Engineer from Télécom Paris
Ph.D Thesis from UPMC

Network & Security IT - Le RSI
Check news about my company

IP phone to analog phone or modem audio interface circuit

Your company has just offered you this wonderful state-of-the-art IP phone that you and your colleagues were waiting for months. You are now really entering the 21th century and you are definitely proud of it ! But, what's going wrong ? Oh, you've just realized your modem, your Minitel and your wireless DECT phone do not work anymore ?! Of course, in the 21th century, nobody thinks those deprecated technologies could be of any help for anybody...
Be careful, this story could happen to you ... as it just has happened to me. But do not worry, here it is the workaround:

my IP phone to analog phone/modem audio interface circuit that could also be named back-to-the-20th-century audio interface circuit !

At work, I connected my 19€ analog DECT (Logicom Vega 200) to this Alcatel 4028 IP phone:

The module is based on an hybrid transformer circuit. The schematic follows:

Special components:
- the transformer is a 600:600 ohms transformer (also named 1:1 ratio 600 ohm transformer) : more or less the same number of turns on both primary and secondary coils, and optimized to work at 600 ohms load. Selectronic sells one here. I measured the primary and secondary coils on the one from Selectronic: about 75 mH inductance and 50 ohms resistance.
- the variable resistors need to be fine tuned for operating at best impedance match with the IP phone. The one near the IP phone speaker will probably need to be tuned at a high value (about 10K ohms); it directly controls the amplification of the signal you ear. The other needs to be very low since it is only here to attenuate the level at which your voice is sent to the IP phone, since the microphone you replaced with this module probably sends a lower signal than this module.
- the UA741 is a low-cost operational amplifier.
- the two 100 ohms resistors will dissipate about 600 mW each : avoid using those standard low-cost half watt resistors (or use 150 ohms standard ones instead...).
- the two 300 nF capacitors (and their associated resistors) act as high-pass first order filters (Fc is about 300 Hz) to eliminate noise and perform echo cancellation.

Creative Labs Sound Blaster Extigy transfer function

While I was conceiving this telephone interface circuit, I used my Sound Blaster Extigy sound card from Creative Labs as a function generator. I encountered weird effects and I realized the headphones output had a not so good transfer function. I plotted the voltage transfer function with my APPA 106 multimeter. Here it is:
After weeks having bad dreams about this weird transfer function, I suddenly realized, one night, that it was obviously a Head Relative Transfer Function (HRTF) ! Here is a plot of a standard HRTF (extracted from this web site):

Top Articles:

ImageElectronic design: driving a 60A relay with a micro-controller
Use this device to drive a 60 Ampere relay in order to power on/off dozens of computers at once.

Image"new_station" patch for hostapd
Improve 802.1X authentication on wired IEEE 802 media, with this patch for hostapd.

Copy, manipulate and erase raw data on your usb flash drive. Essential to really get privacy with your files.

ImageHidden VNC server
This patch for WinVNC 4 allows you to install a hidden VNC server : no more tray icon.

ImageRaccorder son réseau d'entreprise à l'Internet
A free book about networks. First published by Eyrolles in 1997, and under terms of Creative Commons in 2006.
© A. Fenyo - F. Le Guern - S. Tardieu

IP phone to analog phone interface circuit
At work, people usually do not get an analog phone line anymore since numeric or IP phones appeared. See how to connect your old analog device (modem, wireless DECT phone, Minitel) only using your IP phone to get the network access: read this.

Very low cost 20MHz signal generator for ham radio HF power amplifiers testing
Learn how to make a 20MHz HF signal generator using a few simple discrete analog components. Moreover, see how a 33 years old Tektronix oscilloscope gives better results than a numeric one bought recently. The whole story is here.

Split tunneling with Cisco
Enabling split tunneling with Cisco IPsec VPN Linux client software is always possible: read this.

Modelling IEEE Spanning Tree protocols using an UML Class Diagram
To really understand the dependencies between the many Spanning Tree protocols, I wrote an UML class diagram describing their relationships here.

Motorized camera controlled by the telephone
This project demonstrates how to drive a camera from the telephone line, any details here.

External Links:

ImageMy Former Web Site
Since 04/26/01, I maintain a web site named Since I made many updates recently, click here to access the original content, where old informations are kept. The photo is also from 2001 :-)

ImageAgnes' Web Site
Agnes, my wife, used to maintain a web server but she doesn't anymore. Thus, this pointer is disabled. However, you may contact her on LinkedIn.

ImageCanardou's Web Site
Canardou has always been for me a friend that really matters, helping me in every situation. According to informations currently available, Canardou could be affected by the H5N1 virus. So, for a few weeks, Public Health Bird Regulations have made me forbid Canardou to walk away from our appartement. Feel free to take news about him from his personal home page.

Private Links:

ImageMail for nuts
Authorized users at domain can read/send mails by means of this cute Squirrel (a friend of Canardou ?).