Hi,
I am currently working with Peter on defining the format for a VHF modem.
There are some things where I would like to get the help of the
knowledge of the list. It deals with the syncronisation pattern put in
front of the stream.
First some information:
In a digital communication system that is purely sequencial (like GMSK),
there is usually a "syncronisation pattern" that is put in front of the
stream.
The "what does a D-STAR signal really look like" blog article show a bit
how it works (1). Scroll down to the part " A close-up of the beginning
of the stream"
The goal is to allow the receiver to syncronise itself to the stream it
receives: detect the bitstream, give the FM receiver some time to get on
that and get the "bit syncronisation" process started.
As a result of David's comment yesterday about autodetecting "audio
reverse", I was thinking what else we could "autodetect": like baudrates
or even transmission modes.
1/
First baudrates:
- The current gmsk runs at 4800 bps while there has been the discussion
to support other (lower) bitrates
- The gmsk receiver actually samples the audio at 48 Khz, which means
that a bit of information is equivalent to 10 audio-samples.
Option 1:
- allow lower baud-rates, but only baud-rates where the periode of a bit
is a multiple of 1/48000 seconds.
This would allow bitrates like:
- 4800 bps (10 samples / bit)
- 3600 bps (15 samples / bit)
- 3000 bps (16 samples / bit)
- 2400 bps (20 samples / bit)
- 1800 bps (30 samples / bit)
- 1200 bps (40 samples / bit)
(among others)
Now, enter the issue of the "syncronisation pattern".
At first, I was only thinking about bitrates that have a period that is
a multiple of 1/4800 second.
So, specify this:
"the bit syncronisation stream must be at least 32 time "01", running at
the bitrate of the codec".
The idea would be this:
The receiver decodes the stream at the maximum speed: 4800 bps
Depending on the speed of the sender, the received pattern would be as
follows:
- Receiving a 4800 bps stream: "010101010101..."
- Receiving a 2400 bps stream: "001100110011..."
- Receiving a 1800 bps stream: "000111000111..."
- Receiving a 1200 bps stream: "000011110000...."
This would be easy to implement, but would exclude certain bitrates
(like 3600 bps or 3000 bps).
Question:
Does anybody see a reason to support bitrates other then the four
available in option 2?
P.S. I like the idea of a 1800 bps stream. This would be roughly
equivalent of a non-protected codec2-stream running at 1400 bps plus
some overhead for syncronisation.
2/
Modulation systems:
Now, appart of gmsk modulation, there is also the idea of use 4FSK/C4FM.
Some background info:
GMSK -which is nothing more then a variant of 2FSK- has two states per
symbol ("0": voltage > 0, or "1": voltage < 0). This allows for 1 bit /
symbol/
C4FM/4FSK allows for 4 states per symbol: -1, -1/3, +1/3 and +1. This
allows for sending 2 bits / symbol; permitting -either- for twice as
much bits to be send per channel, or reducing the bandwidth of a channel
to half (but without 3db S/N gain).
If a 4FSK signal is decoded with a GMSK demodulator, -1 and -1/3 are
both decoded as "1". +1/3 and +1 are decoded as "0".
By doing some modification of the syncronisation pattern when sending a
4FSK/C4FM signal, it should be possible to make the system determine if
the incoming stream if a GMSK or a C4FM stream.
So, the question is simple:
Is there a reason to forsee C4FM/4FSK on a codec2 VHF format?
- It can double the bitrate of the bitstream, but concidering the
bitrate needed by codec2, is that really needed?
On the other hand, there could be applications like the following:
-> inter-repeater links carrier multiple streams
-> pure-data streams
- It can also half the bandwidth of the channel, as this can also be
done using gmsk at 2400 bps, is there a need for this?
- Perhaps make it optional in this version of the specification? (it can
become mandatory in a latter stage, if needed).
Does anybody see a reason to add C4FM in this version of the specs?
(appart from "I want everthing").
One final remark:
Do note that we are talking about the SPECIFICATION of the codec2 VHF
modem. Do not expect all these wissles and bells to be present in the
first actual implementation of it? This will -in the first stage- be
simply 4800 bps gmsk.
A technical side question:
Can somebody please explain what is the exact difference (if there is
one) between 4FSK and C4FM? Most documents either describe C4FM as "a
special version of 4FSK" or simply equate it.
Who can explain the exact difference between these two?
(1) http://villazeebries.krbonne.net/hamstuff/?p=106
73
Kristoff - ON1ARF
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