Hi David,
Just one thing that came to mind because you're talking about FEC... did
you ever look at optimizing index assignment in codec2 to minimize the
effect of bit errors?
Cheers,
Jean-Marc
On 09/04/2015 05:42 PM, David Rowe wrote:
> Hi Thomas,
>
> It's great to see you have been thinking about these issues. As Codec 2
> is open source, you can experiment and try them out.
>
> Some thoughts:
>
> + FreeDV does not have a hard threshold like other DV systems. It uses
> "brain FEC" - this is part of the design Mel Whitten has been advocating
> for many years.
>
> + FreeDV 700B uses a coherent QPSK modem (rather than DQPSK) and
> diversity - rather than reducing the bandwidth it is spread out in
> frequency to combat fading. This has given significant gains in
> simulations and now proven real world performance exceeding SSB.
>
> + FreeDV 1600 uses partial FEC of selected bits, however we are getting
> equivalent or better results with no FEC with our latest 700B waveform.
>
> + FEC, in general, has problems when applied to narrow band HF DV:
>
> http://www.rowetel.com/blog/?p=4147
>
> + I don't think dynamic reduction of the rx bandwidth alone will help.
> The SNR of the remaining carriers will be the same, each carrier has the
> same power as before and the noise power in each carrier is the same.
>
> Cheers,
>
> David
>
> On 04/09/15 19:11, Thomas Schiller wrote:
>> Hi David, hello List,
>>
>> thank you for your work on FreeDV and Codec2. I'm strongly
>> interested in a better HAM-DV which must be solve different
>> demands rather than commercial DV. One major DV-drawback is
>> the hard decision cutoff at low s/n. While in SSB and even
>> FM the demodulated s/n is rather proportional of the HF s/n
>> all DV-systems have a stong threshold. This is a good approach
>> for commercial but a poor behavior to HAM. We like to use our
>> brain to listen inside of the channel noise to get the last
>> sample biological-approximated to what we often already expect
>> to know or hear (for example in a contest).
>>
>> Your attempt to use a 2nd carrier points in the right direction.
>> I've written a suggestion to a HAM-defined DV-system in the german
>> amateur journal FUNKAMATEUR ("Analoge und digitale Sprachsysteme
>> für den Amateurfunk"; FA12/12, Page 1260-1263).
>>
>> http://archiv2.funkamateur.de/search
>>
>> The idea is to use at least four D-QPSK carriers shifted to minimize
>> interference. The inner should be modulated with the basic parameters
>> and a strong FEC. The carriers with more distance to the central
>> should carry information to enhance the quality and could get a weaker
>> or at maximum center-distance may be no FEC. The distribution of
>> parameter bits to the carriers must be carefully choosen out of a list
>> of significance of your CODEC2 or any other codec. As far as I know
>> nobody has followed this idea before.
>>
>> Doing so, the RX then has the chance to dynamically reduce the received
>> bandwith if the s/n is getting weak. Using only half of the carriers
>> could enhance the s/n up to 6dB which is very much! Well of cause it
>> will sound then more "spacy" below the decision point. But this is
>> much more similar to what we hear on analog SSB or FM if the HF-signal
>> becoms weak. At least one can listen to the behavior of the cannel and
>> it's a nice gift to enhance the covering range at the same tramsmitting
>> power! This is amateur's most wanted feature at all ;-)
>>
>> This is very different to commercial DV-systems which we currently use
>> to "recycle" or adapt to an HAM application. Commercial DV needs a
>> fixed s/n decision due to their cell-design and the quality necessities
>> of the ordinary telephone user. In fact cell-design is not really
>> compatible to what amateurs need on short wave. Hence we will never get
>> below this interesting cutoff level if we don't put dynamic bandwith
>> adaption to the HAM-DV functional specification. Do you agree?
>>
>> It would be very nice to do some experiments to find out what one can
>> win in practice on a real HF-channel.
>>
>> 73, Thomas - DC7GB
>>
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