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Distributing audio over the Internet in a compressed format seems more like the rule these days rather than the exception. In fact, depending on your genre, 90% of your listeners may be listening to your mix in a compressed and or streamed format. While we can lament over the fall of vinyl and even uncompressed CD audio, the fact is that lossy compression codecs like MP3, AAC, and Windows Media Audio are more popular than ever and something you can't ignore if you want your mix to translate well into the outside world.
One thing you want to do when you're working towards a master that you know is going to be compressed for the web is start by selecting the proper encoder. Most of the formats are going to be lossy, that is to say that you're actually losing data in the compression process, unlike let's say, a zip file where you are zipping some data up and then extracting the back out into the exact same 1s and 0s as you had before compressing it.
When you use MP3 or AAC in non-lossless formats, you're actually losing the information forever. So even if you went back to a CD, let's say, you downloaded some of your favorite MP3s and then you've converted them to WAV files to burn to a CD, it's not the same as burning those original WAV files to the CD. You actually lost some information that you can never get back. So selecting the proper encoder is going to give you the best results.
Once you've decided on a format, whether that's going to be MP3 or maybe AAC because you're encoding it for the iTunes store, there are many encoders that you can use. Pro Tools has the Fraunhofer codec built in, in its export. So if I am doing a file Bounce to Disk, or if I'm exporting the clip from the Clips list using Export Clips as Files, if I choose the file type MP3, I'll choose Interleaved and hit Export, I get the MP3 encoding window that comes up.
No matter what encoder you're using, you are going to be given some decisions about the quality of the encoding or how lossy you want it to be, and this is known as the bit rate and it's really all about bit rate. In uncompressed audio, we're always talking about the sample rate and the bit depth, but when we are working with compressed formats like MP3 and AAC, it's all about this bit rate number. I can generally encode in a constant bit rate and that's going to use the same bit rate for the entire length of the file or I can use something called Variable Bit Rate or VBR.
What that's going to do is, it's going to use more bits in the denser sections where more things are going on and fewer bits when less audio information is taking place. In this case, I can choose as low as 16 kb or as high as 320. Now you can see this is also dropping down the sample rate here that 16 setting. Now unless you're really encoding it for maybe audio books or old-school telephone systems, typically we are going to want to be encoding our files at as high of a bit rate as we can.
Now that broadband Internet connections are pretty ubiquitous, I tend to send my MP3s at the highest possible bit rate either 320 or at least higher than 256. However, different sites have different requirements and one exercise that you can do on your own is just take this standard Pro Tools MP3 encoder and take the same track and do a few different encoding settings. So try one at 320 and compare that to the WAV, see if you can tell the difference between the two, and it's actually going to be fairly hard at such a high bit rate, and then start dropping it down.
And as you drop down to, let's say 128 or even 96, you are going to start hearing some fatigue in the high end, you are going to start hearing some aliasing in the symbols, and what it is really doing is that MP3 encoder is figuring out what part of the spectrum it can throw away. So by doing that, it kind of thinks about how humans hear and how that kind of rolls off, and it removes some material from the audio, and a lot of times you can hear that in the high end.
An exercise that I'd like to do all the time is go to YouTube and the stream will start at a very low bit rate. It will start at the 480p stream and then I'll kick it up to the 720 or the 1080 and what you'll notice is that the audio instantly gets betters because on YouTube, the quality of the audio is connected to the quality of the video and what you often hear is as soon as you kick it up to the higher bit rate, all the highs and all the brilliance of these symbols in the top end come back into the mix.
Actually, no matter what you're using to encode, whether it's the Fraunhofer, I personally use the LAME encoder and that can be rigged into almost any program, even iTunes. So just do a Google search for the LAME encoder and you'll find a ton of information. But regardless of whether using VBR or CBR, if you are going to use lower rates, or even if you plan on having your audio uploaded to sites like YouTube, Facebook, Bandcamp, things like that, there's going to be some compression and sometimes this compression is proprietary.
For example, SoundCloud kind of has its own compression and I know a lot of people that kind of use that as a lowest common denominator, how does my mix sound after I upload it to SoundCloud. And some of the things that you can do to optimize your mix for compression is for one, just listen to it with different types of compression, but in terms of making changes to it, a lot of times what's going on the high end and the low end can actually confuse the encoder. So if you have a whole ton of top end, it can actually force the priority of bits over to the range that our ears are less sensitive, that top end and it can sort of waste bits on a lot of high frequency material.
I hear people go either way. Some mastering engineers recommend boosting the high frequencies for compressed formats because you are going to lose some of the top end in compression and I've heard other mastering engineers say, no, I am actually going to carve some of that off. I am actually going to low pass filter above a certain frequency so that that encoder isn't getting confused by any ultra high highs and it's focusing on the critical mid band where our human ears are going to be listening most closely.
Ultimately, there's a lot of experimentation and figuring out what the different codecs are going to sound like and generally, I find I get the best mileage out of just getting a really great sounding mix in the CD 16-bit format, and I find that if I work really hard and get a really great sounding mix that translates to CD well, it generally translates really well to compressed formats also. When you're limiting for something that's going to be compressed, let's say, you're using Maxim; for CD I'll typically set my ceiling to just under 0, because that's really all I need to prevent the CD from clipping.
Now with encoding to MP3 or AAC, sometimes a bit of level gets added during the encoding process. So knocking this down to -1, you're ceiling to -1, in other words, leaving a dB of headroom at the top end of your master can actually make the encoded file sound better because the output of the encoding isn't actually clipped. So again, this is something you can experiment with. Apple released a PDF on guidelines for mastering for iTunes and if you go to their web site and look it up, it's got a ton of great information on optimizing your mix, sample rates, bit depths, how those get encoded into the iTunes formats, how lossless audio is changing the way that were distributing digital music and kind of what the future holds for that.
It's a really great read. I highly encourage you to check it out, but in the end, do the best you can. Many listeners may be able to hear the difference in listening tests, but it's actually kind of funny when asked if they ultimately care, most don't. Remember, people just love music and that's why you tend to hear people checking out music videos on YouTube, listening to songs on their iPhone speaker or their ear buds and they love it. So at the end of the day, make sure your content stands out.
So if you spend all the time worrying about your sound mix and how it translates on YouTube, but you forgot to actually create an interesting mix and an interesting song, that's what's really going to get you. So just put all this stuff into perspective when you're working on your mix and ultimately, your master.
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