"First, the recording must be carefully cleaned
and played to get the maximum out of the grooves."
I can't believe I simplified it that much... actually the sequence of the playback process is something like this, presuming "78" rpm phonograph records:
1) Determine what type of disc is being dealt with... laminated, shellac, soft cut - lacquer (glass, steel, cardboard or aluminum base) etc.
2) Select the correct chemicals and method to clean the surface of the disc to remove embedded dirt (pizza and soda pop) and/or any residual mold release compounds left over from the pressing process.
3) Clean the disc *carefully* to avoid making any additional scratches or doing any other damage to what could already be a fragile item.
4) Determine if the disc was centered properly when it was pressed and the center hole was punched... if not, the disc will need to be carefully centered during playback to avoid adding "wow" to the finished transfer.
5) Examine the grooves to determine the correct size and shape of stylus required to get the best transfer of sound from the disc. Sometimes an odd size of stylus will get above or below areas damaged by previously playing with poor equipment or a worn stylus.
6) "Pitch" the recording, to determine the correct playback speed... many "78"'s were not recorded at 78 rpm. Some varied from 76 rpm to 80 rpm, and an obscure few very old ones could have been recorded as low as 60 rpm or as high as 90 rpm.
7) NOW, you can PLAY the disc to hear the sound as it was originally recorded. A stereo cartridge allows the quietest groove wall to be selected for further digital processing.
Only now can the rest of the Audio Restoration process can continue...
The sound is converted to digital form, in a high quality Analog to Digital converter, and some very heavy-duty DSP (Digital Signal Processing) equipment begins to process the sound, removing the clicks, pops, crackle and even some distortions caused by groove damage. Often, hum was present in the original recordings, and this is removed also.
Probably the most spectacular result of audio restoration is the surface noise removal, and final equalization to make the recordings sound as good as possible on current day equipment.