Matt_G wrote on Thu, 06 July 2006 13:55 |
danlavry wrote on Fri, 30 June 2006 03:54 |
I do not have a problem with someone upsampling, doing some processing then down sampling back. There many way to do things, and at times going X2 or even way up is called for, or provides an advantage.
It is difficult to have a good DA anti imaging analog filter, so we upsample way up.
It is often easier to process near 20KHz with 88.2-96KHz, so going X2 may make sense.
And so on...
But that has nothing to do with the basic sample rate, which determines what music you capture. That should relates to the range of frequencies one can hear. Upsampling does not alter what you can hear, it may make some specific localized tasks work better.
I said many times before. I am sorry it is not simple or easy to understand the distinction between audio sample rate (the rate for conversion, transfer, storage) and "localized rates" for specific reasons. Hack, I know of cases that would require localized upsampling to near 10MHz rates... Followed, of course, by serious down sampling.
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Thanks Dan, it is important to note the differences between recording at high sample rates & upsampling to higher sample rates to clean up non linear processes. Thanks for clearing that up.
So what about capturing the final output from an analog mastering path at 16bit 44.1kHz, compared to capturing at 24bit 96kHz when the final medium is going to be CD? In this case is it better to capture at a high resolution then downsample & dither to 16bit 44.1kHz or simply capture at 16bit 44.1kHz to avoid a further DSP step that an SRC & dither would require? Or perhaps 24bit 44.1kHz would be a good compromise? Any thoughts on this Dan?
Matt
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Hi Matt
I would certainly separate the number of bits from the sample rate.
First about bits:
In the good old days, we were struggling to have clean 16 bits. The analog noise floor was so high, that it served as dither. At very high noise, referenced to the quantization level, the dither action was fine, but the noise penalty (reduced dynamics)was too high.
Next we started having the noise approaching the 16 bit quantization, eventually having the noise lower then the 16 bit quantization. At that point, it became necessary to do something about the unwanted "art effects" (distortions and noise modulation) of low level signals. By now, many AD's are better then 16 bits. In fact, when an AD is a "true 18 bits" or more, and the end result is say 16 bits, it is best to dither. There is no reason to record directly to 16 bits, when you can record to 24 bits (the last few bits are of no value, they are noisy) and then dither to 16 bits.
Of course, a noise shaped dither is better then the older varieties, because the added noise resides in frequencies where the ear sensitivity is lower.
As a rule, keep the word length as wide as you can (say 24 bits), and dither at the last step of the processes (with a noise shaping type if you can).
Regarding the sample rate: take 44.1KHz vs. 88.2KHz. This days, virtually all converters "do their job internally" at much higher rates then 44.1 or 88.2... Therefore, having 44.1KHz requires some down-sampling to take place. So the first question that come to mind is: which is a better downsampler? The one in the AD? An external SRC? This is important because the last stage in the down sampling process is the one performing 88.2 to 44.1KHz. That last stage is the most difficult and demanding one (in terms of down sampling). So the answer depends on specific implementations of the SRC...
Regards
Dan Lavry
http://lavryengineering.com