The following is from a letter to Mr. Tim Neely, author of the Goldmine Record Album Price Guide.
If you will, allow me to point out an area of record differentiation which your guide does not address but which I believe is very essential to the sound of the record.
Let me refer to Fleetwood Mac's Rumours LP as an example of this issue. I believe it is the case that nearly every song on this record became a Top 40 or Top 10 hit in 1977 or 1978. I was very moved by many of these songs on the radio and also on an 8-track copy of the album which my visiting cousins had brought to my house.
Around 1988 or so I visited a friend and played his copy of the LP and the music on that occasion did nothing for me. I was really puzzled at this and disappointed.
What I now believe is that his copy of Rumours was, despite having the same number as the original and early pressings of the LP in the late 70's, a version which had been fed through a digital signal processor before or during the cutting process.
I have read that in the stamping of coins, a given set of molds only has a limited lifespan before it will not produce impressions that are deep and clear enough. Presumably the same is true of recording stampers. The stamping plant needs to produce new stampers from time to time.
Apparently, as the CD era dawned, it became routine to use digital signal processors in the signal path between the master tape and the stamper as it is being cut.
Due to straitened circumstances I have been and remain unable to perform examinations of record grooves by microscope myself. But I think I know what to look for. Because every sonic impulse on a digitalized record occurs at and only at those moments when samples are taken, by adjusting the magnification properly you should be able to identify perfectly regular 'steps' or levels - perfectly regular in the sense that they occur at identical intervals according to the sampling frequency.
Note that for recent LP's that may have been processed with megahertz-rate equipment, you will need to use greater magnification to identify these regularly spaced beats or pulses.
Your guide mentions that in the 2 years or so since the 4th edition you had seen "countless articles about the resurgence in vinyl." I believe I may have been somewhat responsible for this: in late 2004 I posted an item to the users' forum on kournikova.com titled Digital Audio Is Not Music! in which I pointed out that regardless of how fast the sampling rate, there are an equal number of instants between each sample when the digital system is non-functional.
In practice this means that when a sample is not being reproduced the digital system presumably 'rings' - or buzzes as the case may be - in idleness until the next sample occurs. Under even greater magnifications this too should be observable under the microscope.
My hope is that if you validate these observations, you will share your insight with others and the process of distinguishing vinyl by whether it has or has not been digitalized can begin to be developed and recognized. (Conclusion of letter excerpts.)
I urge interested readers who are able to make use of high quality microscopes to look for the features of digital processing I refer to here.