Introduction: External USB Audio Card With Optical S/PDIF POF Interface
The purpose of the project was to create an external USB audio card, which could be able to:
1) serve as usual external USB audio card with headphone/line output and audio line input
2) can transmit the digital audio data at relatively long distance (20m -100m)
3) can receive and process the digital audio data send by the similar card and either transfer it through the USB to the PC, or convert it to analog audio signal
As long distance transfer media was chosen the POF.
A short explanation of the POF technology will be presented:
What is POF?
POF stands for plastic optical fiber with the term plastic being used a vulgarization for polymer materials.
The 1mm fiber diameter is about 500 times thicker than a glass optical fiber. 96% of the cores cross section conducts modulated light for data transmission similar to glass optical fiber applications. The maximal transmission distance amounts to about 100m without active repeaters. Polymer fibers are used for high speed data network in homes, commerce and industry as well as in cars and airplanes. POF is often regarded as an optical home network because POF is easy to install. The fiber is thin, can be shortened to the desired length by a sharp knife and requires no connectors on its ends. Anyone can set up a robust, high performance and Ethernet compatible network without any special tools.
The key advantages of POF networks are:
- No electromagnetic radiation
- Electrically isolated network
- Immunity against electromagnetic coupling
- No electromagnetic cross talk
- Flexible, reliable and maintenance-free
- Low weight
- Resistant to humidity, heat and vibration
- Visible light that is eye-safe
Target Markets and Applications
POF is an excellent solution for multimedia and intelligent home networks. In particular, POF is suitable as network backbones to mutually interconnect TV, set top boxes, NetTV, gaming, PCs, network attached servers (NAS), internet and VoIP. Carrier Networks POF is an attractive solution for telecommunication companies, cable networks and infrastructure providers to offer a completely optical solution (Vertical installations GOF (glass optical fiber), Horizontal (floor level) POF). As an electrically non-conducting and against electromagnetic interference immune “cable”, POF is the only cost-effective solution that suits both to old and new buildings.
POF is also a smart alternative to copper cabling in commercial environment. Compared to copper, POF has the following advantages: POF is thin and therefore easy to install and hide. POF is also easily extensible and upgradeable, robust, electrically non-conducting and reliable. Industrial Networks
POF already has a track record for connecting Ethernet-based industrial applications, such as control units within the manufacturing process, industrial control modules and links in the control room. POF is often the only affordable solution to meet the mechanical, electromagnetic and chemical challenge of an industrial environment in a spark-free manner. Car, Aircraft and Train Networks POF does away with the boundaries of today's copper cables in mobile applications by its unique advantages such as like low weight, requires least space, immunity against vibration and humidity.
Further information about the POF and some high speed solution can be found here:
As data transfer protocol was decided to be used the S/PDIF standard.
An internet research for audio codec suitable for the realization of the project showed, that the best solution would be the Texas Instruments (Burr-Brown) chip PCM2906.
Here can be seen the chip information: http://www.ti.com/product/pcm2906c
For the POF transmitter/receiver part was used the Firecomms Optolock FC3000T. It includes the transmitting LED and the receiving PIN photodiode in one package. More information about the device can be found under the link: http://www.firecomms.com/ . The used Optolock is analog. Under desire a different type can be used (there are some which contain the receiver part embedded inside the same package), but the schematic and the PCB of the current implementation must be changed according the new requirements (in some cases differential to single ended conversion must be done).
The opamps and the linear regulator used are also products of Texas instruments. (For the audio amplifiers better to use OPA2353UA instead OPA2340UA as given in the Eagle files)
The micro switch comes from http://www.te.com . The electrolyte capacitors are tantalum. The resistors metal film devices.
The Schottky diode requirements are: VF ≤ 350 mV at 10 mA, IR ≤ 2 μA at 4 V. I have used BAT43.
The audio jacks are from the type MJ-352WO (MJ-352). Can be ordered at http://www.farnell.com – article # 1267400.
For the project can be used 12MHz quartz crystal, different from the one I used (NMP120 from Saronix), but the oscillator capacitors must be used according its datasheet.
The project was developed by the use of the CadSoft Freeware software Eagle Light edition.
It can be downloaded here: http://www.cadsoftusa.com/download-eagle/freeware/?language=en
The schematics and the PCB views are presented on the pictures.
As download are available also the Eagle files. You are free to use them for your designs and PCB orders. The size of the PCB is almost exactly half euro-board. In this way you can order two PCB-s at once.
The boards were produced by the Olimex. http://www.olimex.com/ - both PCB-s with double sided metal layers cost ~ 50 USD and the quality is very good. (Abrasive disk cutting recommended).
The parts list (BOM) can be extracted from the Eagle database.
The PCB –s are designed for SMD devices. How to solder such kind of devices can be learned in internet. In the instructables.com also exist a lot of instructables explaining this. Even for experienced technicians will be a challenge to solder some of these devices, because they have very small pin pitch. It can be seen on the pictures, than on the one PCB I have put through hole capacitors. If you can not find the SMD ones, you can also use this approach. Another suggestion – years ago was recommended that that the chips must be soldered last on PCB to prevent their damaging. Opposite to this, I will advice to solder first the codec chip, and after that the periphery devices. In this way the free access to all pins of the chip by the soldering iron is not disturbed. Of course all soldering requirements concerning the ESD rules must be considered.
The project can be accomplished in few different forms:
1) Only USB audio card with audio output – the POF transmitter/receiver part and audio input amplifier will be not mounted
2) USB audio card with line input (presented on the picture – the red line shows the border of the populated PCB area)
3) USB audio card with POF transmitter
4) USB audio card with POF transmitter and receiver
The picture shows the implementation for the phase 1) and 2) - Only USB audio card without any optical interface
To be able to transmit the digital audio data at long distance at least two separate boards must be produced (at least one of them in a form 4, the other can be in form 3). For test purpose a single board in loop configuration can be used (only one wire of the POF cable inserted in the Optolock as single loop) , but to prevent the confusion inside the codec the S/PDIF input line must be interrupted, or the threshold voltage of the S/PDIF receiver comparator must be tuned to be close to the supply potential by the trimmer potentiometer.
If right mounted and soldered the USB card is automatically recognized by Windows and no driver installation is needed. For Windows 7 and upper a PCM2906c version must be used. For Windows XP PCM2906b shall work fine too. Also under Linux the card shall be usable.
The sound quality is incredible.
A bord accomplished at stage 3) is shown on the next pictures.
Using the microswitch the S/PDIF data transmisson can be enabled/disabled. At the end of the POF cable is seen the modulated red light. (Not dangerous for the eyes)
A board assembled in form 4 is presented on the pictures. It was connected in loop configuration - it optical transmitter was connected to its own optical receiver.
The wave scope diagrams of the POF S/PDIF receiver are shown on the pictures below.
Different tests with different POF cable length were done.
To achieve the optimal results the value of R32* can be changed for different POF cable lenghts.
An experiments with POF cable over 200m were done, but the signal was very week to be recognised as S/PDIF standard.
Finally two fully assembled boards were connected together through POF cable ~ 20m.
The scope wave diagrams of the POF S/PDIF show the signal form at the PCM2906 SPDIF Dout pin (the green trace) and the signal at the otput of the optical receiver comparator (Din of PCM2906 - second board).
The only adjustment needed at different POF cable lengths is the threshold voltage of the POF receiver comparator. To have this threshold fine adjusted an oscilloscope is needed.
Of course different type of opamps for the optical receiver can be used also - I will recommend to use JFET input OPAMPS (they have low input capacitance) with high BW (over 100MHz) and high SR. If single OPAMP used , the schematics/PCB must be changed and additional fast comparator must be added.