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How do text messages travel from one phone carrier to another?
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tooki:

--- Quote from: tggzzz on January 07, 2019, 03:38:56 pm ---I did. The quality was highly variable. Sometimes the best course was to "hang up and try again".
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Well, that happens even today! :P


--- Quote from: tggzzz on January 07, 2019, 03:38:56 pm ---
--- Quote ---And then to add insult to injury, phones also got worse. The old handsets of the Bell era had nice, big, well-made speakers and microphones in them. The quality of phones post-deregulation plummeted quickly.

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Precisely, although that isn't limited to one carrier and one manufacturer.
--- End quote ---
Who said it was?!? I said “Bell era” to pin down the time, I didn’t mean exclusively “under the Bell system”. ;)


--- Quote from: tggzzz on January 07, 2019, 03:38:56 pm ---The GSM codecs greatly reduce the data rate by encoding the signal, just as mp3 etc does. The difference is the GSM codecs incorporate a model of the human voice tract, so it would be surprising if they transmitted arbitrary sounds well.
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We know they can’t. They’re why hold music sounds so terrible, those codecs are hardcore optimized for voice, and on non-voice they absolutely fall flat and degrade into pure compression artifacts.
madires:

--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

--- End quote ---

There's G.722 with 7kHz for a higher audio quality. Most marketing departments call it "HD voice".
coppice:

--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

--- End quote ---
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.


--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born

--- End quote ---
Most countries only achieved a fully digital telephone network in the late 90s, so unless you are quite young you've probably heard some local calls which were analogue from end to end. They were not too bad, but in the 60s and 70s when most long distance calls were analogue end to end the quality was highly variable, and could be bloody awful. The shift to a digital backbone was the best thing that ever happened to the PSTN.
tooki:

--- Quote from: madires on January 07, 2019, 04:04:26 pm ---
--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

--- End quote ---

There's G.722 with 7kHz for a higher audio quality. Most marketing departments call it "HD voice".

--- End quote ---
I know — I already linked to info about that in the post you’re quoting.



--- Quote from: coppice on January 07, 2019, 04:04:48 pm ---
--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

--- End quote ---
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.
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I don’t think it was quite that bad. (I mean, a bad circuit could be, but a good one was good enough.)



--- Quote from: coppice on January 07, 2019, 04:04:48 pm ---
--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

--- End quote ---
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.


--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born

--- End quote ---
Most countries only achieved a fully digital telephone network in the late 90s, so unless you are quite young you've probably heard some local calls which were analogue from end to end. They were not too bad, but in the 60s and 70s when most long distance calls were analogue end to end the quality was highly variable, and could be bloody awful. The shift to a digital backbone was the best thing that ever happened to the PSTN.
--- End quote ---
Yeah, that’s true, local calls (or at least within the same local exchange) probably were analog.

And yes, no doubt analog long distance was terrible!
newbrain:

--- Quote from: tooki on January 07, 2019, 03:14:28 pm ---And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born
[...]
Anyway, the real issue is standards: two cellphones making a call do not create a high-speed point-to-point data link, nor are they just shooting packets at each other (like FaceTime). They’re running a call via the voice network, which has strict standards that are essentially inviolable. So without upgrading the entire voice network infrastructure, you can’t just easily upgrade the voice quality.

Nonetheless, a few carriers have been doing this, allowing calls made within their networks, when using a supported handset on both ends, to have better sound quality. But support between carriers has been slow to roll out. See https://en.wikipedia.org/wiki/Wideband_audio

--- End quote ---
Old analog lines were, in my personal experience, ranging from horrible to bad.
When the land line was finally switched to a digital exchange (the last mile was still analog, and still is) quality went up quite a bit.

The only compression done on POTS on digital trunks was A-law or u-law (depending on your country), which causes very little loss in quality.

One of the initial phases in a mobile phone call is codec negotiation, where the phones and the network agree on which transcoder to use for compressing voice.
In some lucky case AMR-WB can be selected, but there also cases where the selected codec is worse than POTS quality.
For GSM and 3G, both circuit calls, it is possible that compressed voice is directly exchanged between the user terminals, with obvious advantages, instead of being:
1. compressed in one mobile
2. expanded in the network - where depends on 2G/3G and network architecture
3. transported as A/u-law samples
4. re-compressed before being sent to the other mobile
5. re-expanded in the mobile
Search for "transcoder free operation", but all the planets must be correctly aligned for that to work.


--- Quote from: tggzzz on January 07, 2019, 03:38:56 pm ---The GSM codecs greatly reduce the data rate by encoding the signal, just as mp3 etc does. The difference is the GSM codecs incorporate a model of the human voice tract, so it would be surprising if they transmitted arbitrary sounds well.

--- End quote ---
Exactly, so much that DTMF tones needs to be transported out of band. As above, they might or might not be transformed back to samples in the network.

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