Hi Doug and all Sandpaper fans!Sandpaper distortion is actually the carrier detection circuit switching on and off as it gains and loses the carriers in rapid succession. It is not a function of that actual noise coming from the groove. It is a function of the demodulation process if it's not going right.
The "breaking pencils" or "explosions" aspect only happens if the CD-4 system is not set up correctly. Scratches or dirt sound no different from two channel records in my system. As I have stated elsewhere, I have heard the various CD-4 distortions and actually did have some of them in my system when I re-entered the CD-4 world many years ago, now, but they were corrected, mainly through tracking adjustments.
You're right with Sandpaper, the carrier goes out of limits. Even if there is a small buffer for small dropouts, if the carrier is always pending around the lower limit we will get the sandpaper.
Found a good example at the Doobie Brothers - Stampede.
I recorded all at the same level, only adjusting the horizontal angel to get the optimum.
Pic 1 shows the 30kHz at track 2 (or any other) beforre the the music and at the last track 5. Of course the switch (carrier sense = matrix on) got problems to handle that.
Pic 2 With Audacities's equalizer I added 8dB for the carrier to bring em up to normal volumen - than it plays well without any distortions.
Pic 3 I clip off the carrier in both recs at 20kHz to get the normal sound we hear. You see, there is is no difference in the volumen between a formerly low or high carrier in the sound.
Roberta Flack Suzanne is also an example: they reduced slowly the carrier from the 6th minute till end. so I added the 30kHz 1dB after 7min, 8min and 9minutes to keep em upright = everything OK.