Electronics > Projects, Designs, and Technical Stuff
1200 baud data transfer over audio passband of cellphone. Is that possible?
coppice:
--- Quote from: TheUnnamedNewbie on October 29, 2019, 12:07:21 pm ---Audio on a cellphone tends to also be compressed in very specific ways that are optimized for voice (which is why the 'please wait' music with callcenters is so horrible). I suspect you will find it very hard to send any data through it as it too will be crazy deformed by the codecs.
--- End quote ---
Yep. Speech codecs are so speech specific that most other things, including multiple voices speaking at the same time, sound awful.
coppice:
--- Quote from: ogden on October 29, 2019, 09:44:21 am ---
--- Quote from: coppice on October 29, 2019, 02:00:01 am ---Most of the speech codecs used for cellular assign a single pitch for each 5ms
--- End quote ---
Then use OFDM modulation with symbol length > 5ms.
--- End quote ---
What does the modulation have to so with the codec being used? They are completely unrelated issues. Most forms of OFDM can add a lot of latency to the channel, but that's about the only specific effect it has on the signal.
ogden:
--- Quote from: coppice on October 29, 2019, 01:34:31 pm ---
--- Quote from: ogden on October 29, 2019, 09:44:21 am ---
--- Quote from: coppice on October 29, 2019, 02:00:01 am ---Most of the speech codecs used for cellular assign a single pitch for each 5ms
--- End quote ---
Then use OFDM modulation with symbol length > 5ms.
--- End quote ---
What does the modulation have to so with the codec being used? They are completely unrelated issues.
--- End quote ---
You talked about standard FSK and PSK modulations, their short symbol length (update every 1.7ms or 0.83ms). I offer OFDM modulation because it has slong(er) symbols that can be stretched depending on FFT order used
borjam:
Old telephone modems based on FSK were the V.21 (300 bps in full duplex) or the asymmetric V.23 (1200/75) intended for videotex systems.
The problem with voice compression algorithms is that they degrade the signal in a way that is mostly harmless for speech intelligibility but they can make it impossible for a data modem to work. Especially if you use more complex modulation systems.
I remember when some telco in Spain begun using compressors in some of their voice trunks. 28800 bps modems were unable to link beyond 9600 bps.
ogden:
--- Quote from: borjam on October 29, 2019, 05:03:01 pm ---The problem with voice compression algorithms is that they degrade the signal in a way that is mostly harmless for speech intelligibility but they can make it impossible for a data modem to work. Especially if you use more complex modulation systems.
--- End quote ---
BS. Many research papers prove you wrong, achieves >1200 bps data transmission rates over GSM-FR (worst case codec, rarely used in modern networks). There are some "secure phone" products that uses not only encrypted VoIP but audio channels as well. You may start here:
https://www.researchgate.net/publication/304296823_Data_transmission_via_GSM_voice_channel_for_end_to_end_security
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