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GStreamer: Past, present, and futureGStreamer: Past, present, and futurePosted Oct 30, 2010 0:09 UTC (Sat) by gmaxwell (subscriber, #30048)In reply to: GStreamer: Past, present, and future by dlang Parent article: GStreamer: Past, present, and future There is lots of misinformation on this subject out there. Given unlimited precision samples a signal which has no energy above the the system nyquist is _perfectly_ re-constructable, not just "good". If the signal does have energy above the nyquist then it's not "no hope": the system is under-determined and there are a number of possible reconstructions. Of course, we don't sample with infinite precision but increasing the sampling rate is a fairly poor way of increasing the SNR for lower frequencies if thats your coal. For example, a 1 bit precision 3MHz process can give as much SNR in the 0-20kHz range as a 20 bit 48khz process but it takes about 3x the bitrate to do so. 24bit converters with >110dB SNR are readily and cheaply available. These systems can represent audio as loud as 'dangerously loud' with the total noise still dwarfed by the thermal noise in your ear and the room around you. It's effectively infinite precision. Heck, given reasonable assumptions (that you don't need enough dynamic range to cover hearing damage to the faintest discernible sounds) well mastered CDDA is nearly so too. There has been extensive study of frequency extension into the ultrasonic, and none of the studies I've seen which weren't obviously flawed could support that hypothesis. If this perception exists it is so weak as to be unmeasurable even in ideal settings (much less your common listening environment which is awash in reflections, distortions, and background noise). There also is no real physiological basis to argue for the existence of significant ultrasonic perception Heck, if you're posting here you're probably old enough that hearing is mostly insignificant even at 18kHz (HF extension falls off dramatically the early twenties for pretty much everyone) much less higher. But hey if you want to _believe_ I've got some dandy homeopathics to sell you.
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GStreamer: Past, present, and future Posted Oct 30, 2010 0:36 UTC (Sat) by dlang (✭ supporter ✭, #313) [Link]
Quote: Given unlimited precision samples a signal which has no energy above the the system nyquist is _perfectly_ re-constructable, not just "good".
I disagree with this statement. something can be reproduced, but not neccessarily _perfectly_
also, any time you have more than one frequency involved, they are going to mix in your sensor, and so you are going to have energy above this frequency.
sampling faster may not be the most efficient way to get better SNR, but it's actually much easier to sample faster than to sample with more precision.
using your example, setting something up to sample 1 bit @ 3MHz may be far cheaper than setting up something to sample 20 bits @ 48KHz. In addition, the low-precision bitstream may end up being more amenible to compression than the high precision bitstream. with something as extreme as the 1bit example, simple run-length encoding probably will gain you much more than a 3x compression ratio. That's not to say that a more sophisticated , lossy, compression algorithm couldn't do better with the 20 bit samples, but again, which is simpler?
I am in no way saying that people hear in the ultrasonic directly, However I am saying that some people listening to a 15KHz sine wave vs a 15KHz square wave will be able to hear a difference.
GStreamer: Past, present, and future Posted Oct 30, 2010 14:21 UTC (Sat) by alankila (subscriber, #47141) [Link]
> I disagree with this statement. something can be reproduced, but not neccessarily _perfectly_
This may be confusing two ways to look at it: as mathematical issue, or as engineering problem. Mathematically the discrete representation and the analog waveform are interchangeable: you can get from one to the other. The quality of the conversion between the two can be made as arbitrarily high as you desire -- typically design targets are set beyond assumed limits of human perception.
>also, any time you have more than one frequency involved, they are going to mix in your sensor, and so you are going to have energy above this frequency.
Intermodulation distortion can generate extra tones, and depending on how strong the effect is, they may even matter. Such nonlinearities do not need more than one frequency, though.
This is normally an undesirable artifact, and our ADC/DACs have evolved to a point where they are essentially perfect with respect to this problem. In any case, from viewpoint of a digital system, artifacts that occurred in the analog realm are part of the signal, and are processed perfectly once captured.
> I am in no way saying that people hear in the ultrasonic directly, However I am saying that some people listening to a 15KHz sine wave vs a 15KHz square wave will be able to hear a difference.
The amusing thing is that a 44.1 kHz representation of a 15 kHz square wave will look identical to a 15 kHz sin wave, because none of the pulse's harmonics are within the passband of the system. Do you happen to have a reference where a system such as this was tested with test subjects so that it would be possible to understand how such a test was conducted?
GStreamer: Past, present, and future Posted Oct 30, 2010 16:27 UTC (Sat) by magnus (subscriber, #34778) [Link] Given unlimited precision samples a signal which has no energy above the the system nyquist is _perfectly_ re-constructable, not just "good".Theoretically, you don't only need unlimited precision on each sample, you also need to have an infinite number of samples, from time -∞ to +∞, to perfectly reconstruct the original signal. In practice though, audio signals will have some information (harmonics etc) at higher frequencies and no filters (not even digital ones) can be perfectly brick-wall shaped, so some aliasing will occur plus you will have some attenuation below the Nyqvist frequency. Sampling at 96 kHz might (if well designed) give you a lot more headroom for these effects. I have no experience with 96 kHz audio so I don't know if this is actually audible or just theory+marketing. Since human hearing is non-linear it's also possible that people can pick up harmonics at higher frequencies even if they can't hear beeps at these frequencies. The only way to know is double blind-testing I guess...
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