In a message dated 98-08-05, Peter McConaghy writes: << I am interested in knowing if there is any standardisation in the connection between a telephone handset and the telephone. Also, are all the mic's similar in electrically? ie bias voltage, output impedance, etc. Does anyone know where I could find more info about it? >>
Peter: Unfortunately, there is no "standard" for the handset-phone interface. I have worked on a couple of products that were intended for series connection between the handset and the phone, where the phone could be "any" phone. This necessitated some detailed research into the handset-phone interfaces used by various vendors. Following are some points worth noting: 1) If you intend to insert something in series with the handset, there are four interfaces to consider: transmit&receive looking toward the handset, and transmit&receive looking toward the phone. On the other hand, if you wish only to interface something to a handset, or something to a phone at the handset jack, you will have only two interfaces to consider. 2) The receiver (earpiece transducer) in the handset is usually 150 ohms AC impedance, but can be 300 or even 600 ohms. A compromise drive circuit can usually drive all three acceptably. The receiver is usually on the center two pins of the jack. 3) The transmitter (mouthpiece transducer) can be carbon, electret, piezo- electric, or electro-dynamic. The electrical characteristics vary over a very wide range. It is virtually impossible to have a single circuit that works (without jumpers or switches) with the full range of transmitters. The transmitter is usually on the outer two pins of the jack. 4) Electret transmitters are polarity sensitive. So, even if you know which two pins the transmitter is on, you can get the polarity wrong. 5) Most handsets assign two pins to the receiver and two pins to the transmitter, leaving the two transducers electrically isolated inside the handset. However, some electret handsets have one lead that is common to both transducers, so that the four leads are power, ground, transmit (single ended, ground referenced), and receive (single ended, ground referenced). 6) Taken together, the problems identified in items 4 and 5 above create a lot of uncertainty. The polarity is unknown, and there may or may not be a common lead. The above items are only a few of the issues you may need to address. In general, it is best to try to narrow the applicability of your product to only one specific phone, so that you can design interface circuits that are optimized for that particular phone. If necessary, you can do this with a list of several specified phones. However, what is very difficult is to try to be compatible with "any" arbitrary phone. I hope this provides at least a partial answer to your question. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax) [email protected]
