US2054647A - Sound reproducing system - Google Patents

Description

Sept. 15, 1936. s, BALLANTINE 2,054,647

SOUND REPRODUC ING SYSTEM Filed Nov. 21, 1953 3 Sheets-Sheet l lawpass HY/er luv Sound Pressure F'requency, oic/Y/af/bns oer Jecond :jmwnto o',

Sept. 15, 1936. s. BALLANTINE YSQUND REPRODUCING SYSTEM Filed Nov. 21, 1955 3 Sheets-She'c 2' P 1936- s. BALLANTINE v 2,054,647

SOUND REPRODUCING SYSTEM Filed Nov. 21, 1935 3 Sheets-Sheet 3 Fl/fer Muff n Patented Sept. 15, 1936 PATENT OFFICE SOUND REPRODUCING SYSTEM Stuart Ballantine, Mo

signor, by mesne assi ration of America, ration of Delaware untain- Lakes, N. J., as gnments, to Radio Corpo- New York, N. Y., a corpo- Application November 21, 1933, Serial No. 699,069

' 4 Claims.

This invention relates to sound reproducing systems and more particularly to electro-acoustic sound reproducing systems-such as are employed in radio broadcast receivers.

The invention is particularly useful in connection with radio receivers where conditions of noise on weak signals and other kinds of interference make it desirable to restrict the range of reproduction at the high frequency end, but it is also useful in general with any kind of electroacoustic system in which it is desirable to adjust the relative amplitudes of the high and low frequenoies.

An object of the invention is to provide a sound reproducing system including electrical elements for regulating the tone quality of the reproduced sound. An object is to provide a sound reproducing system of the electro-acoustic type and including adjustable elements for regulating the intensities of the higher frequency components with respect to the lower frequency components. Another object is to provide electrical sound'reproducing, systems which include simple circuit elements for restricting at will the upper limit of the frequency range of reproduction. A further object is to provide a radio receiver including circuit elements for automatically restricting the audio range of reproduction as the antenna signal voltage decreases.

These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Figs. 1 and 2 are circuit diagrams of sound reproducing systems which include adjustable elements for effecting a tone control;

Fig. 3 is a curve sheet illustrating the relation of sound pressure to frequency for different adjustments of systems such as shown in Figs. 1 and 2;

Figs. 4 and 5 are circuit diagrams of other types of tone control systems;

Figs. 6 and 7 are circuit diagrams of radio receivers which include elements for automatically varying the range of audio reproduction in accordance with the strength of the received carrier wave signals;

Fig. 8 is a fragmentary circuit diagram illustrating a modification of the Fig. 7 circuit arrangement; and

Fig. 9 is a circuit diagram of a radio receiver including a different system for automatically controlling the range of audio reproduction as a function of the received signal strength.

In the several circuit diagrams, the sound re-" producing apparatus is shownas a composite system including a loud speaker I for the reproduction of the lower frequencies and a loud speaker 2 for the reproduction of the higher frequencies. The speakers may be directly connected in parallel across a source of audio frequency current, such as the audio frequency transformer 3 in the output circuit of vacuum tube 4. If desired, sharp cut-ofi characteristics may be obtained with speakers which lack such characteristics by supplying power to the speakers I, 2 through a low-pass filter 5 and a highpass filter 6, respectively.

With either system, a tone control adjustment is provided by the switch I which may be opened to interrupt the supply of energy to the high frequency speaker 2. If the opening of the switch 1 produces an undesirably large change in the impedance characteristics of the circuit, a compensating network may be connected across the audio circuit when the speaker 2 is removed. As shown in Fig. 2, the switch I connects a resistance 8 across the high pass filter when the circuit through the speaker 2 is opened, the resistance 8 approximating the impedance of' the reproducer 2 and being of a proper value to terminate the high frequency filter without reflection losses.

In the case of a radio receiver, the switch 1 may be mounted on the control panel to provide a simple and inexpensive means for restricting the range of reproduction. This method of tone control has substantial advantages over methods involving the insertion of networks or filters in the audio amplifier or the radio frequency circuits.

The effect obtained by this tone control will be apparent from a consideration of the sound pressure curves of Fig. 3. The solid line curve AB represents the sound pressure response of the composite reproducer system and it will be noted that the response or audio output is substantially uniform for all frequencies between about 50 and 10,000 cycles when both reproducers are used. The curve AAA" represents the contribution of the low frequency speaker I whilecurve B'Bf'B represents the contribution of the high frequency reproducer 2. When the switch I is opened, the higher frequencies are not reproduced and the system has a relatively sharp cut-01f at about 4,000 cycles.

It is to be understood that the illustrated location of the switch 1 is purelydiagrammatic as the switch may be located in any appropriate circuit where it may be operated to render the reproducer 2 inoperative. For example, in the case of a coil-driven device employing an electromagnet, the exciting current to the electromagnet may be interrupted and, in the case of an electrostatic reproducer, the polarizing voltage may be removed.

In place of suppressing all audio output above a certain limit, it may be desirable in some cases merely to attenuate the higher frequencies. An appropriate arrangement for this method of tone control is shown in Fig. 4 as an adjustable attenuator 9 in the form of a T-type constant resistance device connected in the supply circuit to the high frequency reproducer 2, or simpler types of attenuators such as a variable series resistance or inductance, or a variable shunt capacity may be employed. The remaining circuit elements of the reproducer system may be substantially as illustrated in Fig. 2.

As illustrated in Fig. 5, a control of the relative intensities of the audio energy supplied to the repro-ducers l, 2 may be obtained by providing separate high and low frequency channels for the reproducers. One or both channels include a transmission control element, such as the voltage dividers lil, l l and each channel may include an amplifier I2. In the operation of systems of this type, it is usually sufiicient to set the output of the low frequency reproducer l at some predetermined level by adjustment of the control l0, and to obtain an adequate tone control solely by adjustment of the transmission in the high frequency channel.

In a radio receiver, for example, it is usually desirable to place only voltage divider II on the control panel and to place the voltage divider ID at some point on the chassis which may be conveniently reached from time to time, if desired, for effecting a relatively permanent or factory adjustment of the low frequency transmission.

This method of tone control by variation of the transmission efiiciency of separate high and low frequency channels is especially adapted for the automatic control of the tone quality of reproduction as a function of the received signal energy. In ordinary circumstances, the reception of weak radio signals is accompanied by noise and undesirable distubances of many kinds which are, in general, of relatively high frequency. On the other hand, strong radio signals are not usually accompanied by such disturbing noises. It is therefore desirable, in general, to restrict the audio range of reproduction when receiving weak signals and to employ the full range of reproduction when strong signals are received. Radio receivers which include circuit elements for automatically controlling the range of reproduction as a function of the received signal energy are illustrated in Figs. 6 to 9, inclusive.

As shown schematically in Fig. 6, the radio receiver includes a radio amplifier 13 of any desired type, and a detector and/or audio amplifier I4 across which the low pass filter 5 and the high pass filter 6 are connected in parallel. An audio amplifier I2 is provided between the low pass filter 5 and the reproducer l and, if desired, a transmission control such as the voltage divider I0 of the Fig. 5 circuit may be employed but, in general, such adjustable control is not necessary. The high frequency channel includes an amplifier tube l5 which is associated with an automatic gain control system, and may include a further amplifier stage I2 of the constant gain type. The automatic control system includes the rectifier l6 which is connected across desired points in the radio frequency system, for example in parallel with the detector I4, the rectifier being preferably of the diode type and subjected to a bias voltage from a current source, indicated diagrammatically as a battery ll, which prevents the development of a rectified output so long as the received signal energy is less than a predetermined critical value. The rectified voltage is developed across an output resistance. I8 which is shunted by a radio lay-pass condenser l9, and preferably, this direct current voltage is employed. in the known manner to effect an automatic control of the detector input voltage. The direct current component of the rectifier system is applied to the control grids of radio amplifier 13 through the lead 20, the alternating current components of modulation frequencies being removed by the filter comprising resistance 2| and capacity 22.

In accordance with this invention, the direct current output of the rectifier system is employed to effect an automatic control of the transmission efiiciency of the high frequency channel. The filtered direct current voltage developed by the rectifier system is impressed upon the grid of a tube 23 which has a resistor 24 connected between its cathode and the plate current supply 25. The voltage drop produced across the resistor 24 by plate current flow in tube 23 is applied as a negative bias to the control grid of the amplifier tube 15 through a resistor 26. In the absence of an antenna input, the tube 23 is so biased by a current source 21 that a relatively heavy plate current flow is established and the voltage developed across resistor 24 is sufiicient to reduce the transmission through the amplifier tube I5 to a negligible value. As the antenna voltage increases, the rectifier system becomes operative and the direct current voltage developed across the rectifier output resistance l8 makes the bias voltage on tube 23 more negative, thus reducing the plate current flow and therefore the direct current voltage which is developed across the resistor 24. As the voltage across the resistor 24 decreases, the negative bias on the amplifier 15 decreases and the transmission through the high frequency channel is correspondingly increased.

The tube l5 may be of the tetrode type and a resistance 28 is preferably shunted across the output transformer 29 of the tube to maintain a constant output impedance for varying grid bias voltage.

The described circuit effects an automatic decrease in the transmission of the higher frequencies as the antenna input decreases. Various rates of control and of the relation between transmission and antenna voltages may be obtained by suitable choice of the tubes and energizing potentials applied thereto, as will be apparent to those familiar with the design and construction of radio circuits. I have found that adjustment particularly useful which provides a constant normal transmission through tube I5 for all signal voltages exceeding a certain value and which gradually decreases the transmission as the signal voltage falls below that predetermined value.

While the circuit of Fig. 6 employs a rectifier system for automatically controlling the ampllfier gain and the range of audio reproduction, it will be apparent that the automatic gain control system could be omitted without affecting the operation of the tone control system or, alternatively, the tube 23 could be energized by the potential drop across a resistor in a volume control circuit of the manually regulated type. Whether the variation of volume control voltage is developed automatically or is adjusted manually, the same automatic regulation of tone control with varying antenna input will be obtained.

Another method of automatic tone control isillustrated in Fig. 7. A conventional type of radio amplifier l3 works into a detector I4 which is followed by an audio amplifier. 30. The radio amplifier tubes are initially biased for high gainby appropriate means, such as a current source 3| and means is provided for automatically regulating the gain of the amplifier to maintain an approximately constant input to the detector M.

A gain control system similar to that of Fig. 6 is illustrated, the several parts being identified by corresponding reference numerals, but it is to be understood that the exact design of the radio amplifier and the automatic gain control system is not an essential part of the invention.

The low frequency reproducer I and the high frequency reproducer 2 are connected in parallel across the audio amplifier 30 and, in accordance with the invention, a switch 1 in an energizing circuit of the high frequency speaker 2 is automatically controlled as a function of the receiver input or of the adjustment of the radio amplification. The switch 1 forms part of a relay or electromagnetic switch in the plate circuits of the controlled tubes of the radio amplifier l3. As illustrated, the plate circuits of the controlled tubes are returned to the plate supply source 32 through a tuning meter 33 and the winding 34 of the relay or solenoid which controls the switch I. The winding may be shunted by an adjustable resistance 35 for regulating that value of plate current which is suificient to attract the armature to open the switch 1. An additional contact or switch 35 may be provided, if desired, to control the circuit of a signal light 31 for indicating whether the composite reproducer system or only the low frequency reproducer is in operation. The determination of the critical signal strength at which the tone control switch is to be operated is a matter of judgment and experiment, and will doubtless depend upon the noise conditions existing at the particular receiver location. When the signal strength falls below the predetermined critical value, the increased plate current flow in the radio amplifier results in the attraction of the armature of the control relay to open switch I, thus deenergizing the high frequency reproducer 2.

Another type of control is illustrated in the fragmentary circuit diagram, Fig. 8, which includes only those elements which follow a radio amplifier of the automatic gain control type such as, for example, the elements shown to the left of the broken line in Fig. '7. The audio system following a detector 4 and audio amplifier 30 provides but one channel for the transmission of audio frequency energy to the reproducers but a low pass filter 5 is automatically switched into the transmission channel when the received signal energy falls below a predetermined critical value. The relay elements 34, 35 may be the same as shown in Fig. 7, but double contacts 38, 39 are provided to connect the low pass filter into the audio circuit when the plate current flow rises above a critical value and to by-pass the audio energy around the filter when the plate current flow falls below the critical value. The

filter 5' has an upper cut-ofi adaptedto attenuate the higher frequencies to a desired extent when the filter is inserted in the: audio system. For greater clarity of illustration, the reproducer system is shown as including. the separate speakers I, 2 but it will be apparent that a single speaker may be used if it is capable of reproducing the full range of audio frequencies.

Other circuit arrangements maybe employed for-obtaining automatic tone regulation by the methods illustrated in Figs.v 7 and 8. The system shown in Fig; 9 is generally like that of Fig. 7 but employs a separate tube 40 to control therelay switch system 34, 35, 1', in place of the plate current of the controlled tubes of the radio amplifier It. Like elements of. the circuits of Figs. 7 and 9 are identified by corresponding reference numerals, but will not be described in detail. The relay winding 34 is included in the plate circuit of the tube 40, and the bias on tube 40 is automatically varied asa function of the received signal energy by connecting the grid to the automatic bias lead 20 of the gain control system. The desired quiescent bias on tube 40 may be so adjusted, by a current source 4|, that plate current of tube 40 energizes relay winding 35 to hold switch 1' open until the added negative bias component due to rectification by tube I6 reaches a value corresponding to that critical receiver input for which full range reproduction is desired. This circuit is of particular advantage in permitting precise operation of the relay in the region of weak signals. This result is accomplished by employing a tube 40 which has a relatively sharp plate-current cut-01f rather than, for example, one of the variable-mu variety having a gradual cut-01f, such as would be employed in the radio amplifier l3. The plate current of tube 48 will then remain at zero over a considerable range of signal strength in the region where the latter is weak, and this action permits more precise operation of the relay at the critical signal strength for which it is adjusted.

It will be apparent that the described circuits are illustrative of the wide range of appropriate constructions for obtaining a simple and effective tone control. Many variations will be obvious to those familiar with the design of vacuum tube circuits. For example, relay control systems such as shown in Figs. 7 and 8 may be employed in radio receivers in which the sensitivity is controlled manually by adjustment of the control grid bias on the radio amplifier tubes.

These and other variations or modifications in the current arrangements fall within the spirit of my invention as set forth in the following claims.

I claim:

1. In a radio receiver, the combination with a radio amplifier, a detector, and an audio system including an audio amplifier and sound reproducing means, of means for automatically controlling the radio amplifier gain to maintain an approximately constant radio input to said detector over a Wide range in the magnitude of the radio input to said receiver, and means for controlling the width of the band of audio frequencies which is reproduced by said sound reproducing means; the said gain controlling means and band width controlling means including, in common, rectifier means for developing a direct current voltage which varies with the radio input to said receiver, said band width controlling means including a direct current amplifier, and means energized by the output of said direct current amplifier when said direct current voltage exceeds a critical value for providing the sole biasing of the grid of the audio amplifier in a manner such that transmission by said audio system of the higher frequency end of said band is blocked.

2. In a sound-reproducing system, the combination with a detector, a high frequency and a low frequency sound reproducer, a separate high frequency and low frequency amplifier between said detector and the respective sound reproducers; of means for blocking reproduction by said high frequency reproducer, and means controlled by the radio voltage input to said detector for actuating said blocking means, said blocking means including means for rendering said high amplifier inoperative by providing the sole negative bias therefor.

3. In a sound-reproducing system, the combination with a detector, of a high frequency and a low frequency reproducer, a separate audio transmission channel between said detector and each of said reproducers, and an amplifier coilpled to the high frequency reproducer, means solely biasing the amplifier so that for input signals below a predetermined level the amplifier is blocked, and additional means arranged to control said biasing means so that for input signals above said level the amplifier is operative to transmit signals to the reproducer coupled thereto.

4. In a sound-reproducing system, the combination with a detector, of a high frequency and a low frequency reproducer, a separate audio transmission channel between said detector and each of said reproducers, an amplifier coupled to the high frequency reproducer, and means cooperating with the detector for providing the sole negative bias on said amplifier, said means automatically controlling the bias on the grid of said amplifier to vary the bias inversely with variations in signal intensity.

STUART BALLANTINE.

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