-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 03/03/13 14:25, Jens Christian Hillerup wrote:

I basically just wanted to throw it out here. Does anybody have experience in modulating data? Has this kind of digital one-way communication been done in an activist setting before? Does it make sense to kick off a project aimed at creating a easily usable system capable of modulating and demodulating data at modest bitrates (>15KB/s)?

Hi Jens, Last year I spent some time playing with audio encoding of data for transmission over handheld radios. The state of the art here is dialup modems - on a good day they can get 56,000 bits per second over a channel designed for voice, but that requires advanced modulation and error correction techniques. The radio hams have packet radio (AX.25 and APRS) running at 1,200-9,600 bps over long distances using simple modulation and no error correction. Some early home computers used audio cassettes for storage (300-1,200 bps, CUTS or Kansas City Standard). If you want to support purely one-way communication (no acks), you'll need to forward error correct the data. Hamming codes and parity checks are simple to implement but they'll eat a lot of your bandwidth; Reed-Solomon codes are more bandwidth-efficient but also more complex. Some Java code for modulation, framing and error correction is here if you're interested: http://briar.git.sf.net/git/gitweb.cgi?p=briar/sandpit;a=tree;f=src/net/sf/b... http://briar.git.sf.net/git/gitweb.cgi?p=briar/sandpit;a=tree;f=src/net/sf/b... Cheers, Michael -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iQEcBAEBAgAGBQJRNMBmAAoJEBEET9GfxSfMytEIAK2Y+r+22fKXbXCTFC2ALigw NoeHiHu0wfz4phyPZ5Dqk21dvvOFtP2RJaf+XY70cHKaYw2NW3dWnKNRmshJKtfH oCuZhjRAHhyy8xP+xJFNonEJgQrPkGHe7oFZ6FiqcpmOKL3RPU9G7oyrYYxr1Yas IX5Oxn7N3dG5BmkrN25VLrEieRumLMzakmywkFb2wQE1aQ2RnnQ9/SUPMnTQiPnV tK39FvSCJwbNVzmwZemVGMuiH/xiI3zJeIwtIK86HUEcKX9dS5Nk//b2al4RgXD/ mjwDD2ctBqu1JT7hMgOsETU37cU2BE14KpRfpR0mTzk6Nc8ZtA6KgW88lVku1u0= =Cpee -----END PGP SIGNATURE----- -- Too many emails? Unsubscribe, change to digest, or change password by emailing moderator at companys@stanford.edu or changing your settings at https://mailman.stanford.edu/mailman/listinfo/liberationtech ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org ______________________________________________________________ ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE