decoder to experience what it sounded like. Back in its time when this system was popular, there were also amplifier receivers that had built-in decoders, however as you can imagine, these are not manufactured anymore and consequently what is available is very expensive to purchase. In order to listen to these records in four channels, one requires a special decoder that converts the two (stereo) channels into four channels. Suffice it to say that most of the popular records such as the John Lennon one will be expensive. If you look at places like eBay and search for Quadraphonic SQ, then there is usually a small selection of records to choose from. However, it is possible to buy records that have quadraphonic channels, and these records have the SQ logo on them. The idea of recording just stereo on a groove is amazing, but then to have four channels, is even more amazing. This IC reduces the component count drastically and enables the use of low-value single-track potentiometers, instead of high-value ganged ones that are usually expensive.Ī quadraphonic recording on vinyl records is something most people may not have heard of. The MC1314P is the gain control block containing voltage controlled amplifiers, which enables volume and balance settings of the channels. This type of enhancement is highly desirable and sounds far better than systems that do not have it. The decoding is performed by the MCP1312P, whilst the MCP1315P enhances the back to front separation by to 12 dB. Hence quadraphonic records with the SQ logo can play in stereo as normal records, however if one had the decoder, then the stereo signals can be separated into four channels. The decoder takes a pair of composite signals (stereo) and separates them out into four channels. The MC1312P, MC1314P, and MC1315P are integrated circuits that provide a complete chipset solution for decoding quadraphonic records encoded on the SQ system developed by theĬolumbia Broadcasting System (CBS).
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