As Yoda would have it!
Or maybe not. Ahem. Let's start.
In the post "What is mastering?" we looked at (you guessed) what digital mastering is. And turns out that a big part of it is correcting imperfections in the mix, where "imperfections" are not so much about sound, balance or genre choices, but about how a mix keeps (or not) its distinctive sounds on different playback systems.
An obvious followup question is: "Can I do it myself?" (as opposite to sending the mix to a mastering house).
The answer is generally "no" - and not because it's forbidden or impossible, but because it tends to be kinda pointless.
Why? Let's see.
By definition, as mixing engineers, we want to produce the best mix we can.
Among other stuff, a "good mix" is a mix that retains its sonic signature across different playback systems.
The "sonic signature" being the result of the mixing choices you make: balance, sounds, what to mute, what to put in the spotlight, the reverbs you use, panning, stereo image and so on and so on.
However, sometimes we come up short, and the mix is - alas - less than perfect. There's stuff to fix - because it doesn't play equally well everywhere: bass disappears on phone or computer speakers, high become harsh when heard on large PA system, the mids get tubby on that grotbox and so on.
And how can that happen?
Well, assuming that you're reasonably competent, the only reason for which said imperfections can be there, is because you couldn't hear and fix them during mixing. And you couldn’t hear them, because of the limitations of the monitoring system and the room in which we mix.
And that is why it makes zero sense that a track is mastered in the same room and with the same playback/monitoring system which was used for mixing.
First of all, a mastering room, and the monitoring system in it, have to be better than the average mixing station. That's because the whole point is that stuff that couldn't be heard when sitting at the latter, can be heard in the former; and playback situations that couldn't be tested in the latter, can be te tested in the former.
But mixing stations tend to be already pretty good.
That means that a mastering room must sound better and have better monitoring than something already good.
That is to say, have superlative and customized physical design and construction, really effective room treatment, and top shelf equipment and monitoring gear, which allows to represent most playback systems… in other words, be a very special room with very special kit inside (to say nothing of expensive!).
So, almost by definition, a quite different - and superior - listening room.
It also makes little sense that a track is mastered by the same person who mixed the track.
Not so much for a matter of experience or golden ears (a good mix engineer will have plenty of both), but of familiarity with the sonics of the material.
If a mix is indeed less then perfect. and needs more than removing working headroom, downsampling etc, it will be very hard for the mixing engineer to de-condition him/herself and really hear what's going on, so to be able to fix it.
Remember, imperfections aren't about mistakes or balance / sound choices (these are all artistic decisions): they are about how much the mix plays as well on a tiny phone speaker as on a full blasting stadium PA.
When you know the mix as intimately as you do when you've built it (and listened to it uncountable times in the process), it's almost inevitable that your brain won't allow you to notice imperfections that would be glaringly obvious to an (experienced) person not familiar with the material.
Sure, certain issues will be just as obvious to you, as to anybody else; and it is possible to reset the ears to a degree... but efficiency and time make so much easier (and quicker) to ask someone else to do the listening&fixing: a mastering engineer, who's not heard your song ever before.
Obviously there's also skills: some specific tooling and metering that a mastering engineer will be initially more familiar with than you.. but that's really a minor point, as any skill can certainly be learnt.
The main issue is simply too much "closeness" with the material.
I've mentioned this briefly already in "What is mastering?", but it's worth repeating: from all of the above, it also follows that it makes little sense to ask a mastering house to "check your mix" in sonic or balance terms.
There is simply no "right mix" from other points of view than playability.
Balance and sound choices are artistic ones... unless they impact playability - which is the only reason to change the sound (and even then, change it as little as possible).
So a mastering engineer may warn you that your wonderful stereo reverb collapses in mono, but in general he will not question your choice or reverb, or your kick/bass balance and the likes. It is what it is.
Actually, any "pro" mastering engineers will assume that the sound in the mix is the sound you want, and will be very cautious in changing it, or even suggesting changes.
If asked for a "check", all he/she could do is offer a subjective opinion, which is a risky sport when you're criticizing your customer's work and artistic vision... so they will be very reluctant, in general, to do so. And even when asked, they often will ignore the question or reply with trivialities, unless of course there's glaring errors.
If a given mastering engineer is expert in a particular genre, he might give you a pointer or two on where and how your track adheres (or not) to the genre's convention... but since (future) great songs often break these conventions, they seldom will.
Yes, it's true that nowadays, lots of mixes are produced by beginners. who are still less skilled than seasoned industry pros - and who could certainly benefit from receiving some mix critique beyond the subject of multi-system playability... but the fact of the matter is that in most cases it is impossible to say what is an error and what is an artistic choice.
In the end sonic choices are just opinions. The proof of the pudding is in forums and mixing group worldwide: beyond a basic, minimal level, everyone asking for a "mix critique" ends up receiving lots of opinions: other people's ideas on how their mix should sound.
Very seldom if ever will someone come up saying "hey, this sounds like a totally different mix when played on my phone". And there's no recipe for commercial success or "rightness" of a production: plenty of unconventional mixes have become very successful (and typically establishing an entire genre).
There's nothing wrong with receiving opinions, of course, especially for less confident beginners, who still have to develop their own mixing voice. But it's still opinions, not absolute judgements.
Whereas a master either plays well on most systems, or it doesn't.
And what about online mastering services?
Well, in the other post we already found out that three out of four steps are relatively mechanical.If you can operate a DAW, you can execute them. No need to pay for it!
Again, the fourth step is the one about cross-system playability, and the minimum sonic changes required (if any) to ensure that.
How do online services fare on this fourth step?
You be the judge. Ideally an automated mastering system should do the same as a mastering engineer, that is to say, return a result which is sonically very similar to what you sent in (only louder and with CD sample rate and word length).
By definition, on your playback system (where it already sounded good) it should sound just as it did.
So, if the masters you get back are consistently different from what you put in (brighter, more scooped, with tonal changes, whatever) it means either that your mixes are really really bad (i.e. unplayable across different systems without so drastic changes to make it impossible to preserve your sound) or that the online service is imprinting on your mix its own pre-coded opinion on how things should sound.
Whether or not that's good, it's up to you to decide.
So there you have it. You certainly can perform the more mechanical aspects of mastering by yourself, but you risk to miss the most important human contribution - the ears and experience of someone who can tell how something is gonna sound on different playback system without actually having to try them all.