Cutting of SQ and QS discs

[kfbkfb]

The trick is that both channels must be limited or compressed exactly the same, with the two devices linked. I have some Behringer stereo compressors with link switches for this purpose.

Ever try some matrix encoded content processed through limiters/compressors (set to high compression) - is the matrix decoded imaging affected a lot?


Kirk Bayne

 

[lukpac]

A question. Cutting a stereo disc involves the cutting ‘needle’ moving left and right in the groove of the master on the lathe. But did cutting SQ and QS discs require new equipment for encoding the vertical modulations? I presume CD4 needed totally new lathes.

Actually, stereo masters are cut with the 'mids' (stuff in the center) moving the stylus laterally, and the stereo 'sides' moving the stylus vertically. For the life of me, I can't imagine getting 2 more channels represented somehow!

That's effectively what gets cut, but in terms of physical cutting and reproduction, it's simply 2 coils 90 degrees from each other, each 45 degrees off vertical, one for each channel. They are wired so that an in-phase signal (i.e., same signal in both channels; mono) moves the stylus laterally, while a completely out of phase signal (i.e., mono but with the polarity flipped in one channel) moves the vertically. If there's a signal in one channel but not the other, one groove wall has modulations and the other doesn't, and the effective stylus motion is at a 45 degree angle.

Matrixed formats are just a stereo signal, so nothing changes. CD-4 used an ultrasonic FM modulated signal to encode the rear channel difference information (with both the front and rear channels being cut in stereo normally). In a most basic sense, cutting and playback is no different from the basic stereo cutting described above, except the cutting and playback systems have to be able to handle frequencies up to 45kHz to preserve the FM modulated rear difference signal. Of course, when cutting at half speed, the cutting equipment would only have to be able to handle frequencies up to around 22.5kHz. But as far as the cutting and playback styli are concerned, they are just dealing with stereo signals that happen to have very wide bandwidth. It's the encoder and decoder that combine/split the audible and ultrasonic content.

 

[austinsignal]

I don't think it's all that different for QS records. The important thing to remember is that, unlike CD-4, any cutting lathe that can cut stereo masters, can cut matrix quad (SQ, QS, EV, et.al.) with no retooling needed.

,

That's effectively what gets cut, but in terms of physical cutting and reproduction, it's simply 2 coils 90 degrees from each other, each 45 degrees off vertical, one for each channel. They are wired so that an in-phase signal (i.e., same signal in both channels; mono) moves the stylus laterally, while a completely out of phase signal (i.e., mono but with the polarity flipped in one channel) moves the vertically. If there's a signal in one channel but not the other, one groove wall has modulations and the other doesn't, and the effective stylus motion is at a 45 degree angle.

Matrixed formats are just a stereo signal, so nothing changes. CD-4 used an ultrasonic FM modulated signal to encode the rear channel difference information (with both the front and rear channels being cut in stereo normally). In a most basic sense, cutting and playback is no different from the basic stereo cutting described above, except the cutting and playback systems have to be able to handle frequencies up to 45kHz to preserve the FM modulated rear difference signal. Of course, when cutting at half speed, the cutting equipment would only have to be able to handle frequencies up to around 22.5kHz. But as far as the cutting and playback styli are concerned, they are just dealing with stereo signals that happen to have very wide bandwidth. It's the encoder and decoder that combine/split the audible and ultrasonic content.

wow thanks!