How to build an optical SPDIF bracket

WARNING:

Creating, testing and using home built electrical circuits and connecting these to your computer's motherboard may pose the following risks: - electrical fire - toxious fume inhalation - burning of skin or face by using a soldering iron - damaging perfectly working parts of youyr motherboard or audio equipment - eye damage by looking directly into the light transmitted by the device

The author of this article explicitly denies any liability to any of these or other risks ..however that being said, this procedure should be feasible with average technical skills and little tools and equipment. If in doubt, get assistance. You may find someone with appropriate knowledge and skills to assist at a small fee or return favour.

Introduction

Most MythTV systems these days are built using a motherboard with integrated audio and video. However, the SPDIF signal is unfit to be wired directly to a SPDIF compatible digital audio receiver. The reason is that the signal is a TTL signal, while it should be a signal swinging between -0.5V and +0.5V. This requires that a simple circuit be built, using some resistors and a capacitor, but the signal can not be used to drive a long cable and thus additional components like a 74HCT04 are needed to amplify this signal to acceptable levels.

This article describes how you can build a SPDIF TOSlink transmitter circuit, using just two components.

Advantages - Optical link means no GND loops, total isolation can be achieved with your receiver (however, your DVI cable will contain a GND connection to your TV, and if these share connections, there will be at least a common GND signal anyway) - no electrical interference possible - Simple to build, takes about one hour - cheap, components should cost less than 15 euro's / 20 USD - no exotic tools required, only soldering iron, pliers, drill

Disadvantages - optical TOSlink cable needed - receiver needs to have a optical TOSlink input (most modern ones have that)

Requirements - Motherboard must have a SPDIF header - SPDIF should be unmuted (see other articles about the software setup)

Hardware needed - 1 SPDIF optical transmitter unit, this article describes the Toshiba TOTX173 - 1 8.2K ohm resistor, 1/4 watt 20% tolerance - 4 pin header, commonly supplied with the motherboard for interconnection of SPDIF to DVI - heat shrink isolating sleeve 3mm diameter - thin solid core copper wire - soldering iron - soldering tin for use in electronic (resin core, not acid core)

Literature needed: - Application sheet from Toshiba for the TOTX173, download it here - manual for your motherboard showing the pin layout for the SPDIF. This one is important, don't guess which pin is which, it may damage your motherboard)

Procedure: - Download the Toshiba application sheet and study it carefully. - The pin connections should be strictly adhered to. The pins as shown in the top view are counted when you view the component from the front, in other words, looking into the plughole of the device. - Lay out the tools and components on a table. - The connector header that you will use should exactly fit over the pins on the motherboard or sound card SPDIF connector. There will probably be 3 wires: both the edges will have wires, and one of the middle ones, and the gap will possible be blocked to prevent it being plugged in the wrong way. - Determine from the mothermoard or sound card manual which signals are wired to which pins:

   - GND (this is the pin carrying the reference 0 volt signal, please note that wire color will possibly not be black, while this is standard)
   - 5V  (this pin carries the 5 volt line, and supplies the transmitter with the power needed to transmit light)
   - INPUT

- Write your wiring diagram down on paper, by linking the header pins 1 - 4 to the TOTX173 by drawing lines. Planning is everything. - Determine where in your case you will place the transmitter. A blank plate for covering a PCI slot can be modified to use for it. - Your SPDIF header connector will probably have a connector on the other end. Cut it off with pliers. Leave enough length of cable to reach the the case. if your favorite place is too far, you will need to lengthen cables first. You can use - Cut off small pieces of shrink sleeve.