I am not a specialist in these things, but there are a few important things to remember.When transfering recordings of tapes or old 78s keep the crackly or humming start that you get before the music starts.It is this section that is used for computer cleaning of the recordings.Dependant on how bad the recording is a lot of noise can be removed, but it will effect the final recording and there is a point at which you will get serious distortion of the instrument sound against noise reduction.It is a very fine dividing line.Most people will however put up with some background noise to appreciate playing at it's best and some will listen to some of the most dreadful recordings just to hear a snippet of a great old player.
Jim Ward who lives in Crawley Sussex specialises in cleaning up old 78s and I am going to sit down with him and pass on some of the information used in this process(he has agreed to this).I have spent a few hours with Jim and it certainly is a job which needs experience,he can hear and sort out distortion that I cannot hear.He can home in on a sound wave and point out a wiggle that looks to me the same as the wiggles before and after,straighten it out by hand and the distortion is gone.Scratch noises appear as a straight line (no wave) so they are easy to delete. Some of the records and recordings from the 1920s sound only a few years old after Jim's work.
These old 78s have a fairly deep groove,there are people who can provide a stylus which will play say half way up the groove where perhaps the old needles used were playing at the bottom of the groove
This can make a remarkable difference to the recording sound as can proper cleaning of the 78 before recording.
Al