For Questions That Don't Merit Their Own Thread

[K^2]

20 hours ago, ARS said:

So... I found a stuff on Youtube that compressed a compilation of songs (around 60 minutes of songs) into 2 seconds-video. The idea is if you played the song at super ultra-slow speed, you could listen the entire song. So I'm wondering, if audio files can be compressed into shorter length (for example, to send a secret message in a particular segment of a song), is there a limit of message length that can be compressed into 1-second audio before it starts to be unreadable as a message (aka, just garbled noise no matter how slow it's played, like the pixels on "zoom and enhance" situation)?

Bandwidth

The HiFi standard for audio is 20kHz. For speech, 8kHz is considered good quality, but music already degrades noticeably. You can get it down to just a few kHz, giving you an almost 10:1 compression, and you'll be able to understand, but quality will be horrible.

But that's where compression comes in. YouTube doesn't stream raw audio, as that would take up a lot of your internet bandwidth. You need about twice number of channels of samples per Hz of bandwidth, and you want at least 16 bits per sample, which is 160kBbytes per second just to send audio. You can get decent quality HD video with that much data. So compression is used instead. I don't know if YT still uses MP3 for audio stream, but it's going to be something like MP3 at any rate. The way that particular form of compression works is that you take your audio recording, you run it through spectrum decomposition, you look for prominent spectral lines, and you record just these.

There's a lot of math involved, and explaining why that has impact on "compressing" runtime is going to get convoluted, but you can almost think of MP3 as an automatic MIDI. The way MIDI files work is that you just record notes being played, duration, and volume. When you play back MIDI files, an instrument synthesizer will try to replicate the notes played. If you want to speed up a MIDI file 100 times, you don't record less data. You simply tell each note to play 1/100th of the duration. The size of the MIDI file will be just as large as it was before, but if you then play that file back at 1/100th of speed, you'll hear exactly the same melody as was encoded in that file originally.

MP3 is a bit of a compromise. Because you aren't recording notes, but rather the spectrum, which does change over time for a particular note played even on simple instruments, let alone vocals, you need some of that temporal information to remain. When you speed up and then slow down playback of an MP3 file, it's not going to degrade nearly as much as raw audio would. Depending on the exact implementation of software you've used to shorten duration of the MP3 file, you can get away with no quality loss by simply increasing sample rate, but then the file will remain the exact same file size it originally was. This might be what's being done, in which case, you should get the same quality as before, and it's just tricking YT into allowing the video through, as well as, likely, messing with content ID. On the other hand, if YT re-compresses audio stream, it's going to use a standard sample rate, which will cause huge degradation. If you anticipate that and want to prepare your audio stream by encoding it at a standard rate, you will have to lose a lot of temporal information. For some kinds of music, the difference could be minor enough to where it's still good enough. I suspect, it will do better with electronic music than orchestral.

So to summarize, it's possible to make a compressed audio file that has all the data of original, but is set up to play back in fraction of the time, and if you listen it slowed down, it will sound identical to original. YouTube might not support that, in which case, you can still get most of audio data across compressed in time, which will result in crappier quality, but nowhere near as bad as if it was analog audio.

[kerbiloid]

6 hours ago, magnemoe said:

Yes but in an megabyte sized data stream its way better methods to transmit data. Or other channels. 
And some other effects than just cutting high frequency is used here as described above. 

When a channel is optimized for the speech range of frequencies for verbal interaction, they even send pictures from the Moon (and Duna).
https://en.wikipedia.org/wiki/Slow-scan_television

Spoiler

Also the audiocassettes store data as a sound signal of speech range (can't recall for ZX Spectrum, 700 Hz or so).

All of this is not a megabyte-wide channel, and you need no special equipment to record/restore the 100^x times accelerated speech, you just should know the word of the song at which you should  cut off and zoom the signal in any software like Audacity.

Edited January 21, 2021 by kerbiloid

[JoeSchmuckatelli]

Is there a 'now' that is relevant at Alpha Centauri (or any other place, really)?  Or is 'now' a concept that is tied intrinsically to our reference frame?

[RyanRising]

4 hours ago, JoeSchmuckatelli said:

Is there a 'now' that is relevant at Alpha Centauri (or any other place, really)?  Or is 'now' a concept that is tied intrinsically to our reference frame?

You can come up with a pretty good reference frame, but yes, "now" is tied to your reference frame.

[kerbiloid]

The reference frame is nice, but until the light from the Alpha Cen reaches us, you can't have any info about the physical state of any object there, so their "now" has a probabilistic nature for you. It's an Alpha Centauri of Schroedinger.

So, enjoy your ~0.1 s of perception delay and the 30 000 km range of what you can call "immediately".

Also as the nerve impulse velocity is ~50..100 m/s, and the brain size is ~0.1 m, the light (and a gravity change) passes ~500 km while a nerve impulse is crossing your brain.
So, the "now" is a very limited thing in the Universe.
Of course, something is happening right "now" at teh AlphaCen, but physically you can't physically interact with it to have any information as an Observer, and to resolve uncertainty.
(Now Heisenberg toasts with Schroedinger.)

Edited January 24, 2021 by kerbiloid

[JoeSchmuckatelli]

OK - something esoteric to build off of this: if we had an outpost / space station out there at AC and instant comms (via quantum entanglement or other sci fi tech)... And a ship that could travel from Earth to the station in 7 months rather than 70,000 years - if the Earth dispatch told the AC station that the ship was leaving 'now' 

... Could the station expect to see the ship in 7 months - or is the Earth 'now' still distinct from the AC 'now' such that '7 months' means different things to each observer? 

[kerbiloid]

2 hours ago, JoeSchmuckatelli said:

(via quantum entanglement or other sci fi tech)

And the quantum entanglement is limited with the lightspeed delay, according to current theories.

2 hours ago, JoeSchmuckatelli said:

if the Earth dispatch told the AC station that the ship was leaving 'now' 

Earth can, but they'll get this info still 4 years later.
Until that, the Earth stays fuzzy and probabilistic for them. Not just due to lack of knowledge, but purely physically. Either it still exists, or no, like the cat.

Edited January 24, 2021 by kerbiloid

[RyanRising]

5 hours ago, JoeSchmuckatelli said:

OK - something esoteric to build off of this: if we had an outpost / space station out there at AC and instant comms (via quantum entanglement or other sci fi tech)... And a ship that could travel from Earth to the station in 7 months rather than 70,000 years - if the Earth dispatch told the AC station that the ship was leaving 'now' 

... Could the station expect to see the ship in 7 months - or is the Earth 'now' still distinct from the AC 'now' such that '7 months' means different things to each observer? 

Since both your communications and space ship are travelling faster than light, I think we're outside of the set of questions general relativity has an answer for.

[JoeSchmuckatelli]

5 hours ago, RyanRising said:

Since both your communications and space ship are travelling faster than light, I think we're outside of the set of questions general relativity has an answer for.

Fair enough - I'm just asking about the frame of reference time relativity and trying to come up with a  'what if' to help me figure it out.  Admittedly, this came out of an argument between my father and I from something we read in a novel. 

 

So I take the correction as a fair one! 

@kerbiloid - you are breaking my heart!  I thought entanglement ignored the light speed limits!   (Edit - seriously; I thought you had to move the entangled electron via conventional methods that did not violate Relativity... but once there... the change could be instantaneous)

Edited January 25, 2021 by JoeSchmuckatelli

[kerbiloid]

5 hours ago, JoeSchmuckatelli said:

you are breaking my heart!  I thought entanglement ignored the light speed limits!

This was a drama for me2 to know it.

Now (actually, since the university) all my hopes are on the many-worlds interpretation. Only it can light for me in the night.

[JoeSchmuckatelli]

But then you end up with The OA and are canceled after two seasons. ;/

[kerbiloid]

43 minutes ago, JoeSchmuckatelli said:

But then you end up with The OA and are canceled after two seasons. ;/

I hardly survived the 1st one.

But happily I'm enforced with the Nine Princes In Amber to withstand the multiverse.

[JoeSchmuckatelli]

Solid reference! 

[kerbiloid]

Science and Nature are our hope!

Spoiler

 

[JoeSchmuckatelli]

If Earth had a higher oxygen percentage in its atmosphere - would paper burn at a temp higher than F451? 

 

 

[ARS]

2 hours ago, JoeSchmuckatelli said:

If Earth had a higher oxygen percentage in its atmosphere - would paper burn at a temp higher than F451? 

In the fire triangle, there are 3 things needed to form a combustion: oxygen, heat and fuel. Modifying any of these parameter will change how things burn. In case of oxygen, if you're increasing the oxygen percentage of the atmosphere, it speeds up the process of combustion. Higher oxygen percentage on the atmosphere will create an even faster burning process. However, like in the fire triangle, things have to be balanced: Too much oxygen with too little fuel means the paper will be burned to ash almost instantly before the fire becomes self-extinguishing because there's nothing left to burn. Too much fuel with too little oxygen means the fire will also self-extinguishing before the paper fully burned because there's not enough oxygen to sustain the process. By default, oxygen itself is not flammable, but it does support and required for the combustion process, adding more just accelerate it or even makes things catch fire at lower temperature (that doesn't stop early alchemists and researchers calling it "Fire air"). If you want paper to burn at a temp higher than F451, the parameter you need to change in the triangle is the fuel (aka the paper itself), not the oxygen content (some materials do becomes significantly more combustible (more reactive) than usual in high oxygen environment, look no further than Apollo 1 fire that killed 3 astronauts)

[JoeSchmuckatelli]

3 hours ago, ARS said:

If you want paper to burn at a temp higher than F451, the parameter you need to change in the triangle is the fuel (aka the paper itself), not the oxygen content

So - what I read is that in the higher oxygen atmosphere, we could get paper to light up at F420? 

[kerbiloid]

https://science.howstuffworks.com/environmental/earth/geophysics/fire1.htm

Quote

When the wood reaches about 300 degrees Fahrenheit (150 degrees Celsius), the heat decomposes some of the cellulose material that makes up the wood.

Some of the decomposed material is released as volatile gases. We know these gases as smoke. Smoke is compounds of hydrogen, carbon and oxygen. The rest of the material forms char, which is nearly pure carbon, and ash, which is all of the unburnable minerals in the wood (calcium, potassium, and so on). The char is what you buy when you buy charcoal. Charcoal is wood that has been heated to remove nearly all of the volatile gases and leave behind the carbon. That is why a charcoal fire burns with no smoke.

The actual burning of wood then happens in two separate reactions:

 

 

• When the volatile gases are hot enough (about 500 degrees F (260 degrees C) for wood), the compound molecules break apart, and the atoms recombine with the oxygen to form water, carbon dioxide and other products. In other words, they burn.

 

https://www.processoperations.com/FireExplode/FE_Chp01.htm

Quote

Minimum Oxygen Concentration (MOC)

The oxygen content in atmospheric air is 21% by volume. Oxygen is the key ingredient in the fire triangle and there is a minimum oxygen concentration required to propagate a flame. This is also known as the limiting oxygen concentration. It is the concentration below which combustion, usually in air diluted with an inert gas such as nitrogen or carbon dioxide, does not propagate in a mixture of gases or vapors.

This is an especially useful result, because explosions and fires are preventable by reducing the oxygen concentration regardless of the concentration of the fuel. This is the basis of the prevention technique of inerting (see later).

The MOC has units of % oxygen in air plus fuel. Below the MOC, the reaction cannot generate enough energy to heat the entire mixture of gases (including the inerts) to the extent required for the self-propagation of the flame. The MOC for several chemicals are shown in the Table.

 

Probably, above some oxygen concentration/pressure the fire point show get lower, but not lower than the wood thermal decomposition point.

[JoeSchmuckatelli]

56 minutes ago, kerbiloid said:

https://science.howstuffworks.com/environmental/earth/geophysics/fire1.htm

 

https://www.processoperations.com/FireExplode/FE_Chp01.htm

 

Probably, above some oxygen concentration/pressure the fire point show get lower, but not lower than the wood thermal decomposition point.

Interesting! 

Help me understand the table:  I see Hydrogen with a 4% MOC - which aligns with my understanding of how flammable it is... But CO2 has a lower MOC.  I don't normally think of CO2 as flammable, so I must be reading the chart incorrectly 

 

[kerbiloid]

40 minutes ago, JoeSchmuckatelli said:

But CO2 has a lower MOC.  I don't normally think of CO2 as flammable, so I must be reading the chart incorrectly 

Probably, he mistyped "carbon monoxide".
Idk the CO's MOC value, but when they produce hydrogen from methane or coal, available oxygen tries to oxidize carbon and leave hydrogen alone.
So, it should be lower than hydrogen's.

[JoeSchmuckatelli]

47 minutes ago, kerbiloid said:

Probably, he mistyped "carbon monoxide".
Idk the CO's MOC value, but when they produce hydrogen from methane or coal, available oxygen tries to oxidize carbon and leave hydrogen alone.
So, it should be lower than hydrogen's.

Upon further review... I propose he meant propane.  According to this: https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.cdc.gov/niosh/mining/userfiles/works/pdfs/tloca.pdf&ved=2ahUKEwj2i8fylr_uAhWIXM0KHeWmCI4QFjAQegQIFRAB&usg=AOvVaw0i9utB13oC4evG5MVn4l39

CO seems to have a higher requirement

[JoeSchmuckatelli]

Space Elevator question - I can't see how you get the cable to connect from the ground to the counter weight. 

• I'm guessing you have to assemble the CW in a geostationary orbit, but 
• -- if you try to launch from the anchor - you are just going to wind the leader around the planet on the way to the anchor... And then be unable to unwind it (not to mention annoying the neighbors) 
• -- if you try drop the leader from the anchor, as it goes 'down' it's going to go faster and want to wind itself around the planet...

So do the plans for this require a straight lift into geostationary orbit with no intervening orbits (leo/stability, etc)? 

(like my early KSP attempts 'fly straight up, turn right, burn hard until you stop falling... )? 

 

Edited January 28, 2021 by JoeSchmuckatelli

[StrandedonEarth]

53 minutes ago, JoeSchmuckatelli said:

 

Space Elevator question

 

If you haven’t already, I recommend reading the Red Mars / Green Mars / Blue Mars trilogy by Kim Stanley Robinson. It includes space elevators. Good read

IIRC, construction has to start in GEO, extending up and down simultaneously. Orbital mechanics will want to start it rotating, so that would have to be countered somehow. Electromagnetics would be the elegant way; rockets near the ends would be the brute force way, and would probably require a stiffer cable 

Edited January 28, 2021 by StrandedonEarth

[wumpus]

I'd assume you assemble a space elevator in space, although plenty of prototypes demand a self-assembling tower.  Presumably the CW starts in geosync and slowly winds out to its final position, keeping the center of mass (I think "center of gravity" may be technically correct in this case, and they might be in slightly different orbits).

I've always assumed you'd launch from geosync, but going higher may well be a more efficient way to gain velocity (as efficient as an electric motor).

[JoeSchmuckatelli]

11 minutes ago, wumpus said:

I'd assume you assemble a space elevator in space, although plenty of prototypes demand a self-assembling tower.  Presumably the CW starts in geosync and slowly winds out to its final position, keeping the center of mass (I think "center of gravity" may be technically correct in this case, and they might be in slightly different orbits).

I've always assumed you'd launch from geosync, but going higher may well be a more efficient way to gain velocity (as efficient as an electric motor).

Right - but think about how you get a cable from there to the ground - you would have to brute force the whole way. 

Traditionally, to lower the orbit, you slow down... Then the leader ship is now moving faster (relative to the ground) so it gets ahead of the CW - which no good, b/c you want the anchor dead below the CW... So you keep burning... Literally until you hit the planet. 

 

Even then, presuming you did that... How do you keep satellites from clipping the cable?