The Casio SK series are the most widely bent and most popular of many a circuit bent musician, but have you ever wondered why?
It stems to the fact that the SK series were the first widely mass produced domestic samplers that were made cheaply, and because the technology was not fully developed all the parts of the circuitry were laid out as separate chips, rather than the one surface mounted chip (or even worse a dreaded black blob on a board ) that would do the same job now. It is possible with just a little knowledge to understand roughly how the SK works, and to a sufficient level of detail to allow some really cool modifications, rather than a hap-hazard approach with a bit of wire, although this method can create some incredible results, and some swear by it..but it would be nice to know what you are doing to the thing to create the outcome that is achieved. Just a little bit of research before you start attacking the circuit board can pay huge dividends in the quality and results of the end creation.

The first thing is to understand how the SK series work, they are all different inside ( the SK-5 is my personal favourite ) but use the same principals, and by and large the same chips. So here we go.. Casio SK series 101!

How it actually works

The core of the SK is the CPU chip ( the big square surface mounted one ) which is a MSM6283 xx where XX is the variant for which ever SK series it is fitted into, in the SK-5 it is MSM6283-05GS. The differences are quite subtle, each variant has the same number of pins, but some are not used on some models and used on others.
This chip has 8 analogue outputs which are combined on the main circuit board to give 4 audio channels ( voices ) and then further mixed into a single audio signal and passed to the speaker. It is also worth noting that the original SK-1 has 9 analogue outputs as it uses the 9th for the percussion, and the drum sounds are synthesized inside the CPU, whereas the SK-5 has a separate chip to generate the percussion. These 8 (or 9) outputs come from the CPU on pins 89-96 ( 97 on SK-1 ), but more on this later. Apart from generating sound, the CPU has to know what the user is doing, i.e. pressing keys, sampling, recording etc. All keys and buttons are wired in a grid and the CPU scans each and every key and button sequentially to see if has been pressed. The CPU has a separate set of pins dedicated to this hardware scanning and they are 65-76 which are the Key common signals and 77-88 which are the Key input signals, so for instance it will send a signal out on pin 65, if the ëSampleí button is pressed then the sample button shorts pin 65 to 77 so the CPU sees itís signal back on pin 77. So the CPU can make sounds, and also know what the user is doing, but it now needs to know if a key is pressed what does it do? What sound should be made? Thankfully some clever Japanese boffin at Casio decided, if for instance the A key is pressed, then the CPU sees that it has been pressed as it is scanning the keys and then it looks in the ROM chip to see what to do. The program in the Rom chip will then instruct the CPU to generate a waveform that resembles a piano, and do this 440 times a second to give a 440Hz waveform ( Middle A! ) which it then outputs on one of its 8 analogue channels. Hey presto! You press a key and get a note.
Sadly it is not this simple.. the CPU has no idea how to generate a waveform that resembles a piano, it doesnít know a piano sound from a sheep farting.. Luckily the wonderful Japanese boffins have told the CPU in advance what a piano sounds like, along with what to do when a key or button is pressed. It is all stored in a ROM chip which is a PD23C246EAC. Inside the ROM the piano sound is represented by hundreds of numbers in a long sequence, stored at a certain location. To make the actual sound, the first number in the long long sequence is passed from the ROM to the CPU via the DATA BUS, say for instance that the first number for a piano sound is 42, the CPU will take the 42 and pass it to one if itís analogue outputs. The analogue output is actually a Digital to Analogue converter built into the CPU. If the Digital to Analogue ( D/A for shortness ) converter is given a big digital number such as 53524 then it will output a big voltage, if it is given a small digital number such as 3 then it will give a tiny voltage. The biggest number the D/A can convert determines the ëBití rating, if it can divide 1- 256 into 256 different voltage levels then it is said to be an ë8-bití converter as 2 to the power 8 is 256. Anyway back to our number 42. The D/A reads the 42 and gives a small voltage out say 0.3V, with this job done, the CPU gets the next number in the sequence for a piano, which happens to be 44, so the D/A jumps from 0.3V to 0.35V. The next number is a 47 and the D/A now spits out 0.4V. If you imagined a graph with whatever position we were at in the long sequence of numbers ( pos 1 = 42, pos 2 = 44, pos 3 = 47 etc ) along the X axis, and the output from the D/A on the Y axis going up, with our first three numbers we would have a line going up at about 45 degrees. If we kept going along plotting the output from the D/A on the Y Axis, and the right set of numbers appeared in the long long sequence of numbers, it would be possible to trace out a curve / line / or even a sine-wave ! or in this case the waveform for the actual recording of a piano. You must remember that this happens incredibly quickly, the output from the D/A changes quick enough to generate a real waveform that you can hear, the ROM is sending the CPU the numbers at light speed across the Data bus. When a musical key is pressed you are actually instructing the CPU how quickly to send the repeating sequence of numbers to the D/A, a high note means the D/A has to process all the sequence of numbers very quickly, over and over to generate a higher frequency, at a lower note it still has to process all the numbers but not as quickly, so it loops longer giving a lower pitch sound.

So what Next? Sampling ?

Well we now know that the CPU is important, the Rom very important as is the Library of knowledge for the CPU and the Data Bus that are the physical wired connections between the CPU and the ROM. However there are a lot more other bits and bobs inside an SK.. there is RAM, and Address Bus, hiding inside the CPU is a A/D ( Analogue to digital converter ), we have clock oscillators, Melody circuits and filters. The Next most important bit is the Address Bus and the RAM. I previously mentioned that the long long sequence of numbers that make up a piano sound is stored at a certain location within the ROM. As the Data bus passes the Data from the ROM to the CPU, the Address bus is how the CPU looks in a certain location within the ROM. Going back to our long long sequence of numbers, the first number (42) is at address #446F6 the next number (44) is at address #446F5 and so on. Not only are the values flying across the data bus at such a rate the D/A can make an audible waveform, the CPU has to set the voltages on the Address bus to instruct the ROM to give it the values to make the sounds with. Then we have the RAM, this is where the CPU can store data, but more useful to the SK, it is where the samples are stored. When the SK samples, the reverse of generating a sound happens. The waveform picked up on the MIC is given to the Analogue to Digital converter inside the CPU. The A/D gives the CPU a whole string of numbers, proportional to the voltage of the changing waveform from the MIC. The CPU is really busy as it has to get the value from the A/D, put it on the Data bus, generate an address to store it in, and select the RAM chip rather than the ROM ( Rom is Read Only Memory ) and then send the value across. The whole cycle repeats about 8 thousand times a second, giving a sample rate of 8Khz, in fact the CPU is so busy sampling it cannot read the Keys and buttons until it has filled the RAM with data. Now we have a custom recorded sound in RAM, so rather than play the sequence of numbers that represent a Piano that are in the ROM, the CPU can read the sound data from the RAM chip and play back what it has just recorded.

SK1 BENDING