US2392218A - Announcing method and system - Google Patents

Description

Jan. 1, 1946. 1.. J. ANDERSON ,392, 1

ANNOUNCING METHOD AND SYSTEM Filed Jan. 9, 1945 MWRVQ N NINQU QNEE H mm mm R an INVENTOR. my I :1

BY v

FTTOE/Vfy 2- sesame I I Y and above 6500 cycles per second. It has been on a transformer It over condenser it. The

regular grid biasing resistors 20 and 21 shunted by bypass condensers 22 and 28 are shown in the cathode circuits of the respective tubes to and I1. Anode potential for tube i! is supplied over resistor 2d.

The signal amplified in amplifier I1 is -reproduced by one or more loud speakers such as I The transformer is does shown at'fit and 21.

' not directly feed the speakers 26 anad El, these elements being interconnected by a bandpass filter 2t composed of two series resonant, series connected sections and a parallel resonant shunt 'the level of the signal channel in substantially direct proportion'to the increase in noise in the found that'this proportioning of the frequency bands will permit the control circuit to raise sound reproducing area, while preventing feedback in the system.

connected section. One series section comprises a condenser St and inductance 3i, and the other series section comprises an inductance $2 and a condenser 33. The parallel shunt element is composed of an inductance rs and a condenser as. and is connected from a point between the series sections and the other side of the line.

Located in the area in which sound is reproduced by the speakers at and 2? is a control pickup microphone dd which is connected to an amplifier tube ti through a transformer t2 and a band elimination filter dd. This band elimination filter is composed of two parallel resonant sections connected in series, one section being vcomposed of an inductance 65 and a condenser fil and the other section being composed of i an inductance es and a condenser fit. To complete this filter, a series resonant section, composed of a condenser E36 and an inductance 52, is connected from a point between the parallel resonant sections and the other side of the line.

The output of amplifier ill is impressed on a full wave rectifier composed of vacuum tube rectifiers 55 and 56 over a transformer El, the

output being passed through a capacity resistance filter to, a load resistor 50. The capacityresistance filter is composed of three resistors 6 l,

62, and $3, and two condensers 6t and st, these elements being given values to provide the proper timing desired during increases and decreases in the control signal.

It will be noted that the control grid of variable gain amplifier I t is connected through the secondary of transformer 9 and the rectifier load resistor $0 andback to the cathode of tube id through fixed bias resistor 20.

In this. manner, variations in current through the resistor 60 will vary the bias on tube l0 and thus vary its gain. The circuit is so arranged that increases in the amplitude of the signal detected by the microphone 40 will reduce the bias on tube it to increase its gain, so that the output of-the signal channel at speakers 26 and 21 will be increased proportionately.

To prevent the output of the speakers 26 and 21 from being detected by microphone 40 and thus cause regeneration. feedback, or singing, the two filters 2t and M are employed. Referring now to Fig. 2, the characteristics of the two filters are illustrated, filter 28 being a band pass filter passing frequencies from approximately- 400 to6500 cycles per second. This frequency range is one which will provide the signal with As mentioned above, other signal channels may feed other listening areas as indicatedby the dotted lines 8, these other channels either having similar automatic level control circuits in case the speakers are in noise areas, or having .solely an amplifier or fixed gain in case the sound is reproducedin silent areas, such as omces or similar locations. I

I claim as my invention:

1. In a system including an amplifier connected between a signal source and a reproduclog device, the combination of a band pass filter connected between said amplifier and saidrelying in a predetermined hand between substantially 400 and 6500 cycles per second, a pickup device, a rectifier, a band elimination filter for restricting the output from said pickup device as impressed on said rectifier to impulses of fre-' quencies lying below 400 and above 6500 cycles per second, detecting sound frequencies in said I per second, and means for directly controlling the bias on said amplifier-with the output of said rectifier. v

2. The method of maintaining a substantially constant signal to noise ratio 'in a listening area served by a sound reproducer and subject to varying noise levels comprising selecting for reproduction a predetermined band of frequencies in said signal lying between 400 and 6500 cycles area lying below 400'and above 6500 cycles per second, and directly controlling the level of reproduction of said signal frequencies by the amplitude of said detected sound frequencies.

3. The method of increasingthe level of reproduction of a signal as the noise level in the reproduction area increases without causing regeneration comprising reproducing only a predetermined band of frequencies in saidlsignal lying between 400 and 6500 cycles per second,

detecting only the sound frequencies lying be-,

low 400 and above- 6500 cycles per second, and increasing the level of signal reproduction directly in accordance with the amplitude of the products of detection. p

4. An announcing system comprising a sound pickup device, an amplifier for the output of said device, a reprdducer for translating-the output of said amplifier into sound waves, a band pass filter connected between said reproducer and amplifier for suppressing the impression of frequencies lying below 400 and above 6500'cycles per second in the output of said device on said reproducer, a second sound pickup device acoustically coupled to said reproducer, a rectifier for the output of said second device, a band elimination filter connected between said rectifier and said second pickup device for suppressing the imgood intelligibility for speech transmission and.

dotted lines, while passing frequencies below 400 pression of frequencies lying between 400 and 6500 cycles per second on said rectifier, and

means for connecting said rectifier to said amplifier for directly. controlling the gain of said amplifier in accordance-"with thgamplitude of the output of said rectifier.

. LESLIE J. ANDERSON.

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