Archive for the 'Bending Projects' Category

Quick music sample

Monday, May 12th, 2008

I’ve only gotten 3 bends fully wired up on the PSS-460, because I’ve been having too much fun playing it.

The bends so far are two distortion bends that also tend to have input on other bends when they’re turned on.

The third bend is one of a group of bends that together make up what I’ve dubbed ‘Ornery Mode’. They cause the cpu/sound chip to ignore keyboard input, and react unpredictably when not ignoring input. The result can be bizarrely ambient, but is quite Aleatronic.

Sample: 28 megs, 20 minutes, 192kbps

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Bending a PSS-460: Wiring up a breadboard

Thursday, May 8th, 2008

In part 2 I examined some bend points. There were so many good ones on the sound chip, I’ll spend some time connecting the chip to an external breadboard to make adding more bends easier.

Preparing a ribbon cable
In the past, I’ve done several bends where each bend gets a wire to the pin. Instead, I’m going to attach a ribbon cable to each side of the sound chip. Conveniently, ribbon cables are available for cheap - I’m using a cable for a floppy drive.
First, cut off the connector. Then, count out twice as many wires as you have pins (I have 12 pins per side, so I’ll be using 24 strands of the cable) and tear off the extra. It should tear easily.
Next, strip the odd-numbered wires (counting the red-marked wire as 1) and cut the even-numbered wires shorter. We won’t be using the even-numbered wires; they’re just there to act as a spacer. Don’t strip off too much insulator; you only need a couple millimeters of bare wire to solder a joint and keeping it short will reduce the chance of (unwanted) short circuits.

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Bending a PSS-460: Finding bends

Sunday, April 27th, 2008

In part 1, I opened the Yamaha PSS-460, gained access to the main board and identified some chips and safe areas of the board.

In this entry, I’ll be outlining how to find bends, both of the simple short variety and the resisted short variety.

Finding Bends

Traditionally, bends have been found by using a pair of jeweler’s screwdrivers connected with a wire. Holding both screwdrivers and playing the instrument often requires more hands than I have. Instead, a pair of insulated alligator clips connected with a wire allows hands-free connections.

To start, I’ll be focusing mainly on the sound chip, a Yamaha YM3812. An integrated circuit (IC, or ‘chip’) is like a miniature computer. It interfaces with the rest of the circuit board with a series of wires or other electrical contacts (called pins). Pins are numbered counter clockwise from the top left of the chip. Pins are generally dedicated to one of three purposes:

Power. ICs need power just like the rest of the circuit board. There will be at least two pins that carry power to the chip (one ground and one ‘high’ - 3 or 5 volts.) In datasheets, these are usually denoted as GND and VSS, V5, V3, V+, etc. In general, shorting a power pin to another pin will result in a crash or at least cancel the output of that pin. According to the YM3812 data sheet, the top left pin is VSS, the bottom left pin is GND.

Control signals. Often broken up into Clock, Interrupt (IRQ), and Command. These are used to synchronize the chip with another chip, or pass commands between them. Shorting these usually results in highly unpredictable behavior, including crashes and partial lockups.

Data. Usually in groups of 8, data pins are used to transmit digital information in and out of the chip. In many cases, the same pins can be used for both input and output, and the YM3812 makes use of this. On this chip, pins 10,11 and 13-18 are data pins. Shorting data pins is the most common way to get relatively stable sound modifications.

An easy way to try all combinations of pins
Start at the top left pin. Attach one clip to this pin. This is the ‘fixed’ clip. Using the other clip, try all the other pins in sequence.
Move the clip from the top left pin to the next counter-clockwise pin. Then try every other pin, leaving out the top-left pin, since you’ve already tried 1 shorted to 2.
As you progress, you can skip every pin the ‘fixed’ clip has already tried.

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Bending a PSS-460: The internals of the keyboard

Friday, April 25th, 2008

I obtained a Yamaha PSS-460 on ebay a few months ago, and am starting a series of posts as I bend it.

The PSS-460 is a good example of an 80’s keyboard. It has a separate CPU and sound synth chip, has a drum synth and a headphone jack. It can be both battery-powered and powered via a 12 VDC jack.

Inside the case

	Up top, we have the battery holder (will cut and re-attach the wires later). On the left and right are the speakers. Between the speakers are a pair of circuit boards, one brownish and one green, one mounted above the other with standoffs and screws. At the bottom is the backside of the keys, conveniently colored brown as well. In fact, both brown boards are just key/button mounts (one for the keyboard, one for the control panel) with pullup resistors and ribbon cables. This leaves the main green board, which is the one most interesting to us.
Gaining access to the main board
	First, we have some shielding/padding to get through. This is here to prevent interference from below the device from affecting the main circuit board. For now, I’m removing it, but will add it back when I’m done.
	After removing it, we get a clear view of the back of the main board.