Patent GB-968482-A (GB)
Title: Improvements in or relating to volume expansion circuit arrangements
IPC: H03G7/02
Filed: 1960-09-30
Abstract: 968,482. Automatic volume control. PHILIPS ELECTRICAL INDUSTRIES Ltd. Sept. 30, 1960, No. 33633/60. Heading H4R. An arrangement for the volume expansion of a band of compressed information signals which is accompanied by an adjacent pilot signal, the pilot signal having an amplitude inversely proportional to the amplitude of the original uncompressed band of information signals, is such that the compressed information signals and pilot signal are applied to a circuit comprising a frequency shift stage which increases the frequency distance between the information signals and pilot signal, both signals being applied to an amplitude limiter to provide a series of pulses, the duration of said pulses being inversely proportional to the amplitude of the pilot signal, filter means being provided for deriving volume expanded information signals at the output of the filter. Incoming carrier signals are received by an aerial (Fig: 1, not shown) and supplied to a mixer stage having a local oscillator which is connected through an intermediate frequency oscillator to a demodulator. Speech signals in the band 0.3 to 3.4 kc./s. and an adjacent pilot signal of frequency 3.7 kc./s. are derived from the output of the demodulator. The speech signals are supplied to a low-pass filter 9 (Fig. 3) having cut-off of 3.4 kc./s. and via transformer 17 to an amplitude limiter. The pilot signal is applied through a filter 10 (3.6 to 3.8 kc./c.) to a ring modulator 18 to which is connected through terminals 19 an oscillator of frequency 20 kc./s. A frequency-converted pilot signal in the band 23.6 to 23.8 kc./s. is selected by filter 20 and, after amplification in amplifier 21 applied to the amplitude limiter. This comprises two rectifiers 23, 24 in the control grid circuit of amplifier 22. The limiting bias of rectifiers 23, 24 is derived from a cathode resistor 25. The limited signals are amplified in amplifier 22 and supplied to low-pass filter 27 having a cut-off frequency of 3.4 kc./8. from where volume expanded information signals are obtained. These are applied to a low frequency amplifier and reproducer (Fig. 1, not shown). In a modification, Fig. 4, the pilot signal, after passing through filter 10, is rectified by stage 29 and passed through a low-pass filter 30 having a cut-off of 100 c/s. Modulator 11,of push-pull type, is constructed for modulation of direct voltages. A variable attenuator 31 provides for adjustment of the upper limit of the expansion control range. The upper limit of the control range may alternatively be effected by providing carrier wave leakage in the modulator 11 or by supplying a constant direct voltage to modulator 11. The pilot signal may be selected in the intermediate frequency band and supplied to the limiter together with the detected speech signals. In such arrangement the frequency shift between pilot signal and speech signals is not effected by frequency conversion of the audio-frequency pilot signal but by directly using the pilot signal in the intermediate frequency band. Specification 968;481 is referred to.