Two different circuits are described here: (1) a S/PDIF to TOSLINK Converter; and (2) a TOSLINK to S/PDIF Converter. The first converts a S/PDIF (coaxial) signal to an optical signal, while the second does the opposite. Each converter is built on a separate circuit board and is powered via a small AC or DC plugpack supply.
This is the S/PDIF to TOSLINK Converter board. It accepts digital audio at the RCA socket at left and outputs an optical signal at the TOSLINK transmitter at right. Power is fed in via the on-board socket at top left.
Transmitting audio digitally is great because in most cases there is no signal degradation. The best transmission medium is optical fibre (ie, TOSLINK) because the two connected devices remain electrically isolated. However, it’s not without its drawbacks – the cables tend to be expensive and can not be cut to length.
Also, because there are multiple competing standards (coaxial, TOSLINK and HDMI to name three), you won’t always have the same connectors at both ends.
In fact, these issues are so common that several SILICON CHIP staff members were in the market for digital audio converters. They are commercially available but the retail cost of around $60 for a bidirectional unit seems high, considering that we can put together something similar for much less than that.
The companion TOSLINK to S/PDIF Converter works the other way – ie, it converts an optical signal to a S/PDIF signal and outputs it at the RCA socket at right.
A similar design to the commercial unit was published in June 2006 (Two-Way SPDIF/Toslink Digital Audio Converter). This is a smart-looking little device in a small plastic box. Unfortunately, kits for that project are no longer available and neatly drilling the boxes from scratch seems like too much work.
Also, there is a problem with bidirectional units due to the fact that the shields of the coaxial input and output sockets are typically connected together within the converter. As a result, if both sections are used, there is still the possibility of an earth loop being formed, resulting in hum problems.
Rather than mount them in a case, the converter boards can be sleeved in heatshrink and hidden behind the A/V equipment they connect to.
With these new designs, you can build just one converter or several, depending on your exact requirements. They are designed to be housed inside heatshrink plastic tubing so there is no need to drill a box and this keeps the unit cost low as well as simplifying the board shape.
We have also made some minor improvements over the earlier design. These versions can be powered from a wider range of plugpacks, so chances are you already have a suitable power supply spare from another piece of equipment. They also use less power, making it easy to run several from a single plugpack.
In addition, TOSLINK modules from both Jaycar and Altronics can be used – in fact virtually any are suitable. Some modules require a 3V supply and some a 5V supply. Only a few resistors in the on-board regulator circuit need to be changed to suit either type.