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By NICHOLAS VINEN Two TOSLINK–S/PDIF Audio Converters Do you have a DVD or CD player with a TOSLINK (optical) output but only coaxial S/PDIF inputs on your amplifier? Or do you have the opposite problem? What about hum from your speakers when running digital audio via a coaxial cable? With these simple converters you can easily solve these problems. T WO 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. Transmitting audio digitally is great because in most cases there is no signal degradation. The best transmission medium is optical fibre (ie, TOSLINK) 62  Silicon Chip 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. Advantages A similar design to the commercial unit was published in June 2006 (TwoWay 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 siliconchip.com.au Digital Audio Signal Formats The digital audio signals found in domestic equipment are all in the form of SPDIF (Sony/Philips Digital Interface) bitstreams – either as 400mV electrical signals sent along 75-ohm coaxial cables or as optical signals (pulses of 660nm red light) sent along fibre-optic cables. The optical signal form is often called “TOSLINK”. Although domestic digital bitstream audio is split almost equally between the coaxial and optical forms, they’re both virtually identical in terms of the encoding and serialisation used. So it’s relatively easy to convert between the two, in either direction. 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. 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. Uses The most obvious use for a digital audio converter is when you want to connect two pieces of equipment and siliconchip.com.au 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. 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. 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. one has a TOSLINK connector while the other has a coaxial socket. However, there is another purpose; when either converter is used, the two pieces of equipment will be electrically isolated. This means that as long as you are careful to avoid unintentionally connecting multiple signal earths via the converter power supply, an earth loop can not be formed, regardless of the connection method at either end. Another useful application is for sending an optical audio signal from one side of a room to the other or even into another room. While wall plates are available for sending TOSLINK over Cat5 network cable, they are expensive and require a power supply at each end which will constantly draw power unless an additional wall switch is installed to turn them on and off. With a pair of these converters, you can first convert the TOSLINK signal to a S/PDIF signal and then feed it into a wall plate via a 75Ω RCA-toOctober 2010  63 D1 K REG1 LM317T BR1: W04M 9–20V DC OR 6–15V AC INPUT A + ~ A K A 100 13 2 12 11 14 10 3 3 IC1b 4 5 IC1c TOSLINK TX 7 IC1e IC1f 1 D4 IC1: 74HCU04 A IC1a 100nF 2 K 300 1 100nF 10k D3 100 F 150 10 F 100nF D2 110 220 F 25V – S/PDIF INPUT CON2 K ADJ ~ CON1 +3V OUT IN 6 9 IC1d 8 D3, D4: 1N4148 A SC  2010 S/PDIF TO TOSLINK CONVERTER D1, D2: 1N4004 A LM317T K K OUT ADJ OUT IN Fig.1: the S/PDIF to TOSLINK Converter uses high-gain inverting amplifier stage IC1f and inverting stage IC1e to square up and buffer the input signal. IC1e then drives the TOSLINK transmitter. . RCA lead (composite video leads are suitable). The signal is then carried over standard 75Ω coaxial cable (eg, RG-6/U or RG-59/U) to the other wall plate. From there, it’s then fed via another RCA-to-RCA lead into the second converter and converted back to optical (TOSLINK) format. The power supply at each end (typically a plugpack) can easily be switched off at the wall, along with the sending and receiving equipment, to save power when it is not in use. Performance We tested both converters with Dolby Digital, DTS and linear PCM audio data. The PCM tests included both 48kHz 24-bit stereo and 96kHz 24-bit stereo audio streams. Both units were able to correctly handle all of these streams with one exception: if the TOSLINK to S/PDIF converter is built with a receiver module rated to handle 8Mbps (such as the Altronics Z1602), then it may not work with 96kHz 24-bit linear PCM. This type of audio has a bit rate 64  Silicon Chip of 6.144Mbps (96,000 x 2 x 32) so it seems that the nominal 8Mbps unit should be able to handle it. However, that specification is listed as a maximum rather than typical rating and the measurement conditions involve a cable only 1m long and a stated duty cycle of 50%. In reality, NRZI-encoded data, if considered as being at fixed frequency, has a variable duty cycle. We also tested a 16Mbps receiver (Jaycar ZL3003) and this handled the 96kHz PCM stream correctly. However, unless you are using a DVD-audio player or computer sound card with 96kHz capability, the highest sample rate you are likely to transmit is 48kHz (with a bit rate of 3.072MHz). In this case, either receiver unit is suitable. The data in Dolby Digital and DTS streams is compressed, so their bit rates are lower again. Power supply Either an AC or DC plugpack can be used to power these converters. The acceptable voltage range is 6-15VAC for AC plugpacks and 9-20VDC for DC plugpacks. The current consumption is below 20mA in each case. Power is applied to each converter board via a 2.5mm ID DC socket which suits many but not all plugpacks. In some cases, an adaptor plug may be required or you will have to change the DC connector on the plugpack to suit the on-board socket. If you are the type of person who keeps plugpacks from defunct equipment then you will almost certainly have something suitable. Otherwise, buy the cheapest option which suits the above requirements (eg, Altronics M8922 or Jaycar MP3020) but if it has a fixed plug, check that it’s a 2.5mm type. Circuit description Fig.1 shows the S/PDIF to TOSLINK Converter circuit. Either AC or DC power is supplied via CON1, a PCmount DC connector. If the supply is AC, it is rectified by bridge rectifier BR1 and filtered by a 220µF capacitor to form an unregulated DC supply. If DC is supplied from the plugpack, it siliconchip.com.au D1 K REG1 LM317T BR1: W04M IN + 9–20V DC OR 6–15V AC INPUT ~ CON1 A +5V* OUT K ADJ ~ D2 110 A 220 F 25V 330 * 10 F – 100 F L1 47 H 100nF 3 100nF 4 IC1b IC1: 74HC04 TOSLINK RX 14 5 IC1c 6 3 1 1 IC1a 2 9 2 IC1d 11 * FOR A 3V TOSLINK RECEIVER, CHANGE THE 330  RESISTOR TO 150  & SWAP THE 390  & 220  RESISTORS IC1e CON2 390 * 150nF 8 10 220 * 13 IC1f S/PDIF OUTPUT 150 12 7 LM317T SC  2010 TOSLINK TO S/PDIF CONVERTER D1, D2: 1N4004 A K OUT ADJ OUT IN Fig.2: the TOSLINK to S/PDIF Converter uses a TOSLINK receiver to drive inverter stage IC1a. Its output is in turn buffered and inverted by IC1b-IC1f which then drive the output via a 150nF capacitor and a divider stage. charges the 220µF capacitor directly via BR1 and the connector polarity does not matter because only two of the diodes within BR1 will conduct. Which two diodes actually conduct depends on whether the supply plug is centre positive or negative. Because there are always two diodes in series with the supply, its voltage is reduced by around 1.4V (two diode drops) which is more than the typical 0.7V loss with a single reverse polarity protection diode. However, because the circuit runs at such a low voltage, this doesn’t really matter. The filtered DC supply is regulated to around 3V by adjustable regulator REG1. Its output voltage is set by the ratio of the two resistors on its OUT and ADJ terminals and is (150Ω/110Ω + 1) x 1.25V = 2.95V. In practice, it’s slightly higher than this due to the leakage current from REG1’s adjust pin. The 100µF capacitor provides output filtering for REG1 while the 10µF siliconchip.com.au capacitor bypasses the ADJ (adjust) pin, improving supply ripple rejection. Diodes D1 and D2 protect REG1 from the charge stored in those two capacitors should its input be shorted. That is unlikely because of BR1, however they are cheap insurance and make the regulator circuit virtually “bulletproof”. Signal conversion The S/PDIF audio signal enters the board via RCA socket CON2. It is a bi-phase encoded digital signal (also known as “non-return to zero” or NRZI encoding) which, when terminated with 75Ω, has a voltage swing of about 0.5V peak-to-peak. Its frequency depends on the data format and sample rate but is typically between about 0.9MHz and 6MHz. IC1f is part of a 74HCU04 unbuffered inverter IC and is configured as a high-gain inverting amplifier. The incoming digital signal is AC-coupled to its input via a 100nF capacitor. The 300Ω and 100Ω resistors together set its input impedance to around 75Ω, matching the source and cable impedance for minimum signal attenuation. Diodes D3 & D4 protect IC1f should a higher amplitude signal be accidentally connected to CON2 (or if a high-voltage spike gets in for some other reason). IC1f’s closed loop gain is set by the ratio of the 10kΩ and 100Ω resistors, ie, it is around 100. This is enough so that its output swings fully between the supply rails with a 0.5V input signal while also squaring up the digital signal. This output is buffered and inverted again by IC1e, so that its polarity is the same as at the input (although with NRZI encoding, polarity doesn’t matter). That signal is then sent directly to the TOSLINK transmitter which modulates its output LED to transmit the digital signal over optical fibre. Note that we have used a 74HCU04 October 2010  65 Using The Altronics 3V TOSLINK Receiver S/PDIF TO TOSLINK CONVERTER 4148 300Ω 100Ω © 2010 REG1 LM317T D1 10 µF + D2 IC1 74HCU04 100nF D4 D3 C S IN S/PDIF + 10k 100nF 4004 4004 ~ SC S/PDIF COAXIAL INPUT 220 µF W04M – 4148 POWER INPUT 1 0 1 0 1100 2 1 0µF 180Ω 110Ω + K NILS OT ot S/PDIF FIDP/S to TOSLINK POWER IN ~ + 001210101 102 © 100nF TOSLINK TX TOSLINK OPTICAL OUTPUT Fig.3: follow this parts layout diagram to build the S/PDIF to TOSLINK Converter circuit. It converts coaxial SPDIF signals to optical format. TOSLINK TO S/PDIF CONVERTER IC1 74HC04 100nF 100nF TOSLINK RX 4004 10 µF + 110Ω + D1 D2 150nF SC 01210102 S/PDIF COAXIAL OUTPUT 150Ω 220Ω* 47 µH © 2010 4004 330Ω* REG1 LM317T 390Ω* ~ 2 0 1 0 1 2100 1 0 µF CS 220 µF W04M – 0102 © TOSLINK OPTICAL INPUT + POWER INPUT FIDP/S ot KTOSLINK NILS OT to S/PDIF POWER IN + ~ The parts layout shown in Fig.4 for the TOSLINK to S/PDIF Converter suits a 5V TOSLINK receiver (eg, Jaycar ZL3003). Alternatively, if you are using a 3V TOSLINK receiver (eg, Altronics Z1602), be sure to change the indicated resistor values. Both the Jaycar and Altronics TOSLINK transmitters (Cat. ZL3000 & Z1601 respectively) operate from 3V, so no such changes are required on the S/PDIF to TOSLINK Converter board (Fig.3). S/PDIF OUT NOTE: FOR A 3V TOSLINK RECEIVER, CHANGE THE 330 Ω RESISTOR TO 150 Ω AND SWAP THE 390 Ω AND 220 Ω RESISTORS Fig.4: this is the layout for the TOSLINK to S/PDIF Converter circuit. It converts optical (TOSLINK) signals to coaxial format. Note that you have to swap some resistor values if you are using a 3V TOSLINK receiver. inverter in this circuit rather than a 74HC04 (which is easier to get). The reason for this is that the 74HC04 has a much higher open loop gain and larger phase shift (ie, signal delay) between its input and its output. That’s because each section of the 74HC04 is actually three CMOS inverters in series. This is done to reduce the input capacitance and improve the output drive strength, which are desirable properties in a digital circuit. However, these factors combine to make it unstable in this type of configuration and even a small amount of noise picked up at its input can cause the output to oscillate at a very high frequency (tens of MHz). This increases the circuit’s power consumption when there is no input signal and also causes it to emit more electromagnetic interference (EMI). The 74HCU04 IC is a little different, as each of its sections is just a single CMOS inverter. These devices are primarily intended for use in crystal oscillator circuits but they also work well for amplifying low-level digital signals, as in this case. So while a 74HC04 may work in this circuit, it is undesirable to make the substitution for the reasons stated above. TOSLINK to S/PDIF converter Now let’s take a look at the TOSLINK to S/PDIF Converter – see Fig.2. The power supply is identical to that used in Fig.1 except that its output voltage must be tailored to suit the particular TOSLINK receiver used. For 5V receivers such as the Jaycar ZL3000, 110Ω and 330Ω resistors are used at its OUT and ADJ terminals since (330Ω/110Ω + 1) x 1.25V = 5V. For 3V receivers such as the Altronics Z1602, the same resistors are used as for the other converter (ie, the 330Ω resistor is changed to 150Ω). Inductor L1 and its associated 100nF capacitor form an LC low-pass filter. This isolates the TOSLINK receiver’s supply from the main supply so that switching noise can not be coupled back into it and upset its internal high-gain amplifier. That amplifier is fed from a phototransistor which picks up the bi-phase signal from the optic fibre, converting it to a digital electrical signal at its pin 1 output. This signal is now buffered and inverted by IC1a (part of a 74HC04 hex inverter IC) and then again by the remaining five inverter stages. These are hooked up in parallel to provide enough current to drive a 75Ω load. Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 150nF 0.15µF 150n 154 100nF 0.1µF 100n 104 Table 1: Resistor Colour Codes o o o o o o o o No.   1   1   1   1   1   1   1 66  Silicon Chip Value 10kΩ 330Ω 300Ω 220Ω 150Ω 110Ω 100Ω 4-Band Code (1%) brown black orange brown orange orange brown brown orange black brown brown red red brown brown brown green brown brown brown brown brown brown brown black brown brown 5-Band Code (1%) brown black black red brown orange orange black black brown orange black black black brown red red black black brown brown green black black brown brown brown black black brown brown black black black brown siliconchip.com.au The signal at the inverter outputs is then AC-coupled via a 150nF capacitor, so that it is centred about ground potential, and its amplitude reduced by a resistive divider made from three resistors. This divider also provides the correct output impedance of around 75Ω (actually, 72.5Ω assuming the resistors are accurate). Because the circuit can operate from either a 3V or 5V supply rail (depending on the TOSLINK receiver used), the divider ratio must be configured to provide the correct output signal level. The resistors in Fig.2 are shown configured for a 5V supply rail. The 220Ω and 150Ω resistors in parallel are equivalent to an 89Ω resistor so the 5V peak-to-peak output from the inverters is translated to 5 x 89/(390 + 89) = 0.929V peak-to-peak. This is close enough to the 1V desired. Since the source impedance is 75Ω and the signal is terminated by 75Ω at the other end, the receiver can therefore expect to receive a signal which is a little under 0.5V peak-to-peak. For a 3V supply rail, we swap the 220Ω and 390Ω resistors. The two resistors in parallel then form a 141Ω equivalent resistor and the formula becomes 3 x 114/(220 + 114) = 1.024V peak-to-peak, again within the acceptable range. Construction The two PC boards are the same shape and size and the construction procedure is similar. The S/PDIF to TOSLINK Converter board is coded 01210101, while the TOSLINK to S/PDIF Converter board is coded 01210102. Both measure 74 x 34.5mm. Fig.3 shows how to build the S/PDIF to TOSLINK Converter, while Fig.4 is the parts layout for the TOSLINK to S/PDIF Converter. Whichever board you choose to build, start by checking the copper tracks to ensure that there are no breaks or short circuits. Also check that the holes are drilled to the correct size and that the components fit, especially the three connectors, the regulator and the bridge rectifier. That done, fit the resistors. Check each with a multimeter set to Ohms before installation and remember to change three resistors on the TOSLINK to S/PDIF Converter board if you are using a 3V TOSLINK receiver – see Fig.2 & Fig.4. The discrete diodes can go in next. Be sure to install them with the corsiliconchip.com.au Parts List S/PDIF to TOSLINK Converter 1 PC board, code 01210101, 74 x 34.5mm 1 black switched PC-mount RCA socket (Jaycar PS0279, Altronics P0145A) 1 TOSLINK transmitter (Jaycar ZL3000, Altronics Z1601) 1 2.5mm ID PC-mount DC socket 1 M3 x 6mm machine screw 1 M3 shake-proof washer 1 M3 nut 1 75mm length of 30mm diameter heatshrink tubing Semiconductors 1 74HCU04 hex unbuffered inverter IC (IC1) 1 LM317T adjustable regulator (REG1) 1 W04(M) bridge rectifier (BR1) 2 1N4004 diodes (D1, D2) 2 1N4148 diodes (D3, D4) Capacitors 1 220µF 25V electrolytic 1 100µF 16V electrolytic 1 10µF 16V electrolytic 3 100nF MKT Resistors 1 10kΩ 1 300Ω 1 150Ω 1 110Ω 1 100Ω Alternative parts: W04(M) may be substituted with W02(M), W06(M), W08(M) or W10(M) TOSLINK to S/PDIF Converter 1 PC board, code 01210102, 74 x 34.5mm 1 TOSLINK receiver (Jaycar ZL3003, Altronics Z1602) rect polarity and don’t get the 1N4004 and 1N4148 diodes mixed up on the S/PDIF to TOSLINK Converter board. If you are building the TOSLINK to S/PDIF Converter, install the 47µH axial inductor (L1) next. It looks similar to a resistor but is usually “fatter” and may also be a different colour. Now mount the 74HCU04/74HC04 IC. Check that it is correctly orientated and be sure to push it all the way down onto the PC board fully before soldering all 14 pins. 1 black switched PC-mount RCA socket (Jaycar PS0279, Altronics P0145A) 1 2.5mm ID PC-mount DC socket (Jaycar PS0520, Altronics P0621A) 1 47µH axial RF inductor (L1) 1 M3 x 6mm machine screw 1 M3 shake-proof washer 1 M3 nut 1 75mm length of 30mm diameter heatshrink tubing Semiconductors 1 74HC04 hex inverter IC (IC1) 1 LM317T adjustable linear regulator (REG1) 1 W04(M) bridge rectifier (BR1) (Jaycar ZR-1304, Altronics Z0073) 2 1N4004 diodes (D1, D2) Capacitors 1 220µF 25V electrolytic 1 100µF 16V electrolytic 1 10µF 16V electrolytic 1 150nF MKT 2 100nF MKT Resistors 1 390Ω 1 330Ω (for 5V TOSLINK receiver) 1 220Ω 1 150Ω (2 for 3V TOSLINK receiver) 1 110Ω Alternative parts: W04(M) may be substituted with W02(M), W06(M), W08(M) or W10(M); 47µH axial RF inductor may be substituted with 68µH or 100µH The LM317T regulator is next on the list. To install it, first bend its leads down at right-angles 6mm from its body, then fit it to the PC board and secure its metal tab using an M3 x 10mm machine screw, nut and shakeproof washer. Do the nut up firmly, then solder and trim the three leads. Do not solder the regulator’s leads before securing its metal tab to the board. If you do, you could crack the copper tracks of the PC board as the nut is tightened down. October 2010  67 Using A Single Plugpack With Multiple Converters I F YOU REQUIRE multiple converters in one location, they can be powered from a single plugpack using a “Y-cable”. However, you have to be careful that this arrangement does not introduce any earth loops. It’s just a matter of ensuring that no two converters share a plugpack if one has a coaxial cable connected to a power amplifier while the other has a coaxial cable connected to a signal source (eg, DVD player). The power splitter (Y) cable shown here was made using two 2.5mm ID (inner diameter) DC plugs, one in-line 2.5mm ID DC socket and approximately 1m of twin core flex, which can be salvaged from a dead plugpack (including one of the DC connectors). Begin by cutting the cable into three sections of roughly equal length. Split the wires apart at each end and strip the insulation back. You will need to split the wires by a few centimetres to allow enough length to slip heatshrink over the leads while leaving the exposed ends far enough away so that the heatshrink doesn’t shrink prematurely when soldering. Next, unscrew the plastic shell from each connector and pass one of the cables through it. Slip a 20mm length of 2.5mm diameter heatshrink over one lead and solder that wire to the smaller of the two tabs on the connector. That done, slide the heatshrink tubing over the soldered Now install the bridge W04M rectifier. Make sure that the “+” marking on the top of the device lines up with the “+” on the layout diagram and check that it is correctly seated on the PC board before soldering its pins. Follow this by fitting the three MKT capacitors (note the location of the 150nF capacitor on the TOSLINK to S/PDIF Converter). After that, you can mount the electrolytic capacitors, being careful to check their orientation. The three connectors can now be fitted. Ensure that they are pushed down fully onto the PC board and are parallel with the edge before soldering their pins. The plastic posts on the RCA socket should go most of the way through the holes on the board (you may have to push it down fairly hard to get it to fit). Similarly, the DC socket may need to be pressed down firmly, as it can be a tight fit. 68  Silicon Chip joint and the metal tab and shrink it down. Next, solder the other wire to the larger tab and crimp the metal clamp over the cable to hold it in place. Make sure the two conductors can not contact each other, then screw the plastic cover back into place. Once all three wires have been soldered to the connectors, slide a 40mm length of 5-6mm diameter heatshrink onto the line socket cable and two 20mm lengths of 3mm diameter heatshrink over the indi- Use a generous amount of solder for the larger pins on the both DC and RCA sockets to ensure they are wellanchored. The TOSLINK transmitter on the S/PDIF to TOSLINK Converter board is initially held in place with two plastic posts which snap into the appropriate holes. It is then just a matter of soldering the three pins. By contrast, the TOSLINK receiver on the TOSLINK to S/PDIF Converter is held in place by two large metal pins. They should be soldered first, after which the three signal pins can be soldered. Testing That completes the board assembly which should now be carefully checked for errors. That done, apply power and test the adaptor before encapsulating it in heatshrink tubing. During this test, take care to ensure vidual leads. Twist all three positive wires together (with the line socket cable facing the opposite direction to the other two) and apply solder to the joint – an alligator clip stand will help hold the wires steady. Check that all three centre pins are electrically connected and then shrink the smaller piece of heatshrink tubing over the solder joint. Now repeat this procedure for the three negative wires and then shrink the larger diameter insulation over both joints and the cable is complete. that the parts cannot short against any metal objects, especially on the underside of the PC board. It is also a good idea to check the underside of the PC board to make sure that there are no long protruding pins which may later pierce through the heatshrink insulation. If there are, cut them off short with side-cutters. Once you have confirmed that the converter is functioning correctly, cut the heatshrink tubing to a length of 75mm, slide it over the unit so that it projects evenly over both ends and apply some gentle heat (eg, from a hair drier). Be careful not to bump the heatshrink out of position while doing this and be careful not to overheat it if using a hot-air gun. That’s it! If you need additional converters, just build some more. They should each take no more than about SC 30 minutes to assemble. siliconchip.com.au