There are a lot of myths and personal opinions when it comes to lossy vs. lossless compression formats, and you'll probably get 100 different answers. Here's my view:
1. In a blind test, you may always hear a difference, but that doesn't necessarily mean that you can sort the lossy compression from the control (the unmodified test sample)
But that's the endless debate of hearing
the difference. There is always a technical difference, which can be seen if you compare waveforms. When it comes to digitizing music that you are to process later, you'll want to keep a lossless format for as long as possible until you're done with the audio processing
. Apart from a few very special audio codecs through history (like BBC MPEG1 Layer 2), re-compressing an audio file will degrade the signal a bit more every time you do so.
The answer to the second half of question 1 - "Is [hearing the pre/post compression difference] relevant when the source audio is a vinyl rip rather than a professionally produced CD?":
- I would argue that it is actually more
relevant when ripping vinyl than a CD. The challenges for the compression algorithms are the same, they're just more abundant in the case of vinyl. Let me explain: The MP3 algorithm uses time-window analysis to determine the ear's natural masking in three domains - frequency, time and also phase (for mp3s encoded in "joint stereo"). While fine nuances in frequency and phase may not be too ugly after compression (like diffuse overtone structures or out-of-phase effects), the by far most problematic variable is small nuances in time. A classic example is a simple transient, which is only 1 sample long, but through compression gets blurred out into the full length of the time-window. A vinyl record is full of such transients (pops and crackles), so the noise of the vinyl medium puts more stress on the compression algorithm than a typical "noiseless CD".
2. I know friends who spend lots of time using de-hissers and de-cracklers, and high-end software can sometimes do a surprisingly good job. I generally avoid using such tools myself, since you usually can hear artifacts from the process anyway, and the artifacts sound more disturbing than the original noise IMO. However, I do remove the worst popcorn effects manually, using the pencil tool in Audacity (one may just as well run a heavy low-pass filter across the few samples in question to get a similar effect).
Second half of question 2: Apart from most encoders using a 20Hz high-pass filter before encoding, I don't see any reason at all why compressing an audio file can produce a (technically) better sound. Unless we're talking the effects of dynamic
compression, but that's a totally unrelated topic and is something that we should keep out of this discussion to avoid confusion.