Last updated: Wednesday, May 25, 2000 1:06 AM CST

S/PDIF Coax -> Optical conversion

Table Of Contents Picture Index
Updates
  • 5/26/00 - IMPORTANT! There was a serious mistake with the image of the Sharp transmitter - pin 2 and 3 were incorrectly labeled, and implementing that pinout would reverse polarity! The image has been fixed. Thanks go out to a visitor with a far better eye than myself.
  • 5/25/00 - New pictures added.
  • 5/25/00 - I decided to check the page with Netscape...of course, much of the page had problems. Fixed. I hate Netscape.
  • 5/24/00 - Page updated to reflect battery powered design - new images on the way of the refit unit.
  • 5/23/00 - After receiving some interesting suggestions, I'll work to expand the scope of this page slightly. In the works: addition of multiple optical or coax output stages (with matching impedence), battery operation, and bidirectional conversion. Update: battery operation works perfectly fine, I just had a bad 9v connector, of all things. Currently testing battery life with the unit.

Introduction
Yes, yes...it's yet another set of instructions for constructing your very own digital coaxial to optical converter. First off, why you would need such a device - you have a sound source with a coax digital output, and a digital recorder that accepts optical input (in most cases, everyone's favorite - the minidisc recorder). Total cost for this project will run $25-$30.

Now, does recording digitally make a difference? I would say that while analog recording sounds fine for general use, digital recording is very nice for devices where you would be better off bypassing the DACs. Digital to analog converters do exactly as they say - take digital values and produce a proportional analog voltage to feed your speakers/headphones. DACs vary in quality, and can be affected by the electrical noise of their environment. PC sound cards have to deal with a very noisy environment, and sound can suffer depending on the build quality of the card. However - if your sound card has a digital output, DAC quality makes no difference, because you can directly record the digital signals. A $30 Vortex2 card and a $200 Live Platinum card will produce the same digital signal.

By the way, this is why *recording* CD audio is a doubleplus ungood thing versus *ripping* the audio as wave data. Recording audio is constituted of a CD/DVD-ROM drive reading the disc's data and using its ultra-cheap DACs to send the audio down the miniscule wire connected to your sound card. If you ever wondered why designers used such flimsy wiring for something as grand as CD Digital Audio, you now know...a pure silver 12AWG cable wouldn't be able to compensate much for the sound out of xx-rom DACs (and would cost a tad more). Rip. :)

Why? Why have you done such a thing?
I looked around for plans for a coax -> optical converter, but most plans were just parts of the full device. I wanted a small box with coaxial input, optical output, DC powered via a wall wart (AC adapter), and battery powered - simple and portable. I tested the unit with a Rayovac Heavy Duty (D1604) 9V battery, which yielded 12 hours of operation before I started to hear the pops and static indicating power dropout.

If this is your first electronics project, don't worry much. You shouldn't have much difficulty putting this together, and you'll get a nice feel for the amazing world of electronics. You'll need a few tools to get started:

  • soldering iron (for a small project like this, a pencil type is far more convenient than the larger soldering guns)
  • desoldering bulb/braid/pump - Radio Shack has a nice little desoldering bulb for about $5. This is what I use since I just need something that works. You -will- need one, because you -will- add too much solder at times. Trust me. :)
  • drill (alternately, a soldering iron can just burn through the plastic. Not pretty, but it works)
  • small wire cutter
  • small needle nose plier
  • small philips head screwdriver
  • hot glue gun
On to the important things.

Schematic


References
  • D1,D2 - 1N4001 silicon diode, 50V
  • C1- 10uF electrolytic capacitor, 50V
  • C2- 2.2uF electrolytic capacitor, 50V
  • C3- 47uF electrolytic capacitor, 35V
  • C4,C5- 0.1uF ceramic capacitor
  • R1- 75 ohm carbon-film resistor, 1/4W
  • R2- 10k ohm carbon-film resistor, 1/4W
  • L1- 47 uH shielded inductor
  • S1- SPST switch
  • P1- DC female power jack, 2.1mm
  • J1- female phono (RCA) jack
  • IC1- 7805 5V power regulator
  • IC2- 74HCU04 hex inverter
  • GP1F32T- Sharp fiber optic transmitter
  • 9V - 9V battery
I'll take this time to apologize for the schematic, I wasn't thinking much when I slapped it together...

Circuit Description

The top half of the circuit is power regulation. Power can be supplied either via a wall wart or 9V battery. The diodes in line with the positive leads of each power source prevent backflow into either when both are present (plugging in DC power while you have the battery installed won't start charging the battery - preventing leakage or explosion). A switch controls power input, to accomodate battery operation (energy conservation). The hex inverter and the Sharp transmitter require 5V to operate, and the 7805 provides exactly that. The 7805 needs at least 7 volts of input to send out a stable 5 volts (it can also accept higher voltages, but keep in mind that extra energy is dissipated as heat, so you may want to consider getting a heatsink for the 7805). Signals for coax connections are transmitted with an amplitude of -0.5V to +0.5V, and the hex inverter brings this to the 0V to +5V level (TTL), which the Sharp unit can then convert to light. The Toshiba TX173 or TX176 can also be used - consult ePanorama.net (digital audio or optoelectronics) for information on the additional components you'll need for that setup. I prefer the Sharp for its simplicity and availability.

Parts List

Mouser Electronics:
Qty. DescriptionReferencePart #Price Ea.
1
1
1
1
1
2
1
1
2
1
1
1
1
1
1
1
1
  7805 5V voltage regulator
74HCU04 unbuffered hex inverter
10uF electrolytic capacitor, 50V
2.2uF electrolytic capacitor, 50V
47uF electrolytic capacitor, 35V
0.1uF ceramic capacitor
75 ohm carbon-film resistor, 1/4W
10k ohm carbon-film resistor, 1/4W
1N4001 silicon diode, 50V
47uH inductor
2.1mm DC power jack
Phono jack
SPST rocker switch
14 pin IC socket
Phenolic prototype board
SerPac 9V case (2.60x4.10x1.51))
9V battery snap
IC1
IC2
C1
C2
C3
C4,C5
R1
R2
D1,D2
L1
P1
J1
S1
511-L7805ABV
511-M74HCU04
140-LLRL50V10
140-LLRL50V2.2
140-LLRL35V47
581-UDZ104K1
291-75
291-10K
583-1N4001
70-IM2-47
163-1821
164-4215
107-DS850K-00
571-26403573
574-64P44XXXP
635-211-B
12BC006
 
Subtotal
Shipping (USPS)
Total
$0.60
$0.34
$0.24
$0.22
$0.34
$0.31
$0.07
$0.07
$0.04
$0.70
$1.75
$1.12
$1.29
$0.08
$5.87
$4.90
$0.31
 
$18.60
$3.20
$21.80
  
Radio Shack:
Qty. DescriptionReference   Part #Price Ea.
1
1
  4.25"x6" perfboard *
22AWG solid hookup wire (3 color)
276-1394
278-1221
 
Total
$2.39
$4.49
 
$6.88
 *This perfboard replaces the prototype board from Mouser, and at half the price - a recommended change if you have access to a local Radio Shack.

Fiber optic transmitter:

  • Linus Sweers sells the Sharp GP1F32T transmitter for $10 USD (US) and $12 USD (International), including shipping. Credit card payment is accepted through PayPal - as of 5/22/2000, PayPal has a $5 bonus for signing up, leaving the cost at $5. Reliable service, and quick shipping.
  • In the UK, the same Sharp and the Toshiba TOTX173/176 units can be purchased at SC-Elec for 6 including shipping.
Notes:
  • You can use any type of SPST (single pole, single throw) switch, as long as it fits the case you're using. Mouser also carries an illuminated version of the above rocker switch, if you enjoy light.
  • The parts listed above will do the job, but if you feel like it you can always upgrade to components with better tolerances, appearances, packaging styles, etc. The capacitors are already low leakage electrolytics, along the lines of the Panasonic Z series (Mouser's equivalent thereof). In other words, you don't need to worry about the caps. The resistors can be upgraded to metal-film, which is not a bad idea. I left out things like gold plated IC sockets and jacks, since we're working with a digital signal here.
  • Be creative! Find whatever enclosure you want to use (altoids boxes seem to be appreciated in the small electronics project world :)...add a LED...SCMS killer...bidirectional conversions...etc, etc. Fun stuff. :)

Construction - Putting It All Together

The Case

  1. Drilling. You'll need to drill out holes for the phono and power jacks - arrange the holes so that the jacks are on the opposite 2" faces of the case, to maximize space internally. You'll probably want to stick the TOSlink transmitter next to the coax jack, so remember not to center the coax jack hole.
  2. Burning. Now comes a messier part, creating a hole for the transmitter. If you have a nibbling tool, use it to cut out a square hole just large enough to allow the main cavity and the plastic border of the transmitter face to show through the case. TOSlink cable plugs rest against the bordering plastic. Since I was impatient and didn't feel like waiting until the next day to get a nibbling tool, I just used my trusty, all-purpose soldering iron to burn a hole to accomodate. Messy, as the pictures will testify.
  3. Burning, part 2. We need another hole for the switch. The rocker switch I used requires a rectangular mounting hole, so my soldering iron came to the rescue again. I really should invest in a nibbling tool.
  4. Mounting. The power and phono jack are straightforward - for the transmitter, I used hot glue. I tried epoxy, but the mounting came loose a few days after. This was probably due to poor epoxy, but hot glue has served me well. The rocker switch will snap into the mounting hole and secure itself.
  5. Pass the bare wires of the battery snap through the holes in the battery compartment. Once the wires are inside the case, hold both together and tie a single knot - this will provide strain relief whenever the battery snap is pulled out of the compartment.
Circuit Board
  1. You should be able to be much more efficient in laying out components than I was, because the final design ended up using far fewer components. Start out by cutting a piece of the phenolic board, leaving 22x17 holes intact. This should be more space than you need, but the entire board will still fit in the case. For accurate cuts, you can use a vise and a hacksaw. If you need to make long, general cuts, cutting along a line of holes with a wire cutter will snap the entire board down that line. Be careful with this - if fractures exist along the holes, the crack may spread to the rest of the board.
  2. Start with the power regulator circuit. Take two 5" lengths of wire, strip one end of each, and insert into the top right of the board, vertically aligned. Separate the insertion points of the wires by two holes.
  3. Bend the stripped wire 90 to lay horizontally against the board. Insert components as shown in the images up to C4. The electrolytic caps (C1, C2, C3) are polar, so you'll need to make sure they are oriented correctly. These caps have a stripe with negative symbols indicating which lead is negative - solder that lead to ground and you're set.
  4. After C4, my component layout is horrendous, so you should be able to put the parts a little closer together and cut down on the circuit board size.
  5. Whenever you need to run wire in and out of the circuit, it's best to put stripped wire through the board and use hot glue on the topside for strain relief.
  6. Using 5" initial lengths of wire for the 7 wires connected to the board is recommended, so you have room to work with should anything happen. After the connections to board are complete, fit the board in the case, and start attaching the wiring to the jacks.
  7. Connect the power ground wire from the board to the ground lug of the DC power jack. Then connect the ground wire from the battery snap to the same ground lug, and solder the connection.
  8. Because I added battery power after the initial construction, I installed the diodes inline with the positive leads of the battery and the DC power jack. The color band on the diodes indicates the cathode - orient the diodes so the band is on the board side.
  9. Note that for the sake of the IC chip (the 7404), you should solder the connections to the IC socket and insert the IC after completing the rest of the connections. IC chips can be easily damaged with excessive heat.
  10. On the coax and power jacks, the central pin is positive.
  11. The Sharp transmitter is pinned as follows:


    In order to be sure the wires at the pins would not short, I bent pins 1 and 3 towards the interior of the case and pin 2 towards the plastic face, and then soldered the connections. This should prevent accidental contact.
  12. If you've never soldered before, one useful tip is to tin the wire before attaching to other parts. To tin a wire, heat it with a soldering iron and then applying solder directly to the wire. The solder should flow along the wire on its own. This makes connections much easier, as you can simply heat the wire for connection. For more tips, consult ePanorama's soldering basics links.
Operation
  • After completing the circuit, you'll need an AC adapter to power the unit. Use 7v or higher for input, but remember that higher voltages will be dissipated as heat - so if you only have a 12v adapter, you may need a heatsink.
  • The rest is plug and play...good luck! :)
  • Oh, and please let me know how this goes for you - if by some chance something here is inaccurate, you've found a better case, other possible upgrades, etc, etc. I'm open to pretty much anything.
Useful Links
Contact - nikhilc@mail.utexas.edu