Scope
This article discusses considerations for POTS hybrids. It's a short survey of the history of the hybrid, its glorious advantages, and its limitations.
Understanding the Hybrid
Throughout history, POTS technology has presented a special challenge to the average broadcast engineer. A POTS phone line consists of a single twisted pair of wires which carry all of the signaling voltages as well as both of the callers' voices. In a broadcast situation, if the audio on the phone line were placed on the air unaltered, there would be two instances of the host's voice reaching the console: One instance from the microphone in the studio and the other instance from the phone line itself. This creates echoes and comb-filtering as the two instances are mixed together at the console. Herein lies the problem.
The solution to this problem is a hybrid. In a general sense, a hybrid is a device that takes the input signal and applies it to the phone line out of phase and time-aligned. This nulls the undesired audio from the phone line and leaves only the desired audio. This sounds like a simple task, but there are many factors at hand that make doing this a challenge.
Originally, transformers were used in a hybrid configuration with an extra winding that could be configured out of phase. This is called a passive hybrid and only guarantees a few dB of isolation. Digital Signal Processing eventually arrived on the scene making active hybrids, like the ones from Telos, possible.
Hybrids in Practice
Impedance Matching
In the real world, the phone line is not purely resistive, but a rather complex impedance, causing amplitudes of certain frequencies to vary. Passive, transformer-based hybrids are not capable of automatically matching the impedance, capacitance, or reactance of the phone line. Digital Signal Processing eventually arrived on the scene making active hybrids possible. Active hybrids like those made by Telos have dynamic matching circuits to help with this, allowing up to 30dB of attenuation to the undesired signal.
The cancellation of an undesired signal is sometimes called "nulling." The Telos hybrid sets the timing and phase of the null signal by emitting a burst of noise down the line to coarse align its timing circuits. The null algorithm automatically continues to make fine adjustments throughout the course of the call to improve the amount of rejection.
The amount of rejection of the unwanted signal is sometimes called Trans-Hybrid Loss and there are a number of factors that can reduce this in a system. Only when the impedance of the hybrid is the same as the phone line, and the out-of-phase signal is matched to the original in both amplitude and phase, will full cancellation of the send signal be achieved.
When a phone line has a linear response like the example on the left or a smooth response like the example in the middle, impedance can be easily matched by the active hybrid system. Responses like the one on the right present more of a challenge and may not perform as well.
Time Alignment
Another factor presented in today's mixed world of analog and digital telephony is drifting of the time-base on the phone line. Today, many of the POTS lines provided are not analog at their origin. Instead, they are digital (commonly SIP or VoIP) connections converted to analog POTS by some kind of device, be it a central office switching system or a locally installed ATA or similar device. These devices can often use buffering on the digital connection. Buffering did not exist in the POTS world and is fine as long as the amount of time that the buffer delays is consistent throughout the call.
In the example on the right above, the original signal may have been shifted slightly by a dynamic buffer somewhere in the connection. In this situation, poor trans-hybrid loss occurs until the algorithm in the hybrid can re-adjust the timing of the null. If there is a dynamic buffer on the connection and if it keeps moving the target, the hybrid may never be able to correct the timing of the null adequately.
Also, consider outgoing calls on such a connection. Since there is no such thing as "off-hook" in the digital telephone world, the hybrid will align its null to the fake dial tone presented by the analog telephone adapter. Once a number is dialed and a connection established, the timing on that connection is suddenly different from that of the fake dial tone. This will cause the hybrid's null to be misaligned until it is able to correct itself, which could take up to 20 seconds.
Conclusions
Unpredictable time shifts and impedance matching issues will cause poor trans-hybrid loss in a digital hybrid system. Unfortunately, there has never been a way to correct this with a hybrid. It's also important to remember what is going on with the audio on the phone line. While using an out-of-phase and time-aligned signal is a perfectly acceptable way to reject unwanted audio from the phone line, it's never perfect. The phone company's design goal when producing a hybrid is to get an average of about 12dB rejection, with 6dB on the most difficult lines. These numbers are not really that good, and Telos has been able to exceed these expectations in all cases. However, since digital telephony has separate paths for send and receive, the only way to get perfect send and receive audio with no crosstalk is to move away from POTS technology altogether and implement a phone system that works with SIP or VoIP natively.