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Whether one is producing music, podcasts, game sounds, or film sound effects, Digital Audio Principles provides the tips and techniques that will make the project a success. Author Dave Schroeder explains the basics of digital audio production techniques and covers the essential hardware and software. He also discusses sound theory, frequency response, the range of human hearing, and dynamic range.
Now when you get ready to use audio file compression on your music or music files, you'll be faced with a few different parameters you need to think about and make decisions about in terms of what kind of compression and how much compression you are going to apply to a file. Now it might look similar, but let's remember to not confuse bit rate with bit depth. So bit depth, when we're dealing with audio file compression, it refers to the number of bits per second, and it's represented by KBPS, or the kilobits per second. So, the higher the bit rate, the higher the sound quality.
Another thing is that you can choose between a constant bit rate or a variable bit rate. And sometimes you'll see these as CBR, for constant bit rate, and VBR, for variable bit rate, when you're looking at your settings. Now with a constant bit rate, you're basically in control. Whatever bit rate you set, let's say you set it at 80, then that sound, when it's compressed, it's going to compress it at a rate of 80 kilobits per second for that whole file. But with variable, you're relinquishing a little bit of control, and you're entrusting the computer to make some decisions for you.
With variable you might pick a maximum or minimum bit rate that gets used, but as that compression takes place, the codec will make decisions based on the material that it's compressing, if it should use more bits or fewer bits. If you have a piece of music with a quiet section or not a lot of activity, it might use a lower bit rate. And this in turn means it's going to use less data for that part of the song. Then the end result will be a file that's smaller than if you used a constant bit rate. So if you were to use a variable bit rate on a piece of music that had some loud busy parts and some quiet soft interludes, the codec would decide when it gets to those different parts what kind of bit rate to use.
So if you apply a variable bit rate to compressing let's say a piece of music, and that has some loud sections and some quiet, empty sections, maybe it's an orchestral piece, the codec, as it's compressing that piece of music, will make decisions about what the bit rate should be based on how much information, or how much material there is at that point in time. So in the louder sections, it'll use a higher bit rate and in the lower quieter sections where there's not as much to have to sample, it'll use lower bit rate. So essentially a variable bit rate adapts to what it's compressing, and makes a decision about what bit rate is appropriate.
Now, the net result of using a variable bit rate can be that you end up with a file size that's much smaller than if you were to use a constant bit rate. Now on shorter files you won't notice a lot of advantage on this, but on a longer piece, like a 3 or 4-minute piece of music, using a variable bit rate can generate a smaller file than using a constant bit rate. Of course, the trick is to listen back to your file. Now regardless of whether you use a constant bit rate or a variable bit rate, it's important to always listen to the end result, because sometimes the variable bit rate gets it wrong, and makes bad decisions, and you'll hear that it didn't quite capture things quite right, because it's used to lower the bit rate.
Other times, you'll notice that the difference between the variable bit rate and the constant bit rate, in terms of file size, isn't that different. So it's always a good idea to make sure you listen to something after you've compressed it to make sure that a variable bit rate has behaved the way you want it to, and yielded a good result. Sample rate, on the other hand, is the same as our friend sample rate and the rest of the digital audio world. It refers to the number of samples per second. A higher sample rate, as we know, yields a better sounding file. It takes more samples, and you end up with a better representation of what you're trying to sample.
The trick here though, is that a higher sample rate also generates a bigger audio file, and we're trying to create a smaller audio file. So you have to find a balance between a sample rate that sounds good but also generates a small file. Usually with sample rates and compression, you want to go for 44.1 kHz. It yields good results. If you go lower than that, to like 22 kHz rates, you'll see that the quality is noticeably worse. So when it comes to making decisions about the settings you're going to use for bit rate and sample rate, it really becomes a matter of trade-offs.
You're always battling between size and quality. You want that small file, but you also want a file that people will listen to and not turn off because it's too hard to listen to or it sounds lousy. Generally, the combination of the bit rate of 128 kilobits and a sample rate of 44.1 kHz is considered acceptable quality. I think that's the best way to describe it. If you buy a piece of music from the iTunes music store and download the MP3, you're getting an audio file that's compressed at this rate. Finally, let's talk about the number of channels.
Usually when you're making decisions about your compression settings, you'll be able to choose mono or stereo. Now going mono, or one channel, can definitely help you cut down on file size sometimes. For certain applications, it's definitely fine. If you're doing a voiceover podcast or some sound effects for games, and things like that, mono files a lot of times are actually the way to go. And they actually can be a little bit easier work in certain situations. So don't be anti-mono. At the same time, it's always good to generate one mono version and one stereo version and just A B them, and go ahead and compare things like the sound quality versus the file size.
Then you'll be able to make a decision, and figure out which is the best file the use. Next, we'll talk about a few other adjustments and some considerations to keep in mind when you're compressing digital audio.
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