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#1
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What is a good resource on bit depths?
I am curious to learn more about the differnt bit depths. 8bit really itrigues me because I tried bouncing to 8 bit just to see what it would sound like and I found it odd that I can hear the full frequency range (as I assumme is because I still had it at 48k) and also dynamics were still pretty good but it sounded like FM radio with static, Does anyone know what causes this? I don't understand why it would sound like all of the audio info is still there but with added noise. I post some audio to the internet every week and I am always on the lookout to inprove the quality for the file size. I currently use both mp3 at 16kbps and AAC at 32 kbps, both mono. I am wondering if I can get better frequency response by going with a higher sampling rate (such as 32k) but lowering the bit rate to either 8 or 12.
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#2
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Re: What is a good resource on bit depths?
Dither Explained (White Paper)
Not sure if this is what you're looking for but it might give you a better understanding of how Bit Depth affects sound quality and why there is noise when going from higher bit depths to lower ones.
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Derek Jones Sound Engineer / Producer / Composer Derek Jones Linkedin Megatrax Recording Studios Megatrax Studios Yelp Page A-list Music Artist Page |
#3
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Re: What is a good resource on bit depths?
Resolution (bit depth) has little if nothing to do with frequency response. That is determined by the sampling rate. Without going into too much detail, the basic rule is that the sampling rate has to be at least twice that of the highest frequency that is being reproduced in order to even come close. Since 20 kHz is generally accepted as the highest frequency that most people can hear, the sampling rate of 44.1 kHz found in CD audio is adequate. Really more so since there isn't really much going on between 16 and 20 kHz anyway. Pretty much all good A/D converters band limit the audio to 20 kHz before it gets digitized. Anything higher than 44.1 kHz is just making the digital approximation of the frequency even more accurate.
Resolution determines the dynamic range of the digital approximation. The higher the resolution (bit depth) the wider the dynamic range and the more accurate the digital system is at interpreting and recreating the differences in level of your recordings. The dynamic range of 8 bit audio is around 45 dB which if used effectively is adequate when you consider that the with an average noise floor of 60 dB SPL of most listening environments, which leaves you with around 45 dB to work with before you start approaching the threshold of pain. 16 bit resolution gives you over 90 dB of dynamic range but very few recording engineers really use all of that. The problem goes back to that whole signal to noise ratio of most listening environments. Most of your recording work is done with levels in the upper 40 dB of your 16 bit range which suddenly throws out most of the bits in that 90+ dB. So although no one ever uses the entire dynamic range of 24 bit digital audio, it does provide you with a much higher resolution within the 40 dB or so that you're actually using. Most of the artifacts you hear when converting your 16 bit stuff to 8 bit is caused by levels that fall in place on the 16 bit scale but have no equivalent value in the 8 bit scale so they get bumped to whatever is closest. This can often cause some nasty artifacts. Dither can help with that if it is used properly. That's all I have time to write now but let me know if you have any more questions.
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Jason Romney Sound Design Instructor North Carolina School of the Arts www.cd-romney.com www.ncarts.edu/ncsaprod/designandproduction |
#4
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Re: What is a good resource on bit depths?
I am familiar with the sampling rates and needing to be twice the highest frequency etc. The added info was very interesting about the bits though. I was somewhat familiar with that but now it is a little clearer. I use dither to go from 24 bit down to 16 when I bouce a session but do you know of a dither that will go from 16 down to 8? Also, is using 8 bit for the web even practicle? I am just looking for a way to get higher sample rates without increasing my bit rate of 16kbps and 32 kbps which is of course pretty low quality.
Thanks for the info |
#5
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Re: What is a good resource on bit depths?
One more thing, does anyone know of any software that will dither a 16 bit file to an 8 bit file? Digi's seems to only go from 24 t o16
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#6
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Re: What is a good resource on bit depths?
The Waves Ultramaximizer will do an 8 bit dither.
Be careful not to get the bit rate of an mp3 confused with the bit depth of a standard PCM file. They are two very different things. Bit rate is the amount of data that is being moved every second. This term is much more common when you're talking about internet distribution. How many bits can we send over the wire at a time? The bit rate of PCM audio 16 bit, 44.1 kHz is 705.6 kb per second. I'm not familiar enough with mp3 encoding to go into much detail but the crux of it is that it looks at your file and attempts to prioritize the information. It looks at each bit and ranks it according to how important it is in reproducing the right sound. It then starts from the bottom, eliminating the least important information until it eliminates enough bits to fit within the limit (128 kb/s, 64 kb/s, etc). There's more to it than that but that's the basic idea as I understand it. So if you're really looking to crunch things, it stands to reason that the smaller you can get your file before you encode it (either by dropping the resolution or sampling rate) the less bits will need to be eliminated by the encoder and you can get away with much lower bit rates. I've never been concerned enough about the size of the files I post on the internet to try this theory but it seems logical. Anyone disagree or have any further insight?
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Jason Romney Sound Design Instructor North Carolina School of the Arts www.cd-romney.com www.ncarts.edu/ncsaprod/designandproduction |
#7
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Re: What is a good resource on bit depths?
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-Duardo |
#8
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Re: What is a good resource on bit depths?
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This is much easier to understand with illustrations. Here's a good website that has some pretty good explanations and images. http://www.indiana.edu/~emusic/digital_audio.html I agree with you on the resolution thing. I think we're just using different words to describe the same thing. As you say, with 24 bit audio, you don't need to be as concerned with your levels because you can get an accurate recording without trying to use the entire range. This is due to the added resolution along the scale. You can get the same if not better resolution using only a portion of the 24 bit scale than you get trying to use a larger portion of the 16 bit scale by setting your levels as high as you can before clipping.
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Jason Romney Sound Design Instructor North Carolina School of the Arts www.cd-romney.com www.ncarts.edu/ncsaprod/designandproduction |
#9
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Re: What is a good resource on bit depths?
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There are some digital processes that do work better at higher sampling rates, but 44.1kHz audio can easily be upsampled for processing and then downsampled. There are hardware pieces and software plugins that have been working that way for years. Quote:
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The University of Indiana link is great, but doesn't say anywhere that there's any advantage to sampling at a rate that's higher than twice the frequency you want to capture. -Duardo |
#10
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Re: What is a good resource on bit depths?
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Here's an image from the Indiana link. It shows the source complex wave of 3 kHz (with a lot of harmonics) being digitized at 9 kHz and at 48 kHz. You can't tell me that the top one is going to come out of the DAC as accurate as the bottom one. There's just not enough information there. How is the DAC supposed to assume all those sublte changes along the wave if it's dealing with only three samples? So yeah, the higher sampling rate of 48 kHz is allowing the harmonics to be interpreted but what about the harmonics at the very top of the frequency spectrum? They're the ones that are still suffering even at 48 kHz (which is much more than twice the highest frequency). A sampling rate higher than the 48 kHz will allow those harmonics (that are still within the audio range) to be interpreted more correctly. An interesting experiment would be to put an oscilloscope on each end of a digital system and feed different types of waves into it using a sampling rate at twice the frequency and see how close each type of wave (sine, triangle, sawtooth, etc) at the end gets to the original wave. Anybody done this before?
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Jason Romney Sound Design Instructor North Carolina School of the Arts www.cd-romney.com www.ncarts.edu/ncsaprod/designandproduction |
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