For decades now, a central component of many live television broadcasts has been an interface with telephone lines. Frequently used to send mix-minus signals on a one-way trip back to the originating point for participants in remote locations, the hybrid only uses its send capacity to take an aux out from the control room audio production console. (See Figure 1.) The more demanding application is when two-way conversations are needed for live call-in programs or live inserts from reporters who cannot, at that place and time, set up a video feed. (See Figure 2.) The old QKTs and push-to-talk phone couplers could handle the one-way reports reasonably well, but the telephone hybrid was developed to allow the most natural sounding two-way conversation possible in an interface between broadcast audio gear and the wildly varying conditions on the plain old telephone service (POTS) line.
Managing sound leakage
Even with the advances made in design, the analog hybrids still require a little interaction and gain riding by the audio operator for the cleanest result. An experienced TV sound operator can anticipate the flow of a conversation and slightly duck the studio talent's mic while the caller is talking, and duck the caller when the show host or a guest speaks up.
While it helps to avoid a hollow acoustic echo, especially when studio guests are monitoring the caller through a speaker, this technique is also necessary to combat the traditional bane of the telephone hybrid: trans-hybrid leakage.
Trans-hybrid leakage has been a source of some confusion. The leakage is the distorted, phase-shifted, tinny-sounding talent speech cross talking into the caller signal. This happens because the telephone circuit is a two-wire conduit carrying both receive and send on the same copper pair while the broadcast equipment operates separately in the send and receive roles.
The amount of trans-hybrid loss in the hybrid determines, in large part, the amount of trans-hybrid leakage.
Impedance matching
Impedance matching is key in telephone hybrids. Loading coils, transformers, repeaters and a host of other components cause the impedance characteristics on circuit-switched lines to vary considerably. Through the push of the button selecting the next caller or reporter, the line conditions can change dramatically. Enter the digital signal processing (DSP) hybrid.
DSP has become the primary enabling technology to allow telephone hybrids to take their next big leap in technology. Once the A/D conversion has been made to the incoming signal, mathematical computations can manipulate it on a level impossible with all analog hardware.
The software performing the interface employs a complex technique called convolutional least mean square adaptive filtering. This compares the send and receive signals and produces a leakage profile in the form of an error rate. The error signal drives the impedance matching network components to constantly adapt and fine tune the nulling function on the hybrid to suit the line conditions. On a reasonably good phone line, a digital hybrid with its adaptive DSP function can produce a trans-hybrid loss about twice that of an analog device.
Establishing tone
Another key in the adaptation process is that the reference used to measure momentary trans-hybrid loss in dB must be a broadband signal rather than a tone or series of tones. When a call is established on the digital hybrid, it sends out a burst of noise, which only the caller hears. The initial settings take hold in less than a second, and as the call continues, the voices are used to adjust the balance on the internal impedance matching network, including automatic gain control. Of course, SNR is a critical factor determined by the basic design and quality of the components used in the DSP functions. The bit count in the audio path and the distortion level in amplification also figure prominently in performance and have a profound effect on automatic gain control (AGC), ducking functions and noise gating, which in turn spell out the overall performance of the hybrid.
Freeze gating is essential in the attempt to automate the most natural characteristic possible in the hybrid. In this, the automatic gain will not rise and bring up noise during pauses in the caller's speech. The effect avoided here is known as pumping or breathing and is quite obnoxious when there is a constant background noise level such as a large crowd. When the caller speaks again, there is no initial blast of clipped sound before the compression again takes hold.
The AGC works in concert with the ducking function. This essentially performs the same task that the sound operator used to do in the old analog days of riding gain on the hybrid output and the studio microphones to maximize performance above that provided by the trans-hybrid loss figure alone.
Many DSP hybrids have a manual setting on the ducking function. At the extreme, it can be set to cut off the caller audio completely while the studio sound is active. However, this produces a sound that resembles the echo cancellation typical of many video conferencing systems. The difference is profound.
Take for instance, the smoothly conversational effect between host and remote guests on a program such as the PBS “NewsHour” and the choppy interchange with echo cancellation on a conference room video call. The former uses one-way satellite feeds with hybrids carrying only mix-minus into the guests' IFB earpieces (for perfect acoustic isolation), while the other handles everything on open speakers with no human hands riding gain or anticipating the flow of conversation. (See Figure 3.)
Systems at work
In the real world of live production, there now exists a variety of systems on the market that not only work behind the scenes to balance the necessary impedances, gains and equalization necessary to minimize trans-hybrid leakage, but also offer some intuitive tools to enable easy manipulation of caller and send audio with push-button ease. At the same time, devices today can provide sophisticated features in hardware that can be carried and set up on remote.
Studio devices such as the AVT MAGIC ISDN system interface with computer control to provide visual displays showing the various audio channels in use, dual-tone multi-frequency (DTMF) dialing from the computer screen, call queuing, four-wire ISDN interfaces and all-digital AES/EBU inputs. Each caller can be assigned a priority and receive an instant, fully automated mix-minus audio signal. Call screeners can operate multiple systems remotely, and all the callers can be conferenced. The displays include hold, pre-talk, call drop, forwarding functions and indicators, along with ladder-style audio metering. Priorities such as VIP can be assigned for callers, or the caller with the longest hold time may be automatically switched on-air whenever the call change is activated. Single lines can be locked, and all lines can be dropped at once with a single button push. On some systems, even the callers can exercise a degree of control in situations such as game show formats by using DTMF analysis. The system can read DTMF tones and activate functions either internally or perform tasks completely outside of the telephone system such as activating scoreboard displays.
New hybrid technology has also revolutionized things at the other end of the line. When reporters are out in the field, they can perform many of the audio routing tasks that formerly had to be centrally controlled at the station. Devices like the JK Audio RemoteMix 4 have incorporated a telephone hybrid and keypad into a field mixer along with a private branch exchange (PBX) handset interface and a cell phone interface. Any of the telephone feeds can be sent down an audio line as mix-minus sources, and the unit can connect via Bluetooth to any cell phone or laptop computer. The device even has separate battery compartments for hot-swapping batteries and staying up and running the whole time.
The latest phase of telephone hybrid technology has seen these units migrate from control room rack spaces of their own to light, versatile and portable field units, incorporating all the essential remote audio functions and capability into one physical instrument. At the same time, the studio gear has morphed with computers for unlimited command and control. And to think, it all started with a few little passive black matchboxes stuck on the side of a telephone.
Bennett Liles is a writer and TV production engineer in the Atlanta area.
