Mastering is the final step of the production for a piece of music, the last filter before a song is ready for its releasing, this point is very important because is the last chance to fix those errors that have taken place during the mix, correcting the weak points and standing out the best ones, adding room, punch, depth, brightness, glue for a more homogeneous listening along a single or a full disc, definition and also that amazing warm sound which brings an intelligent/experienced use of top notch analog gear. All this into an acoustically treated neutral room controlled by an experienced engineer with extensive knowledge of his studio and a proper sound system. It doesn't matter if destination is either an Audio CD or a master for digital distribution, your music will sound more competitive and it will translate much better in most audio playback systems.
While it is true that in most cases a mastering for digital distribution or CD can also be transferred to vinyl, due to the peculiarities of this medium, we can make the most of the session if we make the process by keeping in mind the advantages and limitations of the format. When we are performing a mastering for vinyl, some factors like dynamic, stereo image or some extreme on determined frequencies are treated on a different way, which alongside another number of steps result on a much more proper master for this medium. If the sound of your record was intended to sound on a 7", 10" or 12" disc, don't hesitate to request this service to make the most of the sound of your music.
Tools as linear phase equalizers, dynamic EQs, MS processing (Mid/Side) and even multiband compressors, are often helpful when we get into those mixes which need more work yet before being processed through our analog mastering chain, however those tools are not always enough if the result are not 100% satisfactory with regards to our high standard. In such cases, the best solution is usually a process halfway between mixing and mastering, called "Stem Mastering".
When the client requests this service, either on his own or advised by the mastering engineer, he provides to the studio a specified number of tracks, which can be single tracks as "Kick", "Pad", Lead Instrument... or several group tracks.
Unlike what happens in a mix, each track is not worked from scratch, but each file is processed from the settings made by the client, ie, each single track or group of tracks will retain all of the processes you did during the mix, (eq, compression, delays, reverbs, etc), regardless of whether they are inserts per channel or bus sends, engineer will work from them unless he asks to remove some process specifically.
Stem Mastering ensures you a higher (or much higher) final result on those mixes that are not sounding to be mastered yet, over the result you would achieve on a normal stereo mastering session.
DDP - Disc Description Protocol
It's a disk image of the premaster for replication, which offers a number of advantages over the PMCD (premaster Compact Disc), such as faster and more convenient to send, greater security against all types of errors not being a physical media, no additional shipping costs, etc... DDP is the most accepted format for many replication plants these days.
A DDP contains 4 files:
Audio image (.DAT file), DDP Identifier (DDPID), DDP Stream descriptor (DDPMS), Subcode descriptor (PQDESCR)
Optionally, (in DDP 2.0 format), a text file containing track titles and durations can be included, this kind of file is called "CD-Text" and is supported by all those audio players, (soft o hard) capables to read this format.