Patent GB-1018386-A (GB)
Title: Improvements in or relating to stereophonic sound reproducers
Inventor: GOLDMARK PETER CARL
IPC: H04R5/02
Filed: 1963-06-26
Abstract: 1,018,386. Stereophonic systems. COLUMBIA BROADCASTING SYSTEM Inc. June 17, 1964 [June 26, 1963], No. 25335/63. Headings H4J and H4R.  A stereophonic sound reproducer comprises a cabinet containing means for translating sound recordings on twin tracks into electrical signals in two channels respectively and two substantially identical sets of loud-speakers, one set mounted on one side of the cabinet and the other set mounted on the opposite side thereof. In one arrangement each set comprises at least one high frequency loud-speaker and a low frequency loud-speaker, the high frequency loud-speakers being connected with respective amplifiers while a combined signal from the amplifiers is fed to both low frequency loudspeakers. In a second arrangement each set comprises a treble loud-speaker, a medium frequency loud-speaker and a bass loud-speaker, means for connecting the treble loud-speaker and medium frequency loud-speaker of one set in one signal channel and for connecting the treble loud-speaker and medium frequency loudspeaker of the other set in the other signal channel and means for feeding a combined signal from the two channels to both bass loudspeakers. The reproducer may use a gramophone pick-up or a magnetic recording playback head. A stereophonic gramophone includes bass speakers 9 L , 9 R  (Fig. 1), medium frequency speakers <SP>10</SP>L, <SP>10</SP>R and treble speakers 11 L , 11 R . Speakers <SP>10</SP>L and 11 L  are sealed within a compartment 16 constituted by partitions 19, 20 (Fig. 2, not shown) in association with the bounding walls of the cabinet 7. Treble speaker 11 L  is of the enclosed back type. Speakers 9 R , 10 R , 11 R  are similarly arranged. The cabinet 7 houses a stereophonic pick-up P (Fig. 4) the two outputs of which are amplified by amplifiers 17 L , 17 R  respectively. The amplifier outputs are divided by high pass filters HP and low pass filters LP. High frequency signals are applied to speakers <SP>10</SP>L, H L  and <SP>10</SP>R, 11 R  respectively. Bass frequencies (e.g. below 250 c.p.s.) are applied together in phase to speakers 9 L , 9 R  SO that the cones move outwardly and inwardly together. Since the mechanical motion of the cones is opposite, mechanical vibrations applied to the cabinet 7 at points 18 L , <SP>18</SP>R are opposite and tend to minimize acoustic feedback. Perforations 15 in a tray 4 carrying the motor board 3 provide access of bass sound to both sides of the motor board and also assist in minimizing feedback. In an alternative arrangement, signals from stereophonic pick-up P (Fig. 5) are applied to amplifiers AMP1 and AMP2 respectively connected so that, if the two stereophonically related signals at the pick-up are identical, voltages of equal and opposite instantaneous polarity appear at leads X, Y. Treble frequencies are passed by capacitors C1 to speakers TL, TR, mid-range frequencies are passed by band pass filters L 1 , C 2  to speakers ML, MR and bass frequencies are passed by low pass filter L 2  to speakers LL, LR. D.C. supply variations and hum picked up by the leads and amplifiers AMP1 and AMP2, which are closely spaced, result in substantially equal voltages of the same phase on leads X, Y and in little response from the bass speakers. The speakers 10 L , 10 R , <SP>11</SP>L, 11 R  are connected so that their phasing corresponds to that originally recorded. The tray 4 has a flange encased in a rubber gasket 4a (Fig. 3), not shown, which cooperates with a rib 13 on the lid 12 to provide a sealed enclosure. A lateral slit 14 provides good acoustical damping if the seal becomes ineffective. At the front and rear of the cabinet 7 a continuation of the slit 14 extends vertically between the lid and the front (and rear) wall of the main body of the cabinet, also to provide acoustical damping.