# US 4,394,535 A · Split phase stereophonic sound synthesizer

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- Number: US 4,394,535 A
- Document ID: US4394535A
- Status: Granted
- Kind: A
- Application number: 06/241,926
- Filed: 1981-03-09
- Granted: 1983-07-19
- Earliest claimed: 1981-03-09
- Assignees: RCA Corporation
- Inventors: Joseph P. Bingham; John F. Benford
- Web page: https://patentgrep.com/patent/US4394535A

## CPC

- H04S 5/00: Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation
- H04S: STEREOPHONIC SYSTEMS

## Abstract

A stereophonic sound synthesizer system is presented which utilizes a phase splitter in the form of a transformer to develop two oppositely phased audio signals from an applied monaural signal. One of the two oppositely phased signals is applied to a transfer function circuit of the form H(s), which modulates the intensity of a monaural signal as a function of the frequency. The intensity modulated H(s) signal may be applied to an amplifier for subsequent amplification and reproduction. The H(s) signal is also combined with the other of the two oppositely phased signals to produce a difference signal which is the complement of the H(s) signal. The difference signal may be applied to an amplifier for subsequent amplification and reproduction. No differential amplifier is necessary to produce the difference signal because the necessary selective phase opposition of the signals combined in that channel is provided by the use of the oppositely phased transformer output signals. In addition, the transformer electrically isolates the television's electrical system from the stereo synthesizer system's signal outputs.

## Claims

### Claim 1

A stereo synthesizer for producing synthesized stereo sound signals from monophonic input signals comprising:
a source of monophonic sound signals;
a phase splitter circuit having an input coupled to said source of monophonic sound signals and first and second outputs at which monophonic sound signals of opposite phase relationship are produced;
a transfer function circuit having an input coupled to said first output of said phase splitter circuit and an output, and exhibiting an amplitude versus frequency response characteristic including two spaced frequencies of maximum attenuation and a frequency of minimum attenuation intermediate said spaced frequencies within an audio frequency range occupied by said monophonic sound signals, for producing an intensity modulated signal at said transfer function circuit output;
a first output terminal responsive to said output of said transfer function circuit for producing a first synthesized stereo sound signal;
a second output terminal;
means for transferring monophonic sound signals from said second output of said phase splitter circuit to said second output terminal without introduction of variations in amplitude or phase with frequency over said audio frequency range; and
means for transferring intensity modulated signals from said output of said transfer function circuit to said second output terminal without further introduction of variations in amplitude or phase with frequency over said audio frequency range to develop a second synthesized stereo sound signal.

### Claim 2 (depends on 1)

The stereo synthesizer of claim 1, wherein said phase splitter circuit comprises a transformer having a primary winding coupled to said source of monophonic sound signals, and a tapped secondary winding with first and second ends comprising said first and second outputs and the tap point of said secondary winding coupled to a point of signal reference potential.

### Claim 3 (depends on 2)

The stereo synthesizer of claim 2, wherein said means for transferring monophonic sound signals from said second output of said transformer to said second output terminal comprises a first passive network having an input coupled to an end of said transformer secondary winding and an output coupled to said second output terminal; and
said means for transferring intensity modulated signals from said output of said transfer function circuit to said second output terminal comprises a second passive network having an input coupled to the output of said transfer function circuit and an output coupled to said first passive network,
wherein said second synthesized stereo sound signal is developed at the junction of said first and second passive networks.

### Claim 4 (depends on 3)

The stereo synthesizer of claim 3, further comprising a third passive network having an input coupled to the output of said transfer function circuit and an output coupled to said first output terminal.

### Claim 5 (depends on 1, 4)

The stereo synthesizer of claims 1 or 4, wherein said transfer function circuit comprises first and second cascaded twin-tee notch filters.

### Claim 6

In a system which reproduces correlated sound and visual information, including a source of correlated monophonic sound and visual signals; means, coupled to said signal source and including a video reproduction medium, for displaying visual information derived from said visual signals; a synthetic stereo sound system comprising:
a phase splitter circuit having an input coupled to said signal source for receiving said monophonic sound signals and first and second outputs at which monophonic sound signals of opposite phase relationship are produced;
a transfer function circuit having an input coupled to said first output of said phase splitter circuit and an output, and exhibiting an amplitude versus frequency response characteristic including two spaced frequencies of maximum attenuation and a frequency of minimum attenuation intermediate said spaced frequencies within an audio frequency range occupied by said monophonic sound signals, for producing an intensity modulated signal at said transfer function circuit output;
a first output terminal responsive to said output of said transfer function circuit for producing a first synthesized stereo sound signal;
a second output terminal;
means for transferring monophonic sound signals from said second output of said phase splitter circuit to said second output terminal without introduction of variations in amplitude or phase with frequency over said audio frequency range;
means for transferring intensity modulated signals from said output of said transfer function circuit to said second output terminal without further introduction of variations in amplitude or phase with frequency over said audio frequency range to develop a second synthesized stereo sound signal;
first and second loudspeakers located at respective opposite sides of said visual information display means; and
means for coupling said first and second output terminals to said first and second loudspeakers, respectively, for producing a synthetic stereo sound field about said visual information display means.

### Claim 7

A television sound and image reproduction system, including a source of television sound and video signals; a television receiver enclosure, a kinescope mounted in said enclosure for reproducing video information; and means responsive to said video signals for applying video information to said kinescope for reproduction; said reproduction system also including a synthetic stereo sound system comprising:
a phase splitter circuit having an input coupled to said signal source for receiving said television sound signals and first and second outputs at which television sound signals of opposite phase relationship are produced;
a transfer function circuit having an input coupled to said first output of said phase splitter circuit and an output, and exhibiting an amplitude versus frequency response characteristic including two spaced frequencies of maximum attenuation and a frequency of minimum attenuation intermediate said spaced frequencies within an audio frequency range occupied by said television sound signals, for producing an intensity modulated signal at said transfer function circuit output;
a first output terminal responsive to said output of said transfer function circuit for producing a first synthesized stereo sound signal;
a second output terminal;
means for transferring television sound signals from said second output of said phase splitter circuit to said second output terminal without introduction of variations in amplitude or phase with frequency over said audio frequency range;
means for transferring intensity modulated signals from said output of said transfer function circuit to said second output terminal without further introduction of variations in amplitude or phase with frequency over said audio frequency range to develop a second synthesized stereo sound signal;
an amplifier located external to said enclosure, and including first and second amplifying channels having respective inputs coupled to said first and second output terminals, and respective first and second outputs;
first and second loudspeakers located external to said enclosure and subject to being placed by a user at positions on opposite sides of said enclosure, and respectively coupled to said first and second outputs of said amplifier channels for reproducing said first and second synthesized stereo sound signals to develop a synthetic stereo sound field.

### Claim 8 (depends on 7)

The system of claim 7, wherein said kinescope is centered with respect to the location of said first and second loudspeakers.

### Claim 9 (depends on 7, 8)

The system of claims 7 or 8, further comprising a third loudspeaker mounted in said enclosure; and a switch for selectively applying said television sound signals to either said phase splitter circuit or said third loudspeaker,
wherein said television sound signals are applied to said third loudspeaker for reproduction of a monophonic television sound field.

### Claim 10 (depends on 1)

The stereo synthesizer of claim 1 wherein said transfer function circuit exhibits a phase versus frequency response characteristic including phase variations with frequency within said audio frequency range; and
said means for transferring monophonic sound signals to said second output terminal and said means for transferring intensity modulated signals to said second output terminal includes means for substantially equalizing the amplitude of said signals transferred to said second output terminal at frequencies wherein the phase difference between said transferred signals is 180.degree..