I understand the basics of how we get music from an LP. A needle is dragged through the imprinted groove, the vibrations in the needle are transduced into tiny currents by, say, a crystal, then amplified by an amplifier.
But how are two tracks (right and left) contained in the groove? Does each side of the groove have just one channel? How can the sound get such good separation with only one needle (something recorded to play out of only the left speaker doesn’t bleed much onto the right speaker)?
And worse, quadraphonic sound! Four channels in a groove?
Stereo is managed by the groove having different information in the up/down direction than in the left/right direction. One channel is encoded in the grove variations in the plane of the disk, the other channel is in the variations perpendicular to the disk’s surface.
Quadraphonic records still had two channels from the record player cartridge. The two “right” channels and the two “left” channels were encoded and decoded electronically into one physical channel. I’m not sure exactly how they were encoded, but it was done in such a way that the quad record could be played on normal stereo equiptment, and you would just hear CH1+CH3 on one channel and CH2+CH4 on the other.
Not quite. The two channels are perpendicular to each other, but the directions are rotated by 45 degrees to make both channels symmetrical. One channel is (was… I’m feeling old now) encoded in the upper left / lower right movement, the other in the upper right / lower left.
They weren’t, at least at the start. If you played a stereo record with a mono head, it would ruin the record for stereo; the mono head would wreck one of the channels. For this reason, albums were released in a stereo and mono versions (one of the first records I bought was the mono version of “Sgt. Pepper”). Later, they began to develop records that weren’t ruined, but that was about the time mono had pretty much disappeared.
I believe the issue was not mono/stereo but that the needles came in different sizes and you could not play the new smaller groove albums with the older, bigger needles.
Further on quad - there were actually several competing encoding standards from various manufacturers - SQ (CBS), QS (Sansui), CD-4 (JVC) and a couple of others, each with their own decoder boxes to listen to the recordings made with them. Since the whole idea died, the standards war never played out to completion.
SQ and QS were so called “matrix” encodings that utilized normal stereo turntables and cartridges. CD-4, supposedly a superior technique, required special new “super vinyl” recordings and special cartridges. Not to do anything radically different, but to enable frequency response up to 50 KHz. CD-4 recorded the sum of two channels normally, and overlaid it with the difference between the front and back channels encoded as signals above 30KHz so the information could be filtered out and the separate channels reconstructed. A normal stereo system would simply miss the high frequency stuff and play the combined front and back.
The two stereo channels were recorded as L+R on one side and L-R on the other. The electronics would add the two producing 2L, or subtract the two producing 2R. The mono system would pick up ??? (pointless knowledge well runs out at this point)
K364, I think you are thinking of FM modulation which is encode like that to make it compatible with mono receivers. The base modulation is L+R and the subcarrier carries the difference.