Telephone conversations are almost universally digitally encoded these days.
The codec used may be as simple as LPC, or may be one of the perceptual codecs.
There isn't really a guarantee of any undamaged passband. It does not seem to
me that our waveform would pass that. However, you might be able to do use the
FAX pass-through that is generally part of these systems, by using a FAX modem
carrying Codec2 data. Only the FAX data, not the modem carriers, is passed over
the network. This is in generally a one-directional thing, so passing through
data in both directions using existing FAX pass-through is problematical.
Telephones also pass through the very low-speed modem used for burglar alarms.
Touch-tones are passed as data.
You might perform some testing before going farther.
Thanks
Bruce
On April 2, 2014 2:29:23 AM PDT, Kevin Karhan <[email protected]>
wrote:
>Hello everyone,
>
>My Name is Kevin Karhan, I'm currently enlisted as Student for
>IT/Information Science [B.Sc.] at the University of Wuppertal, NRW,
>Germany.
>
>
>I would like to share some troughts I'm having during the process of my
>own project, which is intended to become topic of my bachelor thesis
>since they seemed to be useful.
>
>
>
>As maintainer of the Cryptofon - Project, I've taken a look on the
>needs
>of an Voiceband Modem (which in difference to FDMDV does not use
>multiple 50 baud - links, but instead a single 300 baud link with ~48
>Hz
>difference)
>
>Using a 64-FSK (enabling an 8 bit : 1 baud ratio, the full 2400 bit/s
>for Codec2 are available - so you can either use 2 (syncronized) modems
>at 300 baud, each using 150 baud in a Time Division Duplex.
>
>Regarding a 2400 bit/s transmission, buffered in frames, either useage
>standard 20ms frames, having 48 bit per frame (mean: 6 baud per frame)
>at 50 frames/s, or half the frame rate at 96 bit per frame (mean: 12
>baud per frame) seems useful.
>
>In the case of the Cryptofon, I use a total bandwith of 2400 bit/s -
>including FEC 3/4 - meaning 1800 bit/s of data. Using 300 bit/s for
>Sync
>and other 300 bit/s for additonal Short-burst Data, the system should
>self-syncronize.
>
>In detail, I intent to create frame packets alike these:
>
>bits 1 - 48: (48 bits) Codec2 Frame (1200 bit/s - 40ms frame)
>bits 49 - 60: (12 bits) Short Burst Data
>bits 61 - 72: (12 bits) Sync I/0 (could be likewise the
>Mancester-Sequence of the FDMDV-Modem)
>bits 73 - 96: (24 bits) FEC 3/4 (additional Forward Error correction)
>
>When regarding this 96 bit frame as buffered before transmission, it
>seems useful to have a similar balance at the "subframes" for the baud
>conversion in order to cover lost or incomplete bursts.
>By picking each 12th bit (could be easily pipelined with a LFSR!),
>starting at bit 1, you get 12 well-balanced "subframes", each
>consisting of:
>
>4 bit Codec2
>1 bit SBD
>1 bit Sync
>2 bit FEC 3/4
>
>It seems obvious that creating the FEC 3/4 at this point would make
>sense instead of the frame itself, but I just calulated them in the
>frames for reminder...
>Also instead of FEC 3/4, a similar FEC 6/8 could be used.
>
>
>
>Regarding the primary use of Codec2 for shortwave narrowband / SSB
>amateur radio, the FDMDV modem is still the best choice.
>(Since multipath problems do not encounter on PSTNs, this problem does
>not appear for the Cryptofon in the first place).
>
>
>
>Anyway, I may cross-examine my troughts on this by reading the
>documentation about the FDMDV modem with much more attention than
>before.
>
>
>If the FDMDV modem would offer a total troughput of 2400 bit/s per
>direction and work on PSTN/POTS, too - which I am not shure about since
>it was not designed for it, (in theory, this would generate frequency
>overlays making it hard or even impossible to decode the symbols) I
>could directly integrate it.
>
>
>But ATM I'm focussing of keeping baud rate limited to 300 baud (instead
>of 700 baud) to ensure maximum compactbility even in suboptimal
>constellations (e.g. reversible non-damaging use on a public phone w/
>graphite mic/speaker via self-made acoustic coupler).
>All in all: The Cryptofon has a very bad SNR envoirment it needs to fit
>in...
>
>
>
>
>I'm keeping an eye on the mailinglist...
>
>
>
>Greetings,
>Kevin Karhan
>
>
>------------------------------------------------------------------------
>
>------------------------------------------------------------------------------
>
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Freetel-codec2 mailing list
>[email protected]
>https://lists.sourceforge.net/lists/listinfo/freetel-codec2
--
Sent from my Android phone with K-9 Mail. Please excuse my brevity.
------------------------------------------------------------------------------
_______________________________________________
Freetel-codec2 mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
