Patent GB-1053541-A (GB)
IPC: H04R29/00
Abstract: 1, 053, 541. Determining transfer characteristics; operation techniques. SEISMOGRAPH SERVICE (ENGLAND) Ltd. Dec. 14, 1962 [Dec. 11, 1961], No.44321/61. Heading G1U. In a method of determining a transfer characteristic of a signal transfer system, a test signal is injected into the system and an output from the system is derived to yield a signal representative of the impulse response of the system. The impulse response is then cross correlated against a swept frequency waveform to give a signal representative of the amplitude frequency response of the system. A suitable test signal may be applied in the presence of other signals passing through the system and may comprise a swept frequency waveform. To produce the impulse response of the system, the output derived from the system after applying this input is cross correlated with the input. The significant part of the signal so derived, representing the impulse response, is gated from the rest of the signal, which carries background noise due to the other signals passing through the system, and this gated signal is correlated with the time inverse of the original swept frequency waveform to provide the required amplitude-frequency response. A suitable apparatus for carrying out the determination of the transfer characteristic may be very similar to the disc correlating device described in Specification 1000540. In this apparatus, an insulating disc 14 carries three printed circuit tracks 16a, 16b, 16c respectively concentric with playback heads 17a-17c and erase heads 18a-18c, which are mounted in gaps in the disc 14. An oxide coated disc 20 is arranged to revolve above and in contact with the disc 14. The track 16b represents a linear frequency sweep sweeping from high to low frequency (with the disc 20 rotating anticlockwise) while track 16c represents the same sweep reversed in time. The track 16a duplicates that of 166 except that it has a 90 degrees phase lead at all frequencies. In operation, a large pulse of current is first passed through the track 16a. This impresses a pattern on the disc 20 which when played back via the head 17a and processed via an RC equalising circuit may be re-recorded on the disc 20 via the head 17d, so that the master sweep signal is stored for use. The system under test is connected between the head 17d and the head 17b. The master sweep is then passed through the system from the head 17d, and the system output is passed via the head 17b to the disc 20 and is played back via the correlating waveform track 16b. The output from the printed waveform track 16b represents the impulse response of the system. This output after suitable equalization is supplied to the recording head 17c, whence it is recorded on the disc 20. This recording is played back via the printed track 16c, which produces cross correlation between the impulse response and the sweep waveform reversed in time. This cross correlation output is rectified and smoothed to give a function representing the amplitude frequency response of the system and this is fed to a suitable recording arrangement. Alternatively the waveform before rectification may be compared with the master signal waveform to give the systems' phase frequency response. An alternative arrangement for generating the master sweep frequency waveform at low frequencies is described employing a cam whose periphery is shaped in accordance with the desired waveform. This operates a cam follower linked with a potentiometer, and the voltage picked off the potentiometer represents the required waveform.