Problems in linking telephone hybrid to audio system

Every sound engineer has had to deal with telephone lines at one time or another. Linking the phone conversation to audio system like taking calls to radio studio can be more problematic than you first thought. You can get a good view of the scenario at article Phone Line Basics article from JK Audio.

The main problem in making the audio connection is that the telephone line is full-duplex interface implemented using single twisted pair. When an announcer speaks, his/her voice travels through the phone line output of the phone patch (transformer, analog hybrid, digital hybrid, etc.), to the caller, and back to the studio into the telephone line input of the phone patch. (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!)

Level matching on local and remote voice

Typical commercial telephone hybrid allows the equalizing of levels of local and remote voices. Typically a hybrid needs adjustment for every new connection because of impedance changes. Today automatic digital hybrids are used for equalizing local and remote telephone conversations.

Trans-hybrid loss and announcer voice distortion

Trans-hybrid loss is that portion of the announcer's voice that leaks through the hybrid to its audio output. The higher this spec, in db, the better isolation in the device. This leakage is distorted and phase shifted after its long journey. In the studio, the announcer audio is mixed at the console with the phone patch (caller) output to create the on-air mix. When you use a poor phone patch, 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. Ideally, the output of the hybrid should consist of the caller audio only. Digital hybrids have signal processing electronics to get better trans-hybrid loss figures than which are available with simple analogue solutions. You have to decide what's best for your application and your budget. There are different requirement depending the application (broadcast, teleconference or remote training). For links to telephone hybrid circuit check Hardware for Audio/Video Conferencing at http://www.cs.columbia.edu/~hgs/rtp/hardware.html.

It hase been suggestions that ISDN be used as it is full duplex is a good one, but it might be only practical if both sides of the telco path have ISDN. When calling between plain old telephone service (POTS) and ISDN, the above problems remains.

Echo problem in long distance calls

Echo is caused because of the coupling between incoming and outgoing audio in the telephone circuit and the delay in the telephone line (especially in long distance calls). Echoed back audio is usually caused by an impedance mismatch at a 2/4-wire conversion point (such as a codec-annex-hybrid, analog CO line interface) and by acoustic feedback (feedback from spaker to microphone in handset, acoustic echo in hands-free phones). Thus there is echo; ISDN or other digital telephone set on an all-digital connection would not cause echo because of conversion mismatch, but if normal handset or hadfree telephone is used the acoustic echo is still possible.

Echo doesn't become audible until the delay in the circuit exceeds a certain threshold value which depends on the losses in the circuit. Even milliseconds of terrestrial echo can be annoying, but typically the echo is not annoying if the delay stays below 25ms. Old Bell standards said that on calls of more than 1800 miles, an echo suppressor was used. In general, you need echo cancellation when the delay exceeds some subjective value in the 30-50 ms range.

As it is practically impossible to prevent echo (by perfectly matching the impedance in line circuits and by acoustically insulating all phones), it either has to be suppressed or cancelled when it does occur. For this reason, echo cancellers are deployed by telephone company on long-haul routes that, when used, bring the total circuit delay to above the echo threshold value determined by line loss. These echo cancellers are deployed on both sides of such long-haul routes and the echo canceller at the remote end of the call is responsible for ensuring that you don't hear any echo.

For more information on how echo cancelling works, please consult ITU-T recommendation G.165 or some good telecommunication book. The morale is therefore that if you hear echo, you can't do practically anything about it, as both the cause of the problem and the solution to it lie at the remote end of the connection (typically at the telephone company equipments). If the connection you're talking about is across a private network, make sure that the echo cancellers are correctly dimensioned because wrongly dimensioned echo canceller will be totally ineffective.

Metallic sounding caller voice problem

If your telephone connection is though a digital PBX or digital switch (typical nowadays) then you might encounter a problem that the voice which might sound OK on telephone but sound "metallic" when you connect it to the mixing desk through your high quality hybrid circuit. The metallic sound problem is an aliasing problem cause by the digital telephone system where there is not much filtering after the D/A converter which outputs the sound. The absence of the output filters causes that there are high frequency noise components added to the output audio signal.

The audio sounds fine on normal telephone because it can only playback the normal telephone audio range. The problem is audible with your hybrid circuit of that circuit has wider bandwidth than normal telephone. The solution to make this signa sound normal telephone is to remove everything above 4 kHz by a sharp lowpass filter. You can try if your mixing desk channel equalizers are effective enough to remove this problem. When you start equalizing the signal from telephone hybrid then you can also remove the bass frequencies also (there is usable sound information below 200 Hz on normal telephone line) so you can also get rid of the possible low frequency noise (mains 50 Hz or 60 Hz) which is sometimes present on telephone line.