Hi guys,
Does any body has the source code and/or the algorithm for IMBE vocoder
to decode P25 signals?
I am doing a project on P25 software radio receiver and I need the IMBE
vocoder source code.
Thanks,
Teka Yemane
IMBE is proprietary and the holder of the rights, DVSI, is very very
very unwilling to share. You can try to license it from them, but
experience has shown that they are not interested.
Kelly
IMBE is proprietary and the holder of the rights, DVSI, is very very
very unwilling to share. You can try to license it from them, but
experience has shown that they are not interested.
According to:
the IMBE codec is patented, and here’s the patent number: US 5,870,405.
Which means you are free to read it and understand it, and maybe even
implement it yourself to make sure you know how it works. And if you
live in a country without software patents, then you can probably
even distribute the software.
I’ve also read that a bunch of radios that use IMBE are not
interoperable with each other. Perhaps the company tweaks the format
for
different customers to avoid easy interoperability. So after reading
the patent and writing some code and recording some over-the-air
traffic, you might still need to do a bit of sleuthing to figure out
exactly which bits go where in the signal. That is, they might have
some “trade secrets” in addition to the patent. The fun of trade
secrets is that once somebody figures them out without violating any
contract or law, they get no legal protection.
But that’s the fun of having a “protocol analyzer” for the radio
spectrum – the high and mighty can’t just wave their hand and say
“proprietary, proprietary” and leave you stuck. If they transmit a
signal, you can receive it in high resolution, and your mind and your
software can noodle about how to make sense of it – whether the
manufacturer likes it or not.
John
On Mon, Sep 29, 2008 at 12:58 PM, LRK [email protected] wrote:
I would like very much to use GnuRadio to do simulations so if anybody
figures out how to do IMBE in software, please post it here.
It looks like there are documents for sale which describe exactly how
to become inter-operable with P25:
TIA-102.BABA "Project 25 Vocoder Description"
TIA-102.BABA-1 "Project 25 Vocoder Description Addendum 1"
Brian
On Wed, Sep 24, 2008 at 08:49:08PM -0700, John G. wrote:
implement it yourself to make sure you know how it works. And if you
live in a country without software patents, then you can probably
even distribute the software.
Best I can tell, this patent covers the use of error checking to send
bits generated by vocoders of several types including IMBE. I do not
see a good description of IMBE vocoders.
Since the basic vocoder method of sampling speech and analyzing the
sample to produce a data stream to generate something sounding like
speech at the receiving end of the circuit was used in military systems
in the 1960’s, it is not clear what was ‘improved’ to patent IMBE.
In any case, we built a prototype ‘conference bridge’ around 1966 to
connect three of these vocoders in a conference because they could not
be conferenced at an analog point. Doing the vocoder steps twice gave
an uninteligible collection of sounds.
Since analog patching of P-25 systems is part of the National Emergency
Communications Plan, I have tried to determine if any work has been done
on inteligibility of such patches but inquiries to APCO and SAFECOM have
been unproductive.
I would like very much to use GnuRadio to do simulations so if anybody
figures out how to do IMBE in software, please post it here.
–
LRK
[email protected]
teka yemane wrote:
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Discuss-gnuradio Info Page
Hello,
In the P25 Training guide there is some information on the P25 frame
structure.
When it describes the Voice Data (VC1 = VC9) in the frame on Page 48,
the interleaving, error correcting, etc are done by the IMBE Codec.
These are described in TIA-102.BABA. The current Codec chips decode and
encode to baseband so it handles the whole stream.
Chapter 7 gives some overview on how the Codec works. There are some
other patents that describe IMBE used in other services and the error
correcting techniques are similar. I like using the Patent Search in
Google.
Patent 5,805,612 Mechanism for repeater error mitigation Partial
discussion of how the P25 voice frames are encoded.
Patent 5,491,772 Methods for speech transmission The appendix describes
the IMBE codec used in INMARSAT M. Different frame rate but the
techniques are similar.
If you search on IMBE in the Google Patent search there are many other
patents.
73 Eric