There is an endemic problem on the production side WRT digital between approx late 1960s through early 1990's... That is, because of the historical processes of backing up tape from analog to analog, and then analog to digital, an important step in digital production has been skipped. This step is something that costs money, and the distributors are notorious cheapskates. Now, my description might be detailed, but worthwhile to read. I'll try to organize it so that it doesn't become a contorted mess...
History before digital... Copy DolbyA analog tape to analog tape... Just copy the tape.
History when digital came in: Copy DolbyA analog tape to digital tape... Just copy the tape.
Now, in this digital world, we simply do the appropriate mastering, the produce the digital copy. This mastering is done in the digital domain, and 'converting to analog->DolbyA decode->convert back to digital' has a problem... It takes time, and beancounters don't like spending time -- especially on re-issues.
So, what is the effect? Well, without doing a DolbyA decode we end up with a generally harsh sounding recording, too much high end (not really -- it is the compression associated with DolbyA partially flattening the spectrum.) How do we fix the problem CHEAPLY? Do an EQ of between -3dB or -6dB at 9kHz (Q=0.707 is usually used.)
How does the digital result sound? Still not exactly the same as vinyl... Doesn't sound super-bad, but is not super-good either. The EQ helps a lot, but doesn't fix the compression (which is hard to detect because it is between -20dB and -40dB, only 10dB or 15dB range, and 30msec-60msec release time for the HF bands.)
So -- some number of recordings made between late 1960s and early 1990's, when digital versions are distributed, the material has not been properly decoded. The historical vinyl creation process has OPTIONALLY included DolbyA decoding, but less often digital recordings got DolbyA decoding (however, I have found at least one copy of undecoded vinyl.)
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I have some demos online, in the middle right now producing some decoded (well 60second snippets) of Brasil'66 and Herb Alpert examples. I already have 99Red Balloons and some ABBA online right now. These examples are before/after decoding and comparing the DHNRDS decoder along with an actual DolbyA (by virtue of properly decoded CDs and vinyl releases.)
Tried to keep the demos a sane length -- I want the IP owners to make more money -- not subvert them... (I don't want the stuff to be expensive either -- but I am just rambling like I usually do.)
Here are some examples, with Brasil'66 and Herb Alpert decoding examples to come (might notice the great lack of hiss in these, because most releases that I have heard are hissy -- probably compressed and not decoded both... Really bad.)
Note that my decoding examples are NOT perfect because consumer recordings don't have DolbyA reference tones. I did the best that I could without tones. Brasil'66 and Herb Alpert examples are coming in about 1-1.5Hrs -- I am in the midst of decoding them with the highest quality that I can do. (I am NOT mastering them -- so the results will be very raw.)
https://www.dropbox.com/sh/mduka8faqv1nva7/AAATBBBRIFDht8pVsDN5Dv7Aa?dl=0