[old_tv_nut]

A blanket statement that you aren't hearing the original quality just because the YouTube posting is digital is wrong. It depends on what was done to make the video.

If the sound for the youtube video is being picked up by a microphone in front of the loudspeaker, then the mic arrangement and room acoustics can affect the result much, much more than digitizing the signal ever could. However, the audio may have been processed to reduce noise, which would make a big difference as well.

So, are you interested in the overall result using the vintage phonograph? That may be what you have here. If you want to know the quality of the record and cartridge output, then you need a recording of the equalized signal. Again, straight digitizing without additional processing will not contribute any audible effects.

Disclaimer: using too low a bit rate for the audio could produce audible effects.

In this video you can definitely hear the level of distortion typical of recorded music of that time, which the digital processing does not contribute to in any audible way as far as I can tell.