Vinyl Records in Stereo!

Last month we had a little focus on how vinyl records have been making a come-back. I said I would follow up with how stereo came about and judging by the bursting email box, that’s what I will do.

Most of us have been blessed with two ears. As a result, to be able to listen to a truthful rendition of recorded sound, two channels need to be reproduced, for ears left and right. Stereo recording had been around since the ’30s. But, getting this onto and off a record was not an easy task. Many different ideas were tried, including this one which I find amusing.

A U.S. company took a double-sided master disc and designed a head cutting machine that would cut the left channel on one side of the disc and on the other side, the right channel. However, getting the two cutting heads to start and track each other with 100% accuracy was a nightmare. The other problem was how to play the dam thing. The best solution was to play the disc vertically, i.e. standing on edge. A bit like a ‘50s Juke box. Two pick-up arms were counterbalanced, one set to play side 1 (left) the other set to play side 2 (right).  Unfortunately the whole mechanism was very messy. The pick-ups had to hit the lead-in groves of each side of the disc at exactly the same time. If not, you would hear left and right channels that could be adrift by one or more revolutions of the disc, making the reproduction nonsense. Eventually it was decided that, given the mechanical and technical headaches and Joe-public’s cack-handidness, this idea would be binned.

Another idea was to modulate the cutting of the grove in two directions. In mono (one channel), the grove was modulated (wobbled) from left to right. When the record is played, the stylus (needle) would be wobbled from side to side, with a signal being created by its physically connected magnet and coil. With this new idea, the grove was modulated up and down as well. The design of the pick was such that two coils would detect both axis of movement of the stylus, producing the left and right signals to the amplifier.  This sort of worked, but the main problem was distortion and interference between the two channels. The stylus in the grove would get “confused” with the up-down and left-right movement, resulting in a muddled poorly defined sound. This became much worse on loud music passages when the grove was modulated to its fullest limits.

A company called Westrex came up with a modified grove cutting head, similar to the above, but it was turned through 45 degrees. This meant there was an equal left and right channel modulation of the grove, a complex wiggle pattern if you want. The pick-up stylus was designed with coils and magnets arranged at 45 degrees to detect this signal. In fact, one coil detects the horizontal movement produces a signal that is the sum of left + right, whilst the other coil picking up the vertical movement creates a left – right summed signal. A lateral coil in the pickup is used to remove the added signals, resulting in a good quality left and right signal with a high degree of separation.

The other benefit of this principle, if you couldn’t afford stereo, was that the record can be played using a mono pickup cartridge. This is because the stylus, connected to the signal generating parts, coil, magnet or crystal, recognised the overall movement as a sum of the left and right channels. As a result, you did not lose any of the recorded signals. It is amazing that this 45 degree cutting technique was first developed by a brilliant British engineer called Alan Blumlein, working for EMI, 30 years earlier. Alan was instrumental in developing stereo sound recording and playback on records and cinema. During WW2, he went to work on developing radar and was tragically and mysteriously killed in a plane crash whilst on a secret testing mission. His work on stereo was shelved, to be adopted eventually by Westrex.

So, there we have it. A record cutting principle was put forward 80 odd years ago and still used to this day. CDs eat your heart out. Next month, another article to keep you on the edge of your seat.

Seth Pittham. Zeta services.