How to build an optical SPDIF bracket
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.
- 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
- optical TOSlink cable needed
- receiver needs to have an optical TOSlink input (most modern ones have that)
- Motherboard must have a SPDIF header
- SPDIF should be unmuted (see other articles about the software setup)
- 1 SPDIF optical transmitter unit, this article describes the Toshiba TOTX173, article 181404 at Conrad
- 1 8.2K ohm resistor, 1/4 watt 5% tolerance, see the Wikipedia article on how to identify it's value, it should have a grey-red-red-gold band coding.
- 3 or 4 pin header. 4-pin headers are commonly supplied with the motherboard for interconnection of SPDIF to DVI
- heat shrink isolating sleeve 3mm diameter, for example article 531618 at Conrad
- thin solid core copper wire
- soldering iron
- soldering tin for use in electronic (resin core, not acid core)
Where to get the optical transmitter
I went to the Conrad store in Cologne and paged through their store counter catalog and happened to find the Toshiba TOTX173 transmitter in the pages describing optical devices. Reading some more I figured this must be a TOSLink transmitter, and after asking the assistant to show me one, it was clear from the dummy plug shape that this should work, even if the catalog did not list it as a TOSLink transmitter. it cost me €5.22. I had loads of resistors lying around, so that was no problem at all. These cost a couple of cents a piece. The sleeving, wire, soldering stuff is available at quality electronics stores.
The part is also available on Digikey.com:
If you'd rather not use the TOTX173, you can use an RCA to Toslink adapter, avaialble from Monoprice.com:
Coaxial (RCA) to Optical Toslink Digital Audio Converter
- 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, or even melt wires!
ASRock AliveNF7G-HDReady shows the header, find your motherboard to get the correct information.
- 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 motherboard 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, also referred to as VCC (this pin carries the 5 volt line, and supplies the transmitter with the power needed to transmit light)
- 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 shrink sleeving to isolate these joints from any other metal parts.
- Place the transmitter in a vice grip VERY GENTLY or grip it in pliers and hold the handles together with a rubber band, exposing the pins such that you can solder without it moving.
- Cut off small pieces of shrink sleeve.
- If no joints are needed, then shove a piece of shrink sleeve tubing over the wire and move it towards the far end, far away from where tyou will be using a soldering iron. The objective is to shrink the sleeving using the soldering iron so it fits tight over the joint, thus isolating it.
- Now, using your diagram, wire up the transmitter, by soldering the wires from the connector to the transmitter. Do GND and INPUT.
- For the 5V line, referred to as VCC in the Toshiba document, you will need to join the resistor's one leg to the 5V wire first. Twisting them may work, but any other method is OK< as long as the final result is that both are wired to pin 3 of the transmitter.
- The other end of the resistor is the fitted with a sleeve tube as well, making sure there is a small piece sticking out to solder this leg to the pin 2.
- When these wires are joined, you can move the sleeves over the joints.
- Apply heat to the sleeve, using the soldering iron, a gas lighter, or any other method without damaging the transmitter.
- Remove the dummy plug from the transmitter and plug in your optical cable
- Do not remove the protective cover from the cable, they are normally transparent, so one will be able to see red light being transmitted.
- When MythTV is setup correctly, the optical cable will show a red light shining.
- Connect the optical cable to your receiver and proceed with testing from MythTV. Your receiver should be configured for optical input.
- If you do see a red light emitted from the optical cable, but no sound from your receiver, see the Configuring_Digital_Sound_with_AC3_and_SPDIF article.
- use a clean tip. Use a wet cloth to clean the tip when your soldering iron doesn't have it's own cleaning sponge
- make sure the soldering iron is at working temperature
- use the soldering iron with one hand and solder wire with the other hand.
- apply solder to both ends to be joined first, so that you can then join two parts together by applying heat only. This may help if it is impossible to fasten both ends, like when joining wires.
- do not heat the transmitter more than 3 seconds at a time. If you need a second go, wait 60 seconds to allow cooling.
- Hold the joint still at least 10 seconds after soldering.
- A good solder joint us shiny, uses as little as possible solder (no blobs) and as large as possible contact area.
- Solder is not strong, as it is made from a mixture of lead and tin, so a lateral joint should be used (two wires or wire and pin should overlap)
Commercially Made Cables
- If you aren't up to creating your own optical SPDIF bracket cable, a commercial one available for purchase can be found here: