Wacom digitizer screen

Anyone who has already tried to use some kind of tablet device for writing should know that there are fundamental differences between screen types.

The most common is the capacitive type, where you use a finger or some kind of conductive pen to write on a glass surface, while the touchscreen device captures movement of the capacitance change through a grid of transparent electrodes on the backside of the glass. This works, but it sucks for writing precise text or drawing sketches. You can find these screens in almost every modern smart phone, tablet PC or kitchen appliance. They are cheap!

Next is the resistive touchscreen, where a small, hard point presses down on a plastic surface. The touch element is composed of two pieces of clear foil, coated with a conductive material. While the two layers stay isolated when there is no pressure applied, the pen forces them together in a certain point, forming a conductive path. By knowing the specific resistance of the surface coating, the circuit can determine the position of the pen tip by measuring path resistance from different edges of the screen. This type was pretty popular in PDAs (which have by now been fully replaced by smart phones, what a shame 😉 ) as well as the almost equivalent navigation assistants – and is not that common anymore. Writing performance is fair but not exceptional, though.

The third kind is the most interesting one. Real tablet PCs (the ones with the flip-over display) have this normal-looking pen with the nylon tip, which you can use to accurately write on the glass/plastic display surface. Many even feature some buttons on the pen, some kind of eraser on the backside – and they are damned accurate! They have another thing in common: Most of them use technology by a company called Wacom, also producer of digital writing and drawing pads for artists.

This type is called a “digitizer screen”, and it uses a sensing panel *behind* the actual display to recognize and track the pen. The digitizer panel contains an amazing set of surface coils to provide an alternating magnetic field through the screen. Inside the pen, there is a resonant circuit which uses the field energy to transmit the button states and even pressure on the pen tip back to the coil. By monitoring the strength of the resonance through different surface coils, the digitizer then calculates the position of the pen above the surface. In other words, you get a high-res info about the pen position (easily above 25.000 points resolution along the surface edge, depending on the digitizer type!), you know the pressure applied, button presses on the pen and even where the pen is when it is not yet touching the surface.

I recently disassembled a trashed tablet PC (Toshiba Portege) with a broken motherboard for interesting parts, and came across this:

tablet01

Fig. 1: Backside of Toshiba Portege LCD module. Wacom digitizer (red/orange foil) and control logic (PCB in top left corner) are visible. LCD control board on the bottom.

The LCD panel is a LTM12C328T type. Attached to the backside is a SU-010-X01 tablet pen digitizer, and the marking on the ICs clearly suggests that it is made by Wacom. This would make a fine graphics tablet – but how to attach it to any other PC? [Read More]

Adding S/P-DIF to your soundcard

This is something really simple I did a while ago…but wanted to show you nonetheless.

Most current notebooks still don’t provide an optical or even coaxial digital sound output, also known as S/P-DIF. This is something I can’t really relate to, as it might well be the only way to get encoded surround sound out of that thing without using an extra USB sound card or some crappy stereo upmixer. HDMI is also not the key to the problem since the digital audio stream is tightly embedded into the rest of the signal – an extractor circuit costs about 300 Euros over here!

Instead, save that time and take a look at your soundchip. Most feature S/P-DIF (especially if the containing device features HDMI) natively, but the corresponding pins are simply not connected to anything – which does not mean they are not active! Try to google for the part number (here ALC262), for most chips you will find datasheets right away. Once downloaded, head straight for the pin assignments section and keep an eye out for descriptors like “SPDIFO”.

Edit: If an output pin is available, chances are pretty good that it is useable even if the digital output is not shown in the mixer program. Some applications can turn off the optical output by software, but mostly that means the transmitter device while the data keeps on streaming.

In this case, the original docking station had the optical output built in even though the pin was left unconnected within the notebook, which made things a little easier – although it once again proves that manufacturers sometimes abandon pre-planned features for whatever reasons, which I as a customer generally don’t approve of.

Finding the corresponding pin on the dock connector was done using a generic multimeter. There should be no transistors or other obstructing parts in the signal path as most optical transmitters have built-in logic that only requires a TTL signal to switch the LED emitter. Out of the three or four pins on the transceiver device you should be able to identify one as ground, one as VCC and one of the remaining two will be the signal input. The remaining fourth one (if present) is most likely an enable pin and is tied to either VCC or GND.

Realtek ALC262 with upgrade

Realtek ALC262 with upgrade

Once the correct pin on the package is determined, carefully solder a thin enameled wire on top of it. This is the only tricky part and requires a thin tip and steady hands. Try not to heat it up for too long and check for accidental connections between the neighboring pins afterwards.

Aaaand you’re done. The signal you just tapped can be fed into an optical transmitter (e.g. TOTX… type) directly. By the way: If you are building/designing a digital to analog converter for audio purposes, this trick can be used to implement a very simple USB connection. Just get one of those cheap USB-plug soundcards. The chips inside mostly feature digital outputs and can be wired into an open TTL-compatible input of your DAC project.