| 摘要 |
A test unit is used to check the speech circuits of telephones. It can determine the quality of mouth or ear pieces without using any acoustic facility or equipment. The unit has two loud-speakers connected to a test circuit. The test circuit is also connected to the base of a telephone tone tested. The test circuit has transformers differential amplifiers, and switches. The circuit may be connected to the loud-speaker opposite the mouth-piece or the ear piece and produce feedback of a signal injected into the lines of the telephone by one of the transformers. A detecting circuit comprising a light-emitting diode is used to detect any instability. |
