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Old 10-21-2019, 08:34 AM
Machine Elf is offline
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why do astronaut audio transmissions from orbit still sound so bad?


Back in the days of the Apollo missions, NASA was working with 1960s analog radio equipment, so it's not surprising that the audio quality of their transmissions to ground control was even worse than that of a landline telephone call.

In the 50 years hence, there have been some improvements in technology. I can place a VOIP phone call to someone on the far side of the earth and enjoy remarkably high audio fidelity, courtesy of digital compression and transmission; to my ear, it sounds like something close to the full frequency range of human hearing, with perhaps minor digital compression artifacts being apparent.

So why do astronauts in orbit still sound like shit? Is computing power and transmission bandwidth really that limited? That seems doubtful, since there are often video connections between ground and the astronauts on the ISS. What's the deal?
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Old 10-21-2019, 08:53 AM
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Can you point out a recording where it sounds bad? Like you say, there is two-way bandwidth of tens to hundreds of megabits/second between the station and the ground. There are also VHF telephones, which aren't going to sound that good if there is interference, so it depends on which system the recording was made from.
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Old 10-21-2019, 09:35 AM
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The phone calls you're placing never have to punch through the entire atmosphere. They mostly go through wires, except maybe for a cell call, which only has to traverse a few miles in the air.
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Old 10-21-2019, 09:35 AM
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Good question; I was just discussing this with my wife. Practically every transmission from space that I've heard sounds like its going through a megaphone or a cupped hand. Nowadays, I'd expect a broadcast from space to sound as clear as a Saturday Night Live sketch on TV.
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Old 10-21-2019, 09:50 AM
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I read somewhere that the Space Shuttle, for instance, used 32 kbps digital audio on S-band, so even in the best case that will not sound crystal-clear unless some form of compression is used. Still, it's not that low.
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Old 10-21-2019, 10:40 AM
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Originally Posted by DPRK View Post
Can you point out a recording where it sounds bad? Like you say, there is two-way bandwidth of tens to hundreds of megabits/second between the station and the ground. There are also VHF telephones, which aren't going to sound that good if there is interference, so it depends on which system the recording was made from.
Here's the entire recent all-female spacewalk. Audio from GC sounds clear as a bell (presumably the recording was made at GC), but if you click around and look for transmissions from the astronauts, they sound just like the Apollo days.

The cost of the ISS was somewhere around $150B, so as far as cost is concerned, ISTM that VOIP-quality audio would have been a drop in the bucket.
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Old 10-21-2019, 11:07 AM
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The ground does not sound to me clear as a bell: I swear I can hear digital compression, but that could be my imagination. As for the spacewalking people, apparently EVA comms use VHF, and, granted, there should not be too much interference since they are right next to the radio relays in the space station, but that's exactly the same type of narrow-band FM radio technology used in the Apollo days, so why should it sound different.

As to why they use it instead of something more high-tech? Good question, but it's a relatively simple technology, and it works.
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Old 10-21-2019, 11:11 AM
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The phone calls you're placing never have to punch through the entire atmosphere. They mostly go through wires, except maybe for a cell call, which only has to traverse a few miles in the air.
They manage to transmit video from orbit, so the distance doesn't seem to be an obstacle to high-bandwidth digital transmissions.
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Old 10-21-2019, 01:04 PM
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I would assume that the considerations are similar to a Bluetooth headset.

The time it takes to compress is too long for conversation, which is why your headset doesn't work with mp3.

I would wager that video is of a lesser immediate importance than live audio transmission, so especially for external activities the delay is just too much to allow for.

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Old 10-21-2019, 01:58 PM
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Just a WAG, but for an astronaut on an EVA you want extremely reliable communications with minimum lag. Analog degrades gracefully, digital (especially with high degrees of compression) is either there or not.
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Old 10-21-2019, 04:30 PM
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Originally Posted by eldowan View Post
The time it takes to compress is too long for conversation, which is why your headset doesn't work with mp3.

I would wager that video is of a lesser immediate importance than live audio transmission, so especially for external activities the delay is just too much to allow for.
As they say in French: that’s a negative, good buddy.

Modern compression—audio, video, or generic zlib stream compression—is easily done in real time. A headset might use an ASIC for that, I guess, but an FPGA or even an older general-purpose CPU is fast enough for real-time audio compression.

Also, MP3 compression implementation was limited much more often by patent issues than by processing power. Fraunhofer’s patent (USPTO 6,009,399) expired less than three years ago. IIRC, Fraunhofer often required a per-device licensing fee for MP3 compression, and many non-audio-player device manufacturers decided the fee wasn’t worth it.
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Old 10-21-2019, 04:36 PM
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One explanation worth considering: NASA’s audio quality is a throwback for the same reason most commercial aircraft entertainment systems are a throwback: certification for flight is very expensive.

In other words, current voice quality is acceptable and no one has the time or money to certify the electronics required for modern compression algorithms.

I’ve worked on plenty of instruments with chips/boards designed in 1982. They look like they belong in an Atari 2600. And my employer kept them in place as long as those chips were available because getting new chips/boards certified was prohibitively expensive and time consuming.
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Old 10-21-2019, 04:51 PM
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One of the factors is that the microphones are fitted to the astronauts caps (which are different sizes for each astronaut) inside a closed helmet and can't be adjusted easily - or at all if the astronaut is on an EVA. The caps and microphones can shift around inside the helmet and their sweat and other moisture can affect the equipment. The next generation of space suits is probably going to address this by making the microphone a part of the space suit itself.
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Old 10-21-2019, 05:27 PM
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As they say in French: that’s a negative, good buddy.
So now you know why I don't gamble.

Seriously though, I didn't know about the patent expiring, I guess what I thought I knew is quite out of date

The more you know

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Old 10-21-2019, 09:07 PM
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Don’t feel too bad—your point was definitely valid in the past. In 1997, my 150 MHz Pentium took about an hour to encode a 3-min pop song, IIRC.
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Old 10-22-2019, 09:09 AM
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The problem may be entirely at the space station end. Crappy, poorly placed mics. Lousy signal between mic and the transmitter to ground. Stuff like that.

One complication may be the multiplexing system used to allow people to talk and be heard within the space station.

The space station to ground might have tons of capacity to carry digital audio but the local system may be limited, esp. if they don't want a ton of stray radio cruft messing with more sensitive stuff.
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Old 10-22-2019, 09:48 AM
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The spacesuit currently used by NASA astronauts is the Extravehicular Mobility Unit, developed for the Space Shuttle program and first used in 1981. So it's 1970s technology, with only minor improvements since then.
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Old 10-22-2019, 09:58 AM
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It's all marketing. People expect a voice transmission from space to sound awful, helps convince people that we need to keep spending money on human space travel.
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Old 10-22-2019, 03:47 PM
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Quote:
Originally Posted by EdelweissPirate View Post
One explanation worth considering: NASA’s audio quality is a throwback for the same reason most commercial aircraft entertainment systems are a throwback: certification for flight is very expensive.
I'm not an engineer, but I'm willing to bet this is the reason, akin to the reason why the Omega Speedmaster is still the only watch certified for EVA. Surely the technology would be there - high-bandwidth internet access via satellites is a commercially available off-the-shelf product, and there, too, the signals have to punch through the atmosphere (with a much longer distance between ground and geostationary satellites than between ground and the ISS). But before you instal something of that sort on the ISS, you need to get it approved, and for that purpose you need to get the technology certified. Which is an arduous bureaucratic process. The existing certified solutions are not so unsatisfactory as to create a pressing need to get a successor technology certified.
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Old 10-22-2019, 04:08 PM
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...But before you instal something of that sort on the ISS, you need to get it approved, and for that purpose you need to get the technology certified. Which is an arduous bureaucratic process. The existing certified solutions are not so unsatisfactory as to create a pressing need to get a successor technology certified.
Someone at NASA doesn't decide something needs to be used and NASA certifies it because it is needed?
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Old 10-22-2019, 04:24 PM
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Someone at NASA doesn't decide something needs to be used and NASA certifies it because it is needed?
Certification has nothing to do with need. For something that's essential for safety (like communication equipment), it must be proven to be safe, reliable, have sufficient redundancy, etc. For something non-essential (like a science experiment), it just needs to be safe (i.e. not endanger people or the station under any circumstances).

Last edited by scr4; 10-22-2019 at 04:25 PM.
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Old 10-22-2019, 05:49 PM
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Originally Posted by Machine Elf View Post
Back in the days of the Apollo missions, NASA was working with 1960s analog radio equipment, so it's not surprising that the audio quality of their transmissions to ground control was even worse than that of a landline telephone call.

In the 50 years hence, there have been some improvements in technology. I can place a VOIP phone call to someone on the far side of the earth and enjoy remarkably high audio fidelity, courtesy of digital compression and transmission; to my ear, it sounds like something close to the full frequency range of human hearing, with perhaps minor digital compression artifacts being apparent....
There are several factors.

The Apollo lunar missions were coming from up to 1/4 million miles away, and it required a 210-ft diameter steerable dish for full bandwidth (see cars for scale): https://honeysucklecreek.net/other_s...klet/dss14.jpg

For most earth/space transmissions they used "unified S-band" at around 2.1 Ghz, and the voice was embedded with telemetry. The S-band signal was received at the big dishes, then the voice filtered from that, then injected on the NASA terrestrial network. With analog voice radio, each link in the chain typically degrades quality a bit.

Since you don't need more than about 3Khz to clearly convey the human voice, voice transmissions during the Apollo era were filtered to about 3 Khz audio bandwidth. This enabled better clarity for given amount of RF power.

Despite this the overall audio quality wasn't that bad. Listen to the Apollo 15 astronaut's voices from 1/4 million miles away when landing on the moon in 1971. They are apparently using VOX (Voice Operated Mic) so some of the syllables are clipped. This is normal. https://youtu.be/XvKg68DcTZA

In the shuttle/ISS era the audio during live transmissions didn't sound much better. During the latter shuttle era the TDRSS satellites were available so in theory this could have improved quality but to me it sounded about the same.

There is nothing inherent about audio recording from space that makes it sound bad. Listen to this tour of ISS starting at about 00:25, where the audio was apparently captured on the on-camera mic of the camcorder. Despite the background noise, it sounds quite good when the camera is close to the astronaut: https://youtu.be/QvTmdIhYnes?t=25

This indicates when live audio is transmitted, the lower quality is due to the modulation and encoding system. There is generally limited benefit in intelligibility to exceeding much over 3 Khz audio bandwidth. A 6-8 Khz audio bandwidth might aesthetically sound better but for a given amount of RF power and system noise, its not necessarily any better at conveying dialog.

With any "safety of life" application involving a deployed system there is always the changeover problem. E.g, aviation still uses AM radio for voice because changing that to SSB, FM or digital would require maintaining the existing system while installing new radios and antennas in every aircraft and ground station on earth, then somehow switching over or running them in parallel. Additional RF spectrum would also be needed.

We have lots of technology today and the ability to send digitally-encoded voice, but that doesn't always result in better quality. Modern cell phones use digital voice codecs and have the 200 times the computation power of a Cray-1 supercomputer but audio quality is often worse than an old analog cell phone or even a short wave radio.
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Old 10-22-2019, 05:54 PM
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Random browsing turned up the following document, which is instructive in the way these things work:
https://ntrs.nasa.gov/archive/nasa/c...9720022495.pdf (Multi-EVA communications system analysis, Final Report, June 30, 1972).

This 225-page report, prepared by RCA for NASA, lays out and evaluates in comprehensive detail the functional and technical requirements of NASA's proposed system. The conclusion is that it would basically work, but would be not that great and too expensive to implement.

The appendix with all the specifications is interesting: everything from voice dynamic range, frequency response, and noise requirements, to temperature/vacuum/acceleration/humidity/vibration/etc tolerances and testing procedures.

There is much more to it than simply downloading an MP3 codec from the Internet.
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