Mastering's kinda a funny process in that different people think of it in different ways, and it seems pretty ambiguous as to exactly what its actual role is. So, I'm gonna say a bunch of stuff that is mostly obvious here but from my perspective as a mixer....
To me, mastering is just taking all the separate mixes and putting them together into the final product - the master - that can then be sent to the factory to make all the copies you can sell.
That's pretty simple, so I like thinking of it like that. Anything else stems from that basic goal.
The thing is, a mastering engineer will do whatever he needs to to achieve that. At the very least he's got to take the mixes and assemble them onto a master CD (or a vinyl master in the old days) so that the tracks all start in the right place and the gaps between the songs are appropriate.
That's pretty simple too, which I like.
To make that master disk, the mastering engineer needs to be aware of the technical requirements of the format (ISRC cods,red book standard for CD, sample rate, bit depth, running length, or for vinyl the limitations in low end and stereo spread that are needed to make sure the needle doesn't jump out of the groove). This is all well within their remit as a mastering engineer, so that's all cool - you don't need to worry about any of it because you're paying them to master your mixes.
That's a bit more complicated, but I like it because I can sit back and watch them do what I paid for.
Now, it just so happens that mastering engineers listen to music pretty much constantly, so they hear loads of different genres and have a very good understanding of the norms and expectations within that genre in terms of overall volume and tonal balance. They do most of their listening in their very expensive rooms with very expensive gear, so when they hear your mix they'll notice if something's a bit out of whack relative to the music they usually hear. They can use their experience and the tools at hand to process your stereo mixes to mitigate certain problems, bring the loudness into the expected range and subtly bring the stereo mix more in line tonally with what's expected from the genre. They can also use processing to make all the mixes sound like they belong together on the same CD, and control the relative volume and tone between different tracks to achieve this.
This is as far as a mastering engineer goes before I start to feel like I've messed up somewhere.
Because good mastering engineers have great sounding rooms and gear that they know intimately, and many mixers these days have sub standard gear, monitoring, recording chops and taste (I'm talking about myself here guys
) you end up in the slightly embarrassing situation where you've done a mix that you think sounds good, then you take it to the mastering studio and the mastering engineer immediately points out a bunch of issues that need resolving, and just to give some examples I've been guilty of myself I'd submit these;annoying resonances in acoustic guitars, harsh upper midrange peaks on voices, clicks in the audio, muddy bass, overly dull or bright mixes, badly set bus compressors, and mixes that are totally out there in terms of volume between sections.
In all these cases, the mastering guy was able to do a lot of corrective work with the stereo masters, but realistically it would have been better if I'd addressed them in the mix - especially because addressing them would probably have caused a knock on effect where I made a load of other changes - but my monitoring just wasn't up to snuff for me to notice them, so I was living in happy ignorance. And while listening to your mixes on your home and car stereos etc can help you identify problems, I really think that only takes you so far. I mean, have you tried *mixing* on a car stereo? It's fucking impossible, you just don't know what you're hearing.
My experience with mastering, both attended and remote sessions, has been that it's an important part of the process essentially as a quality control check and to get a fresh set of experienced ears on board. I've learned more in a single day at a good mastering room, just sitting and listening to my mixes over a £20k set of speakers while a pro talks me through his thoughts and process, than I've learned in 6 months of blindly twisting knobs in front of my old shitty £60 Tannoys.
Some things muddy the waters though;
- Mastering plugins. What the fuck are they for? They don't give you a master CD ready for replication, they don't upgrade your monitoring so you can actually hear the problems you need to solve in the first place, and they, despite what their marketing teams say, aren't smart enough to make the decisions that will enhance your mixes. All they are are a bunch of presets and a tool to make your existing mix louder, wider, and try to fit your mix into the sonic profile of an arbitrary commercial sound. Thing is, if your monitoring or lack of experience has caused you to make mistakes in the mix, what's to stop you making more mistakes as you adjust the mastering plugin?
- Automated mastering like Landr etc. Fine if you don't care about the sound of your music very much, but I'd challenge anyone to actually go and sit with a mastering engineer for a day and see the number of judgements they need to make in order to both deliver a commercially viable sound but also enhance the artistic intent of the artist. A computer just won't do that, not until the machines become self aware and exterminate us all.
- Stem mastering. Some cheaper mastering engineers offer stem mastering because they know most of their clients are going to give them shitty mixes, pay them peanuts and expect to receive back fantastic sounding masters. With stems that's a bit easier because they can basically reach deeper into the mix to fix issues and finish the mix themselves. Which is fine, I guess, but I don't want my mastering engineer to mix my record; I want them to master it.
- Online distribution. If you don't intend to release a CD then part of the technical reason for mastering is null and void. At that point you need to decide if your mixes are going to benefit from the experience of a mastering engineer to make them better, which becomes a value judgement that's quite difficult - at least when you leave an 8 hour album mastering session clutching a master CD, you feel like you've paid for a THING. When you just get a stereo file emailed back to you for you to upload to soundcloud, youtube etc, all you know is that it's louder and it sounds a bit different over the same monitors you used to mix it. Is different better? If it sounds better now, why didn't you just mix it that way? It becomes a lot less well defined, though hopefully the mastering guy can talk you through what he actually did, any problems he identified, and convince you that they were worth what you paid them.