Now you really have my attention. We must listen to callers on open speakers. Can we do that with Telos hybrids?
We pay very close attention to the problem of monitoring callers through open speakers. In addition to excellent trans-hybrid loss providing the first line of defense against feedback, our DSP includes a two-stage, feedback reduction process we call Digital Echo Elimination. The first stage is digital dynamics processing that intelligently and instantaneously determines appropriate gain values in the send and receive paths. The second stage is a subtle, inaudible pitch shift to the input audio before it is sent to the telephone line. The two elements of Digital Echo Elimination significantly reduce the potential buildup of feedback.

How does Telos get results that are so much better than other hybrid manufacturers?
Hybrids must deal with complex and erratic phone line impedance characteristics across the phone line's frequency range. Impedance variations are caused by nearly every piece of equipment and run of cable between your studio and the caller's telephone. To cancel the send audio, primitive analog hybrids use simple resistor-capacitor "balancing networks" to attempt to match the impedance of the phone line. It is a rare phone line that has a smooth, unvarying characteristic, so analog hybrids are often hopelessly ineffective.

How does Telos' digital processing hybrid work?
Superior trans-hybrid loss is achieved using our advanced digital signal processing (DSP) technologies. All functions are totally in the digital domain. We have found ways to fine-tune the adaptive process to achieve very fast nulling, stability of adjustment, and, of course, maximum trans-hybrid loss. No other analog- or digital-hybrid equipment comes close.

To offer a technical description: Telos digital hybrids use a very advanced time-domain convolutional adaptive filter algorithm to synthesize a transfer function for the balancing network. A feedback loop continuously adjusts the filter to conform to changing line impedances. In the Telos ONE, an error signal is used to adjust the amplitude and phase cancellation signal at a large number of frequency points. In the Telos 100 Delta, a gradient search technique is used to minimize the discrepancies between the synthesized transfer function and the telephone line's transfer function. The result is a very close match to the phone line impedance curve for optimum rejection.

Must I manually adjust the hybrid so that each call begins with optimum trans-hybrid loss?
Telos hybrids perform all adjustment automatically and require no "tweaking" once installed. When a call is initially established, a brief mute/adapt period (between one-quarter and one-half second) causes the system to adjust to the phone line before that call goes on the air. The caller hears a "noisy tone," but the tone is not heard on the air because the hybrid output is muted during this time. This muting has the incidental benefit of removing the line-switching "clunk." As the conversation proceeds, adaptation continues, using the send audio as the driving signal.

Why is this dynamics control so critical? Levels from caller to caller can vary as much as 30dB. The smart response of the gain control in our hybrid products uses the least amount of processing required so that the natural characteristics of the caller's voice are retained. To further achieve this end, the 100 Delta has a digital, logarithmic (dB linear) compressor that uses a feed-forward topology. This sophisticated compressor provides level-independent operation for outstanding performance regardless of caller level.

The secret to Telos' exceptional performance is the use of advanced digital signal processing (DSP) for all audio functions. Here is a peek inside the DSP of the Telos 100 Delta.