Phone Line Basics: Revisited

Radio Guide Magazine - March, 2002
by Joe Klinger

Back in 1996 I wrote an article for Radio Guide entitled "Phone Line Basics", which attempted to explain audio connectivity through the phone system. At the same time I placed a reprint of the article on our web site. Almost immediately I began to find links into the article from all over the world. After six years it is still the most popular article on our site. One of our competitors even asked if they could place the article on their web site. As it turns out, it is not easy to find accurate information about the phone system available in condensed form.

Accurate is a key word here because many of the articles I found while searching the web were filled with opinions and speculation as to how the phone system works. Condensed is another key word because, lets face it, nobody wants to look through binders of telecom specs. Much of what was outlined in the article had to do with getting audio in and out of proprietary phone systems. Yes, I said proprietary, as in little or no published information. So when Ray asked me to consider writing for Radio Guide, I immediately suggested an update to "Phone Line Basics". I'll warn you right now, this is not a primer on ISDN protocols or the latest coding algorithms. This article addresses the most popular phone line configuration, the one at your next remote, press conference, or hotel room.

PBX Telephone Lines
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. Today's "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 where the telephone lines come into the building. Special telephones are connected point-to-point back to the PBX punch block.

Electronic PBX wiring is typically 4 to 8 wires using RJ-11 or RJ-45 modular telephone jacks. 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 back to the phone. This control information is required to set up or answer a call. In many cases, the tones you hear when you dial a number are there for your own feedback. The "real" digits are sent as data bits to the PBX which completes the call.

Dry Pair
The voice path on an analog PBX is typically referred to as a dry pair. Dry refers to the lack of DC current or ring voltage found on regular phone lines. If you can get access to this audio channel, you can use the telephone base to select, dial, and put calls on hold. This gives you multi-line call handling with audio on one pair. In other words, you can have a multi-line studio phone system without the high cost.

The downside is the proprietary nature of the PBX. Do not expect any support from your phone system manufacturer, in fact they will probably warn against tampering. Also, proprietary means they can make changes without telling you, so any documentation you find may already be outdated. This type of hacking should obviously be left to an engineer who is comfortable with telephone equipment and willing to deal with potential warranty problems.

The digital PBX is not as friendly. Here the voice is digitized right at the handset and sent back as data to the PBX. You do not have access to this voice path. Luckily, most PBX installations still provide an outside line for use with fax machines and modems, and of course your remote broadcast console.

ISDN is just one protocol for a digital PBX. In fact you can have individual ISDN lines that go all the way back to the phone company, or ISDN lines that go to a PBX system within your building. There are a wide variety of ISDN CODECs that allow you to send studio quality audio over the phone line, but as I mentioned earlier, that's a different article.

Analog Phone Lines
The term "outside line" refers to a direct connection to the telephone line outside of the building, also referred to as an "analog line", or "POTS line" (Plain Old Telephone Service), in other words 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, and a 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. Taking a phone off-hook creates a DC signal path across the pair, which is detected as loop current back at the central office. This drops the voltage measured at the phone down to about 3 to 9 volts. An off-hook telephone typically draws about 15 to 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. These circuits provide from 200 to 400 ohms of series resistance to protect from short circuits and decouple the audio signals.

To ring your telephone, the phone company momentarily applies a 90 VRMS, 20 Hz AC signal to the line. Even with a thousand ohms of line resistance, this can still pack 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.

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 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 aliased 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 coding and compression algorithms. Crosstalk from other conversations is another form of noise. The phone company uses 8 bit mulaw nonlinear coding which yields about 12 bits of dynamic range. The bottom line is that you can never count on more than about 45 dB signal to noise ratio.

Signal Levels: -9 dBm average speech across tip/ring.
Speech peaks out to +4 dBm can be measured at the phone, but anything over 0dBm at the central office will be clipped. 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 signal on a tip/ring pair is full duplex, balanced bi-directional audio. This design allows signals to travel for miles without expensive shielding by using common mode rejection to remove noise that is induced onto both wires. In order to send and receive audio through the pair you must use a two wire to four wire hybrid circuit which converts the pair 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 provide at least 1500 volt isolation and surge suppression from lightning strikes.

Commercial hybrid couplers provide familiar audio connections for full duplex transmit and receive audio. The primary difference between couplers is the amount of trans-hybrid loss or echo from the hybrid. When you send audio into a hybrid, some of the audio leaks back into the receive audio mixed with the caller's voice. The amount of return leakage depends on the type of hybrid and how well it matches the characteristics of the phone line.

Within a telephone, the biggest contributor to poor audio quality is the handset microphone. Keep in mind this low cost microphone element is designed to survive years of close proximity spitting and shouting as well as the occasional drop to the floor. 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.

There are a wide variety of audio couplers called Handset Interfaces that replace the handset of a telephone to provide send and receive audio. These interfaces work with analog telephones as well as digital PBX telephones.

Analog Line Survival?
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.