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.

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