Sending sensitivity

Telephone transmission performance

To assess the transmission performance of an individual telephone, or telephone network, a number of parameters have been estab­lished. The voice operation of a telephone can be broken down into the following elements:

1. The path from talker's mouth to the transmitter.

2. The path from the telephone receiver to the listener's ear.

3. The near end path between the transmitter and the receiver.

3. The telephone connection from transmitter and receiver to the distant end.

Different methods exist for comparing the performance of various telephones and for defining the standard to which a particular telephone or system should perform. The four key parameters in determining telephone performance are:

1. Sending sensitivity.

2. Receiving sensitivity

3. Impedance the telephone presents to the line.

4. The line impedance for minimum sidetone.

Each of these parameters can be a function of the frequency range of the telephone and the available line current. Algorithms exist for converting the sensitivity of the transmission path or receive path into a loudness rating i.e. a single number for a given frequency, sound pressure level, and line length.

A loudness rating is a standardised method of measuring the transmission loss of a speech path. It is a single value related to the loudness with which the listener perceives, speech that has been emitted by the talker. The performance of a telephone is assessed by measuring between the telephone and an impedance representing the telephone termination, at the telephone exchange, over varying line lengths and over the full frequency range in which the telephone operates. Known sound pressure levels are applied to the micro­phone of the telephone and the resulting voltage changes appearing at the simulated exchange terminations are measured.

In the other direction signals are applied to the simulated ex­change termination and the resulting sound pressure levels are measured at the telephone receiver. These measurements are nor­mally carried out with the telephone in a sound absorbing box or in an anechoic chamber, with the telephone clamped into a jig which has an artificial ear and an artificial voice. These are used to detect and measure the received signal and generate the tones to stimulate the traasmitter.

The sending sensitivity is defined as the ratio of the voltage measured at the terminating impedance of the exchange feed bridge and the sound pressure level injected into the microphone. This ratio is normally measured over the full frequency range. The sensitivity depends on the frequency response of the microphone, the acoustic path to the microphone, and the transmit gain of the telephone circuit. The sending sensitivity and frequency response are designed to have a rising characteristic within the speech band. The reason for this is that although most of the power in speech is at the lower frequencies, the sharpness of speech is created by the higher fre­quencies. However, this is the region were losses in the cable are highest, therefore by increasing the sensitivity at the higher frequen­cies this tends to compensate for the effect of the cable and increases the clarity of the speech. Above 4kHz the sensitivity should fall off rapidly, to prevent unwanted out of band energy to be transmitted to the network, which might interfere with PCM circuits. At low frequencies the sensitivity should roll off to prevent mains hum pickup. The gain introduced into the sending path is chosen to ensure that the telephone send sensitivity is in accordance with the transmission plan for the telephone network. A typical send sensi­tivity response is shown in Figure 58.7.

Дата добавления: 2015-03-31