Electronics > Projects, Designs, and Technical Stuff
1200 baud data transfer over audio passband of cellphone. Is that possible?
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sv3ora:

--- Quote from: Siwastaja on October 30, 2019, 07:26:36 am ---
--- Quote from: Psi on October 30, 2019, 06:14:30 am ---i wonder if you could craft the signal so the audio compression algorithm couldn't compress it a huge amount.
And get extra bandwidth out of the system that way.

--- End quote ---

No, as shown by the earlier replies, GSM compression is a fixed rate (of 13 kbit/s).

You'll get the most bandwidth out of the system by carefully designing and optimizing the modulation scheme for the GSM compression.

--- End quote ---

The problem is not the bandwidth, but the compression that seems to be applied by the codecs, which do not let the supposed tones pass for more than a few seconds.
sv3ora:
Do you think that I could pass the 1200 baud 1200Hz/2200Hz through the wired telephone instead, or will I have the same problems as the GSM?
jhpadjustable:

--- Quote from: sv3ora on October 30, 2019, 08:42:55 am ---Do you think that I could pass the 1200 baud 1200Hz/2200Hz through the wired telephone instead, or will I have the same problems as the GSM?

--- End quote ---
Worth a try. The fewer switches and long-haul links your call has to pass through, the better your odds.


--- Quote from: sv3ora on October 30, 2019, 08:41:28 am ---The problem is not the bandwidth, but the compression that seems to be applied by the codecs, which do not let the supposed tones pass for more than a few seconds.

--- End quote ---
Bandwidth is sometimes informally used to refer to data rate.
amyk:
It's worth seeking out the official GSM specs (they are dense but free) on codecs and reading them carefully, and also playing around with a PC-based simulator, it will save all the hassle of manipulating actual phones (unless you like that.)
_echo:
Hello,
Not exactly 1200bps, but if you're trying to get binary data through the audio channel of a cell phone (or any phone for that matter), you might try borrowing the portions of the FSK encoding used by TTYs/TDD. Although their use is declining, this baudot infrastructure is still active today.

The core encoding is half duplex FSK, traditionally it's 45.4 bauds/bps. There are a few faster encodings out there, it's possible that those were included.

Telecom vocoders usually have hooks to play nice with TTY, some require a menu option to enable, but it's certainly present in most cell phones.

Sorry, I don't have any additional details. I thought that it might cause some others to remember the encoding.
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