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16 Bit and 24 Bit Audio
Discussion of the mysteries behind bit-depth, sample rates and sound quality
This article is going to be kept as simple as possible. It's just designed to get the new person up to speed on the issues and provide a strong sense of perspective on what really matters. We are going to talk about bit depth and sample rates, how these translate into storage requirements, and then talk about the subjective differences between the two methods of recording your music. In short, what is the relation of 24 bit recordings to the "sound quality" we all want.
|Never refer to 16-bit or 24-bit audio as the "bit rate". Its properly called the bit depth.|
When I first ventured into digital audio, it was a much simpler world. Products that recorded and generated digital audio were all 16 bit. Compact discs, the main method for music distribution, has digital audio that has a bit depth of 16 bits and a sample rate of 44.1 kHz.
Gradually, products began to appear with a higher bit depth--an 18 bit drum machine, 20, then 24 bit effects processors. Then recorders made the leap to 24 bit. Today, your audio interface is probably 24 bit and allows you to select sample rate of your choice, 44.1, 48, 88.2, 96 and even 192khz. Multi track recorders vary between 16/44.1 and 24/96. When you buy one you have to decide which way to go and get it right the first time.
So what do all these number mean and how important are they? That's where we are going to go. First, you have to have these definitions under control.
Bit Depth refers to the number of bits you have to capture audio. The easiest way to envision this is as a series of levels, that audio energy can be sliced at any given moment in time. With 16 bit audio, there are 65,536 possible levels. With every bit of greater resolution, the number of levels double. By the time we get to 24 bit, we actually have 16,777,216 levels. Remember we are talking about a slice of audio frozen in a single moment of time.
Now lets add our friend Time into the picture. That's where we get into the Sample Rate.
The sample rate is the number of times your audio is measured (sampled) per second. So at the red book standard for CDs, the sample rate is 44.1 kHz or 44,100 slices every second. So what is the 96khz sample rate? You guessed it. It's 96,000 slices of audio sampled each second.
So lets put it all together now. This brings us to the Bit Rate, or how much data per second is required to transmit the file, which can then be translated into how big the file is. Your CD is 16bit, 44.1 so that is 44,100 slices, each having 65,536 levels. A new Audio interface may record 96,000 slices a second at nearly 17 million levels for every slice. If you think that is a lot of data, well, you are right, it certainly is. The Bit Rate is usually expressed in Mbit/sec. But you don't need to do all this math. I'm going to do it for you. This is not an important area in the recording process to get sidetracked on. What is important for you is how this translates to your hard drive storage.
Space required for of stereo digital audio
|Bit Depth||Sample Rate||Bit Rate||File Size of one stereo minute||File size of a three minute song|
|16||44,100||1.35 Mbit/sec||10.1 megabytes||30.3 megabytes|
|16||48,000||1.46 Mbit/sec||11.0 megabytes||33 megabytes|
|24||96,000||4.39 Mbit/sec||33.0 megabytes||99 megabytes|
|mp3 file||128 k/bit rate||0.13 Mbit/Sec||0.94 megabytes||2.82 megabytes|
So you see how recording at 24/96 more than triples your file size. Lets take a 3 minute multi-track song and add up the numbers. Just to put the above into greater relief, I included the standard MP3 file's spec.
|Bit depth / sample rate||Number of mono tracks||Size per mono track||Size per song||Songs per 20 GB||Songs per 200 GB|
|16/44.1||8||15.1 megs||121 megs||164||1640|
|24/96||8||49.5 megs||396 megs||50||500|
|16/48||16||16.5 megs||264 megs||74||740|
|24/96||16||49.5 megs||792 megs||24||240|
Note that these size do not count any out takes you may have recorded but didn't delete and they assume a linear recording method from beginning to end of the song for every track. They also don't account for hard disk sector size which will leave a lot of space on the drive empty. Real world mileage may vary.
So you should be noting two things now:
- Recording at 24/96 yields greatly increased audio resolution-over 250 times that at 16/44.1
- Recording at 24/96 takes up roughly 3 1/4 times the space than recording at 16/44.1
Should you record at 24 bits?
Now lets get to the subjective side of how music sounds at these different bit depths and sample rates. No one can really quantify how much better a song is going to sound recorded at 24/96. Just because a 24/96 file has 250 times the audio resolution does not mean it will sound 250 times better; it won't even sound twice the quality. In truth, your non-musically inclined friends may not even notice the difference. You probably will, but don't expect anything dramatic. Can you hear the difference between an MP3 and a WAV file? If so, you will probably hear the difference between 24 bit and 16 bit audio. It gets real subjective hear as our ears are different. But lets try to be a little objective here.
Lets talk about sample rate and the Nyquist Theory. This theory is that the actual upper threshold of a piece of digital audio will top out at half the sample rate. So if you are recording at 44.1, the highest frequencies generated will be around 22kHz. That is 2khz higher than the typical human with excellent hearing can hear. Now we get into the real voodoo. Audiophiles have claimed since the beginning of digital audio that vinyl records on an analog system sound better than digital audio. Indeed, you can find evidence that analog recording and playback equipment can be measured up to 50khz, over twice our threshold of hearing. Here's the great mystery. The theory is that audio energy, even though we don't hear it, exists as has an effect on the lower frequencies we do hear. Back to the Nyquist theory, a 96khz sample rate will translate into potential audio output at 48khz, not too far from the finest analog sound reproduction.
So should you record at a high sample rate? Its going to depend on who you ask. Some people say "It's all going to end up as 44.1 any way" when the cd is burned. What about SACDs, FLAC files, etc.? Others will tell you that when an audio interface processes and mixes sounds at 96 kHz the result is better and remains better even after the final conversion to 44.1, even though the only change in such instances is the addition of numerous zeros--nothing more. Some say 16/44.1 is good enough for CD its good enough for me. Others say do 24/44.1 because it's not that much more space and it increases the signal to noise ratio. There is one argument that says 24/88.2 is superior to 24/96 because it is an even number conversion going back to 44.1.
So what's that? You want to hear what "I" think. Ok, this is the gospel according to Tweak! Use 24 bit for every recording if you have this feature. Again, 24 bit audio has a much lower "noise floor." I was a believer for the past decade that 16 bit was the way to go and I have absolutely changed my mind. No matter of what you are recording this is true. If you have a nice mic, a very good preamp and a clean audio system and are recording highly dynamic instruments such as acoustic guitars, classical orchestras, acapella vocals, the difference will be there. Quiet passages will be less likely struggling to stay above the noise floor on your system. One can record with no compression. You can record at lower levels, with more headroom. This ensures that the occasional peak is not truncated at the top and it will give converters some room the breathe. Because you are not pushing the limits of your bandwidth, your instruments will sound clearer, and the vocals may sound "cleaner", the song will mix better and there will be less noise.
Regarding the sample rate, unless you have a good reason not to, use a sample rate of 44.1. If you are doing audio for video you might want to use 48 khz as many editors only use that rate. Some kinds of music seem to benefit slightly from the high resolution of 88.2 or 96 khz. Acoustic instruments, like guitars, percussion, and of course vocals, i.e., things with delicate high frequencies seem to benefit but its subtle. Lets say you had a $399 audio interface with onboard preamps. Replacing your preamp and converters with better ones will give you more of a sonic difference at 44.1 that recording at 96khz will.
Once inside the sequencer, audio files may be converted to 32 bit for processing and converted back to 24 or 16 on the way out. So, my advice is to record at 24 bits/44.1 at minimum and go up to a sample rate of 88.2 or 96 if you think your material warrants it (and you have the disk space.)
So How do we achieve Sound Quality?
People ask me all the time, and I wince every time they do: "What gear (fill in the blank...soundcard, preamp, cable, recorder, sequencer) will give me the "best" sound quality?" I wince because I imagine the person going out, buying a $500 preamp and the stick it in their studio and connect the mic and crestfallenly realize it "sounds the same" "maybe a little better" um, "hard to tell". Why the heck is that? The main reason is that it is all a big system and it is only as pure as its dirtiest pan pot. One humming cable can obliterate the gains made by otherwise great sounding gear. A pristine mic preamp connected digitally to a soundcard with the jitters will be defeated. Your whole chain has to pro quality, from preamps, in and out of the converters, into top quality monitors. But if you do have a great signal chain, even your 16 bit recordings will sound better than 24/96 with average preamps and typical converters and monitors.
But lets be real clear: it is not just gear. It's technique. Someone with a $2,700 Neumann U87 and a top of the line Great River preamp will still have an awful recording if the mic is not placed well, or if the room brings out undesirable characteristics. If you apply to much compression on a vocal while recording its going to sound bad no matter what gear you have. This is all to say, positively, that when you know how to apply eq, processors and plugins you will be on your way to achieving a mix of high sound quality. This brings me to the point. A person who really knows and works with their gear can deliver greater sound quality at 16 bit with lesser gear than a person who just plugs in and goes on top of the line 24/96 recorders. Technique is as important as gear which is more important than bits and rates. And there is only one thing more important that technique and gear.
The most important point is saved for last, and I've said it before, and will again. Great music recorded on a crappy cassette deck will win more hearts than a turd polished at 24/192. Your talent is more important than anything else, and you can't buy that. So remember to practice, work on your technique, when you can afford to, upgrade your signal path, and then when the pearly gates of Sound Quality open, then consider 24/96 as you would a nice finish on a well-crafted piece of art.
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