You can't get audio quality from a poor mastering
Mastering is the process of taking an audio mix, and to manipulate it to get a balanced sound ready for distribution. Today it is done via the digital domain, or via physical product connected to a computer (DAW, Digital Audio Workstation), and it consists of fine-tuning of volumes via compression and equalization, and the use of various effects.
Mastering is the art of compromise; knowing what's possible and impossible, and making decisions about what's most important in the music. When you work on the bass drum, you'll affect the bass for sure, sometimes for the better, sometimes worse. If the bass drum is light, you may be able to fix it by "getting under the bass" at somewhere under 60 Hz, with careful, selective equalization. You may be able to counteract a problem in the bass instrument by dipping around 80, 90, 100; but this can affect the low end of the vocal or the piano or the guitar - be on the lookout for such interactions. Sometimes you can't tell if a problem can be fixed until you try; don't promise your client miracles. Experience is the best teacher.
Before judging an hifi component or a whole hifi system, did you know that also the recordings can be so manipulated as much as Photoshop can do with images? Do you know the secrets of audio mastering? The stereo image is only the result of a proper recording and mixing, often it is not something real. The more real it seems, the more clever was the audio engineer. But remember: its not real.
The best way to know what I'm talking, is to use just for fun an audio editing software to edit some audio file.
The waveform of a song, opened into an audio editing software
Music production software
Mixing vs Mastering
Autotune and the work of the audio engineer
Below a video about the possibilities of audio mastering to improve the voice of a singer. It is obviously exaggerated, but it is useful to quickly understand what can do an audio engineer, and also why so many "artists" always sings in playback:
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