Phone Line Basics

Radio Guide Magazine - June, 1996
by Joe Klinger

Every engineer has had to deal with telephone lines at one time or another. If the application is in your studio, you can take your time and make some calls to determine the right equipment to buy and the correct phone line configuration to order. Out on the road it's a different story. Many offices and stores have installed low cost electronic PBX (Private Branch Exchange) or Key telephone systems.

The terms "PBX" and "Key" both refer to hardware that enables several telephones to be connected to a smaller number of telephone lines. The term "Key" is now used to describe any small system but it was originally used to describe the manual keys or push-buttons on systems like the 1A series Key telephone. Todays "Key" systems are more like small PBXs with programmable features such as distinctive ringing, hunt groups, and automatic line selection. A PBX consists of a switch box and punch block located somewhere in the building where the telephone lines come in. Special telephones are connected point-to-point back to the PBX switch box.

Electronic PBX wiring is typically 4 to 8 wires using RJ-11 or RJ-45 modular telephone jack. These are not standard telephone wires! Even a simple analog PBX line does not look like a standard phone line. On an electronic PBX, two wires are often used as control lines which send keypress data to the PBX, and ringer and LED data to the phone. This control information is required to set up or answer a call. Luckily, most PBX installations still provide an outside line for use with fax machines and modems.

The term "outside line" which refers to a direct connection to the telephone line outside of the building, along with the term "analog line", or "POTS line" (Plain Old Telephone Service), all refer to a standard residence type phone line. The POTS line is the line you will need for your remote broadcast console, telephone hybrid, analog telephone, cordless telephone, fax machine, or modem. The POTS line consists of two wires called tip and ring. These two wires provide: DC current to power the telephone electronics, AC current to ring the telephone bell or electronic ringer, full duplex balanced voice path.

This is a closed loop, balanced system not referenced to earth ground. The POTS phone line, with all phones on-hook, should measure around 48 volts DC. This drops down to the 3 to 9 volt range when a telephone on the line goes off-hook. An off-hook telephone typically draws about 20 milliamps of DC current to operate, at a DC resistance around 180 ohms. The remaining voltage drop occurs over the copper wire path and over the telephone company circuits where there is usually 200 to 400 ohms of series resistance to protect from short circuits and decouple the audiocircuits.

To ring your telephone, the telephone company momentarily applies a 90 VRMS 20 Hz AC signal to the line. Even with a thousand ohms of line resistance, this is still a bit of a shock, so be careful when you are probing around trying to find a POTS line. Why the primer on telephone lines? The world of communications is certainly changing but there are still millions of analog telephones, cordless telephones, fax machines, and modems that use POTS line. Even though the POTS line has been around for years, its old technology is still misunderstood by many engineers. Lets face it, unless you are a telecom circuit designer, there isn't much coverage of "old" technology in schools, publications, or journals.

Quite often our customers ask questions that relate more to the telephone line than to any specific product. Questions about bandwidth, signal to noise, signal levels, and two wire to four wire hybrids are the most common, so lets walk through a few:

POTS Line Characteristics:
Bandwidth: 180 Hz to 3.2 kHz

The low end is rolled off early to stay away from the 60 Hz region. Also, telecom isolation and hybrid transformers would be much more bulky, (and expensive) if they had to carry signals down to, say 20 Hz.

The high end cut off is more critical. Voice on the telephone network is digitized at 8 kHz sampling rate which means that any signal above 4 kHz will be alaised back as noise in the voice band. Most voice CODECs roll off at about -25dB at 4 kHz with a -3dB down point around 3.2 kHz. The phone company decided years ago that the 180 Hz to 3.2 kHz range would be sufficient for speech intelligibility while allowing them to multiplex many calls over coax and twisted pair. Signal to Noise: Approximately 45 dB

This is not as easy to quantify because noise comes in many forms such as electrical interference from fluorescent fixtures or hiss from the many amplifier stages in the voice path. Speech correlated noise can be introduced from non-linear speech digitizing and compression methods. Crosstalk from other conversations is another form of noise. The bottom line is that you can never count on more than 45 dB signal to noise ratio.

Signal Levels: -9 dBm average speech (at tip/ring) Speech peaks out to +4 dBm are common but will start to clip. The FCC requires that all telephone audio interconnect equipment limit speech to -9dBm, averaged over 3 seconds. Consult FCC Part 68 requirements for all the details.

Hybrids and Telephones
The voice on a tip/ring pair is full duplex balanced audio which requires a two wire to four wire hybrid circuit or transformer to convert it into separate transmit and receive audio paths. Bulky and expensive hybrid transformers have been replaced in most telephones by ICs which perform the same function. Whether it is a transformer or IC, the hybrid must also provide 1500 volt isolation and surge suppression from lightning strikes.

In a telephone, the biggest contributor to poor audio quality is the handset microphone. Keep in mind that this microphone element is designed to survive years of close proximity spitting and shouting as well as the occasional drop to the floor and cookie crumbs. It has to be designed for rugged use but at a low cost. Unfortunately the result is a sturdy element that has considerable distortion, a jagged response curve, and substantial dynamic compression. Beyond the microphone, most telephones perform well on a wide variation of telephone line conditions.

Strangely enough, fax machines and modems will keep analog lines available even in buildings with ISDN and digital PBXs. In addition, data transmission is less tolerant of compression algorithms, line noise, and distortion, so the phone company must keep this in mind when considering further "squishing" of voice channels or loosening of transmission equipment tolerances.