Patent GB-1384761-A (GB)
Title: Audio frequency apparatus
IPC: H03G5/02
Filed: 1972-02-18
Abstract: 1384761 Equalizing amplifiers and loudspeakers to rooms UNITED RECORDING ELECTRONICS INDUSTRIES 18 Feb 1972 [24 Feb 1971] 7701/72 Heading H4J An amplifier-loud-speaker system is equalized with respect to a listening room by radiating a test signal from a source 10 (Fig. 1) through the loud-speaker system 13. A microphone 15 and amplifier 16 having a flat frequency response receive the signal and pass it through a bank of contiguous filters 18 covering the audio frequency range. Each filter is adjusted to obtain a substantially flat frequency response for the overall arrangement. A conventional input source 26 such as a tuner or tape deck is then applied through the adjusted filter bank 18 for radiating through the loud-speaker system 13. As described, a stereophonic arrangement utilizes a test signal source 10 which generates a sawtooth wave of repetition rate 10 Hz or, preferably, a square wave of repetition rate 5 Hz, thereby providing a test signal in the form of a comb of audio frequencies. This is fed through an attenuator to a switch 12 which permits the test signal to be fed to a left or right loud-speaker system 13, 14, for radiation into the room. The microphone 15 and preamplifier 16 pick up and amplify the radiated sound and the preamplifier output is fed to a switch having three positions. Switch 17 is ganged with a switch 12. In the first position, the pre-amplifler output is applied to left channel loudspeaker system 13. In the second position, the preamplifier output is applied to a right filter bank 22 while the test signal is fed to right channel loud-speaker system 14. In the third position, left and right channel signals from an input signal source 26, 27 (e.g. a tuner or tape deck) are applied to the respective filter banks 18, 20 and through respective summing amplifiers 19, 23 to respective loud-speaker systems 13, 14. The filter banks 18, 22 are preferably made up of bridge T null networks in a feedback loop (Figs. 2, 3, not shown) to provide a peaked response and there are preferably three filters per octave. The filters in the filter banks 18, 20 have equal Q which means that bandwidth is proportional to the centre frequency and also that they provide a 3dB per octave gain in spectral power. Since the amplitudes of the harmonic's from the square wave or sawtooth generator decrease exponentially at a rate of 6dB per octave the overall decrease is at a rate of 3db per octave. The test signal source 10 comprises a free running pulse generator 40 (Fig. 4) whose output triggers a triggered pulse generator 41 whose output triggers the sawtooth or square wave generator 42. The output of the filter bank 18 or 19 is fed to instrumentation 25. This comprises a full wave rectifier 43 and a square law network 44 which provides a signal representative of the power contained in the bandwidth of the selected frequency band. A reset integrator 45 is reset and cleared by the P2 pulse from the triggered pulse generator 41 and a zero order hold circuit 46 is fed to a voltage comparator 47 which also receives a scaled reference voltage by a 3dB per octave scaling network to compensate for the overall decrease of amplitude at the 3dB per octave rate previously mentioned. This output is applied to a voltage indicator 40. The equalizing procedure consists of first normalizing the system using filter 11 and the 1 kHz filter on the left filter bank 18 to give a null reading on voltage indicator 40. The switching network 20 is then positioned at the highest frequency filter and this filter is adjusted until the voltage indicator 40 again gives a null reading. All other filters of the filter banks 18, 22 are adjusted in the same manner. In an alternative embodiment (Fig. 6), the output from the square wave or sawtooth generator 10 is applied to a tunable band pass filter 51 having filters similar to the left and right filter banks 53, 54. The equalizing procedure consists of first setting filter 51 to 1kHz and radiating a comb of frequencies passed by the filter 51 into the room through the left channel loud-speaker 13. A voltage indicator 40 (Fig. 4) in the instrumentation network 25 is set to a null reading by adjustment of attenuator 11. Once the apparatus is set up the band pass filter 51 is set to the various filter values in turn and adjustment of the corresponding left bank filters (53) and right bank filters (54) is made to give a null reading in the instrumentation network 25. A 3dB per octave amplifier 52 in the testing circuitry compensates for individual filters passing more spectral power at the low frequencies than at the higher frequencies.