Getting the best from analog telephone lines

<< The following questions have been culled from those asked most often over the years. Most are from broadcasters, but we feel that the answers are also relevant to similar problems encountered in teleconferencing, distance learning, and other applications. If you have a question you think should be on the list, please email>>


The voices of our announcers sound different when we have our older telephone hybrid turned on. Is the hybrid to blame?

Good question. Many overlook this problem when evaluating their telephone interface system. Indeed, the quality of the caller audio and the ability of the hybrid to create full-duplex audio are critical. But announcer voice distortion, caused by poor trans-hybrid loss, is a key indicator of hybrid performance.


I've heard the phrase "trans-hybrid" loss. Exactly what is it?

When the announcer speaks, his voice travels through the hybrid's phone line output, to the caller, and back to the studio into the hybrid's telephone line input. (You can hear this "leakage" in the earpiece of your telephone handset. Just listen to how much of your own voice comes back to you!) Trans-hybrid loss is that portion of the announcer's voice that leaks through the hybrid to its audio output. This leakage is distorted and phase shifted after its long journey. Ideally, the output of the hybrid should consist of the caller audio only.


Just how does trans-hybrid loss cause announcer distortion?

In a broadcast studio, the announcer audio is mixed at the console with the hybrid (caller) output to create the "on-air" mix. When you use a poor hybrid, its output includes a distorted, phase shifted version of the announcer signal. When this "leakage" is combined with the clean announcer audio, a "hollow" or "tinny" sound is produced as some frequencies are more affected by phase cancellation than others. The greater the trans-hybrid loss, the less announcer audio that leaks into the hybrid output and the less the announcer voice distortion.

The diagram below shows the potential problems caused by poor hybrid performance. Telos' all digital approach reduces announcer mic distortion and speaker feedback while enabling natural sounding, two-way conversation on-the-air.




Are there any other problems caused by poor trans-hybrid loss?

Unfortunately, yes. In systems using multiple hybrids to conference several callers, poor trans-hybrid loss will cause a serious "singing" feedback, especially on low-level callers. Poor trans-hybrid loss can also cause feedback when the caller must be heard on an open speaker in the studio.

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