To match the tone, volume and dynamics of a client's project, to releases of a similar genre.
The original job of the mastering engineer was very technical in nature. They had the very difficult task of converting the 'electrical signals' of the analogue master tape, to the purely 'mechanical' ones of the lathe's cutting head. Mastering engineers required many years of training with other senior mastering engineers, dealing with problems such as: stereo signal phase, quick volume changes and frequency response ranges. Mastering to vinyl also had additional issues with the physical location of the songs on the disk. Songs near the outer edge had an easier time than those placed near the inner groove. The only fun they had, was the 'inner groove' writings that sometimes materialized.
The art of the traditional mastering engineer was a world of an almost hoogie boogie, technical magic!
Once the era of the CD became standard, many of the traditional limitations with the transfer from an electrical signal to a mechanical one, ended. Whether mastering from analogue tape, digital tape, or computer files. Signals could now: be out of phase, contain frequencies from 2 hz to 22 khz and could even have bass panned hard left or right! The transfer to the CD media was basically flawless. However slowly, a problem arose that we're still dealing with to this day, the "loudness wars". This is where compromises to the dynamics and frequency response, have made almost all modern releases un-listenable, for long periods of time. I always ask my clients what volume they want their master at. I also explain my true volume goals in: dBFS = 0VU, true RMS or the new LUFS standards.
Dragonfly Collaborative Mastering Service:
In the modern era of musical production, most projects are mixed with a DAW's. The resulting digital files are then sent to the mastering engineer, either by physical mediums such as data disks or internet based FTP / Dropbox services. The key point here is that DAW based mixes can very easily be "recalled".
My method of working with a client is to very carefully listen to your mixes on my system and make all the adjusts I can. There is a point however of diminishing returns, where no mastering engineer can for example lower the level of the centre panned electric guitar compared to the lead vocal, etc. At this point I send the client a first "Mastered Mix" version, with a list of comments about - levels, eq's or dynamic changes that would greatly enhance the final listening experience. This method allows you the client, to review the mastered mix and notes. You can then re-open your DAW project, make some quick changes and send a revised mix for mastering and a possible further review. There are no additional costs involved with this method.
The end result is a fantastic sounding master, prepared for any output media you choose.