Alan Meyerson wrote on Tue, 13 September 2005 13:45 |
Hey Folks- A quick..(I'm sure it's been done a million times before) discussion about upsampling from 44.1. AM I BETTER OFF AT 88.2 BECAUSE OF THE MATH or does it really matter. Can I go to 96? I've heard the arguments for both camps. I just have a decision to make in the next day and want to mull it over again. Thanks, Alan
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1. As a rule, you are better off to stay within a ratio of 2:1 (such as 44.1 to 88.2) than to go for a non integer ratio such as 2.176870748.... (such as 44.1 to 96KHz). The integer ratio requires much simpler sample rate conversion - an asynchronous conversion. The non integer ratio takes much more computations therefore it has greater potential for errors.
2. To take full advantage of the above, one should make sure that they have a SYNCHRONUS converter. There are few such devices o the market, and most devices are asynchronous (to be able to address any rate to any rate).
3. So what if you lack a SYNCHRONUS conversion, and have an ASYNCHRONUS one? That question is more subtle. I have not examined all the SRC's out there but a few implementations I saw for an ASYNCHRONUS SRC do yield better results when operated at 2:1. That may not be the case for all devices at all rates.
3. Why up sample? Say one may (for example) wish to mix a 44.1KHz material with 88.2KHz or 96KHz. One could down sample the high rate, or up sample the low rate. The argument for up sampling the low rate is based on the fact that at least in theory (with perfect up sampling), one does not loose anything. The data contained by the fast rate is left alone. Down sampling the fast rate data to 44.1KHz would remove the energy content above 22.05KHz (assuming it is there and that you need it).
But while up sampling from 44.1KHz to higher may have practical or commercial reasons, one should keep in mind that the processes does not really improve the audio data at all. The starting point is 44.1KHz so the energy content is limited to 0-22.05KHz. You can not expect an up sampler (or any other processes) to create something from nothing, and the best of SRC’s can not insert what is not there. You can examine the audio under 22.050KHz all day long, and find no clue regarding what happens over 22.05KHz. Contrary to all sorts of hype, there is no processes that can recreate that energy.
Some people argue that an up sampled data sounds better, and I will be the last one to argue with or negate such observations. As a DA designer, I can see scenarios where an X2 up sampled data can be better processed by a particular DA than an X1.
I can also see scenarios where the X2 data will be inferior after up sampling by X2.
The bottom line is HRADWARE IMPLEMENTATION (or software implementation). But true to form, many in audio jump into drawing conclusions about what is good or bad, based on tests limited to specific hardware. In some cases such conclusions may be true for a couple of years, and when hardware architecture and approaches changes, the conclusions does not.
Those that believe that an X2 up sampling is a good thing need to answer one question:
The data is going to be up sampled anyway, by any DA in any playback system (unless we go back to the pre early 1990’s). Chances are that the playback DA will up sample by yet another X2, or X4 or X1...or 256.... Therefore, the argument about the benefits of up sampling become about quality of implementation. The studio person may have a better up sampler than a typical commercial DA, and indeed the first X2 up sampling stage is the most critical one, (of course up, up sampling in the studio requires a release format beyond red book CD).
Having said the above, I am not against up sampling for good reasons. I do have a problem with “notions” that up sampling adds some audio or recreates something. Upsampling is done to help remove the DA unwanted image energy, and to overcome the flatness response problems of a X1 44.1KHz DA (a non issue for the last 10-15 years).
For deeper technical explanations, feel free to go to my company web at:
www.lavryengineering.comUnder the support section, look at the paper:
“Sampling, Oversampling, Imaging, Aliasing”
Regards
Dan Lavry
www.lavryengineering.com