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Bit rate, sample rate, and channels

From: Digital Audio Principles

Video: Bit rate, sample rate, and channels

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.

Bit rate, sample rate, and channels

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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This video is part of

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Digital Audio Principles

110 video lessons · 26815 viewers

Dave Schroeder
Author

 
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  1. 50s
    1. Welcome
      50s
  2. 39m 10s
    1. What is sound?
      4m 15s
    2. Hertz and frequency response
      5m 34s
    3. Phase
      2m 39s
    4. Capturing audio
      3m 39s
    5. Sample rate
      6m 16s
    6. Bit depth
      9m 47s
    7. The waveform
      5m 3s
    8. Audio file formats
      1m 57s
  3. 7m 25s
    1. What is a digital audio workstation?
      2m 59s
    2. Typical DAW signal flow
      4m 26s
  4. 50m 33s
    1. What microphones do
      1m 57s
    2. Element types
      5m 0s
    3. Pickup patterns
      6m 51s
    4. Axis
      2m 52s
    5. Frequency response and the proximity effect
      5m 10s
    6. Phase issues
      1m 41s
    7. Microphone types
      8m 44s
    8. Miking vocals
      5m 39s
    9. Miking amplifiers
      2m 17s
    10. Miking drums
      10m 22s
  5. 16m 39s
    1. Cables and connectors overview
      2m 42s
    2. Balanced and unbalanced cables
      3m 19s
    3. Common cable types
      7m 13s
    4. Cable tips
      3m 25s
  6. 12m 16s
    1. What is an I/O device?
      1m 41s
    2. Analog to digital conversion
      3m 10s
    3. Tour of an audio interface
      4m 49s
    4. Interface considerations
      2m 36s
  7. 21m 5s
    1. What is a preamp?
      3m 21s
    2. Input levels
      5m 29s
    3. Padding
      2m 18s
    4. Phantom power
      2m 37s
    5. Phase reverse
      3m 4s
    6. Preamp demo
      4m 16s
  8. 12m 56s
    1. What is a mixer?
      5m 55s
    2. Input section
      1m 17s
    3. Channel strips
      3m 16s
    4. Master section
      2m 28s
  9. 18m 21s
    1. What is monitoring?
      2m 11s
    2. Speakers
      4m 47s
    3. Room considerations
      5m 43s
    4. Headphone types
      3m 50s
    5. Monitoring levels
      1m 50s
  10. 15m 23s
    1. What role do computers play?
      1m 36s
    2. Performance issues
      4m 11s
    3. Hard drives
      4m 38s
    4. Mechanical noise
      2m 10s
    5. Authorization
      2m 48s
  11. 6m 54s
    1. Planning for recording
      54s
    2. Doing a system check
      1m 26s
    3. Planning your inputs
      1m 42s
    4. The recording environment
      2m 52s
  12. 25m 52s
    1. Types of digital audio software
      38s
    2. Multi-track recorders/sequencers
      4m 56s
    3. Two-track recorders/waveform editors
      4m 55s
    4. Loop-based music production software
      5m 44s
    5. Plug-ins
      6m 56s
    6. Other varieties
      2m 43s
  13. 18m 59s
    1. Common components
      46s
    2. The transport
      2m 4s
    3. The toolbar
      3m 19s
    4. The Edit/Arrange window
      4m 42s
    5. The mixer
      5m 8s
    6. The file list
      3m 0s
  14. 19m 17s
    1. Setting up a session
      3m 30s
    2. Assigning inputs and getting signals
      3m 19s
    3. Input modes
      3m 28s
    4. Overdubbing and punching
      5m 14s
    5. Bouncing down
      3m 46s
  15. 19m 42s
    1. What is editing?
      1m 21s
    2. Waveforms
      2m 53s
    3. Making silent cuts and trims
      7m 1s
    4. Fades and automation
      8m 27s
  16. 1h 23m
    1. What are plug-ins?
      3m 0s
    2. Using plug-ins
      6m 11s
    3. EQs
      7m 4s
    4. Dynamics pt. 1: Compressors, limiters, expanders, and gates
      5m 40s
    5. Dynamics pt 2: Applying dynamic effects
      7m 2s
    6. Pitch shifting
      6m 14s
    7. Reverb
      9m 28s
    8. Echo and delay
      6m 23s
    9. Modulation effects: Phaser, flanger, and chorus
      9m 39s
    10. Sound tools pt. 1: About, gain, normalize
      7m 39s
    11. Sound tools pt. 2: Reverse and time compression/expansion
      6m 29s
    12. Sound tools pt. 3: Noise reducers, dither
      8m 11s
  17. 23m 43s
    1. What is MIDI?
      3m 6s
    2. Keyboard controllers
      1m 23s
    3. Computer-based virtual instruments
      1m 6s
    4. Control surfaces
      1m 6s
    5. Recording and editing MIDI
      12m 4s
    6. Virtual instruments
      4m 58s
  18. 27m 29s
    1. What is mixing?
      1m 54s
    2. Some common objectives
      3m 4s
    3. Some useful techniques
      5m 59s
    4. A quick mixing demo
      16m 32s
  19. 18m 48s
    1. What is mastering?
      2m 24s
    2. Sonic maximization
      9m 43s
    3. Final preparations and exporting
      6m 41s
  20. 13m 34s
    1. What is audio compression?
      2m 16s
    2. Popular formats
      2m 9s
    3. Bit rate, sample rate, and channels
      5m 42s
    4. Other adjustments and considerations
      3m 27s
  21. 15m 6s
    1. Essential gear
      7m 36s
    2. Voice recording setups
      1m 43s
    3. The voice production process
      5m 47s
  22. 10m 4s
    1. Analog vs. digital
      2m 48s
    2. Tube vs. solid state
      5m 6s
    3. The continual upgrade
      2m 10s
  23. 18s
    1. Goodbye
      18s

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