Again, why 20 kHz? The patent application only lists examples of greater than 40kHz -- most are in the range of 50+ kHz. My point isn't that this tech is going to work and change the world just that it's incredibly easy to be pessimistic and to justify it with small incorrect assumptions.
> I don't 100% get the band limited signal bit. How does band limiting imply that there's only a single possible reconstruction of the digital signal?
I agree that was a little unclear. I think what he's saying is that since humans can't hear above ~20 KHz, frequencies above that are lost. The 'wobbles' in the square wave are what happens when you take a square wave (which is a summation of infinite sine saves with frequencies an integer multiple of the fundamental) and drop the frequencies above the ~20 KHz cutoff.
In other words, it has nothing to do with any analog/digital conversion. It's just what happens if you ignore frequencies above 20 KHz, which we can't hear anyways. We can't hear any difference between a 'perfect' square wave and the bandwidth limited one.
I also don't completely understand about their being only one possible solution. It makes sense if you know that the signal is a single sine wave, but since in general the signal is a summation of an unknown number of sine waves, it's not clear. I guess this is something he didn't have time to get into more depth for the quick overview :)
> Isn't there also an argument that frequencies above 19khz
The rule of thumb I've heard is 20 KHz. I also wondered if this is an issue. It'd be interesting to know the distribution of how many people can perceive frequencies how much higher than that. I think though that the 20 KHz is already fairly far along the long tail, and most people's cutoffs are actually lower.
> humans can only hear frequencies between 20 Hz and 20 kHz
I can hear up to around 21.5kHz, which I know because I have a 48kHz DAC. However those frequencies are almost always just unnecessary to include in a signal anyway.
Absolutely. 20kHz, from an electrical engineering perspective, isn't really a wide bandwidth. You might get some fairly strong distortion but almost always it will still work, unless we're talking about something like a piezo.
No, 100 dB SPL is not a fundamental law of physics, but it's certainly a fundamental limit of human tolerance (it's about the same sound pressure as standing next to an operating jackhammer), and even if you can't hear it at ~20 kHz, any small animal in the vicinity will likely be tortured (seriously, PETA should take an interest in this technology). And since we're talking about investing in a consumer technology, any fundamental limit in human tolerance or even FDA guidance should be a deal breaker from a business perspective. My objection is that no one did such a sanity check before investing a large sum into this concept.
The statement that frequencies above 20kHz don't matter rests upon the assumption that the ear is linear. If the ear is not linear (I don't know whether it is not not) then frequencies above 20kHz will matter, as the ear will be able to mix higher frequencies down to less than 20kHz. For example, if we have frequencies of 56kHz and 59kHz, the ear MIGHT be able to discern a difference frequency of 3kHz. No doubt this effect could be reproduced by signal with a sampling rate of 44.1KHz, but only if the analogue systems, before the sampling stage, reproduce any non-linearity in the human ear.
Incidentally, you can get speakers that create a localised beam of sound, that the person sitting next to you cannot hear. They work by transmitting frequencies above the audible range. These high frequencies can be beamformed by a relaitively small speaker array, so the sound is localised. They then rely on the non-linearity of the ear (or maybe the air around the ear?) to mix the ultrasonic frequencies down to audible frequencies. I guess there must be non-linearity in the human auditory system!
On the subject of 24-bits my understanding is that 16-bits is adequate, provided the levels (scaling) are set correctly in the recording. What 24-bits delivers is the ability to do a crappy job of the mixing, and still end up with the full dynamic range of the human ear. 24-bits is probably a temporary solution though, as manufacturers will engage in the usual Loudness War [1], and push the signal to the top of the dynamic range. Before long 24-bit audio will be equivalent to 16-bits (since the 8 least significant bits will be unused) and the next big thing will be 32-bit audio.
Having said all that, I'd guess that the speakers will be the limiting factor in most sound systems, not the recording format.
Quite a few people can hear 20kHz. I can still clearly hear 18.5kHz (or what my laptop's speakers emit when I feed a 18.5kHz sine) and 19-20kHz gives me headaches.
I always understood ~20Hz to be relegated more for inaudible movie effects like you mentioned. Usually contributing to the feeling of dread, fear, and anxiety in horror movies, etc.
It’s news to me that one might need 20Hz for purely an audio track, especially in the home. I would understand at a concert or something but to demand it from a single small self-contained unit like that sounds like looking for love in all the wrong places.
That's not what I'm getting at. For most purposes the 22KHz limit is a hard one. What I mean is that the eardrums do not have a linear response at any frequency.
> One nefarious method malware could use to get data off the device without RF would be to play sub 20kHz audio through the speakers, assuming there was a device with a microphone near by that's able to receive the signal, and of course that the speakers can play a frequency that low.
did you mean to say "sub 20 Hz"? or rather the ultrasound between 20kHz and 22kHz?
it's a bit ambiguous cause both 20 Hz and kHz are lower and upper ranges of human hearing, respectively.
if you meant the lower, then no. mobile speakers already almost entirely lack power in the musical bass frequencies (50-150Hz roughly), let alone the infrasonic ones. if the speaker would even have a response to such a signal, it's not going to have a lot of range. .. a sub-20Hz audio signal has a wavelength of more than 17 metres in air, I'm not sure if that's physically impossible, but I'm having a hard time imagining that tiny tweeter to manage such a signal (my knowledge is mostly with digital signal processing, not entirely sure about what speakers can do, Bose has done some pretty amazing room-shaking bass with very clever tiny speakers, that seemed impossible to me as well, has to be some trick there). And then, doing it without getting harmonics in the audible spectrum! :)
There will always be people with slightly wider sensory perception than others, but are you honestly saying that you can easily hear sounds "well above" 20kHz? Have you tested this in a double blind test? I know that sounds like a lot of effort, but it would be interesting. You would be somewhat unusual if you could hear, say, 22kHz.
There are people who can hear up to 22khz and beyond, so I've never understood why many of these arguments aren't easily resolvable on that basis alone.
no, it addresses it fairly clearly (albeit briefly):
> So the math is ideal, but what of real world complications? The most notorious is the band-limiting requirement. Signals with content over the Nyquist frequency must be lowpassed before sampling to avoid aliasing distortion; this analog lowpass is the infamous antialiasing filter. Antialiasing can't be ideal in practice, but modern techniques bring it very close. ...and with that we come to oversampling.
if you accept that the limit of hearing is around 20 kHz, then you must also accept that frequencies above that can freely be removed without loss of fidelity to the human ear.
the article notes that higher frequencies can be heard, but only in the form of ultrasonic intermodulation distortion. (i.e. not in fact the higher frequencies at all)
I don’t find this argument particularly compelling. You can hear up to 27kHz if the sound is loud enough, under laboratory conditions. However, from a psychoacoustic standpoint, you are unlikely to be able to hear those frequencies under normal listening conditions, and typical program material doesn’t make use of those frequencies anyway.
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