Hi David,
On 09/04/2015 06:26 PM, David Rowe wrote:
> For the VQ-ed 1200 bit/s mode - no not yet. But happy if some one wants
> to take a look at that.
I only have limited time, but if you give me an existing codebook with N
vectors, I can give you a reordered version of it that should minimize
the effect of bit error.
Cheers,
Jean-Marc
> On 05/09/15 07:26, Jean-Marc Valin wrote:
>> 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
>>>>
>>>> ------------------------------------------------------------------------------
>>>> _______________________________________________
>>>> Freetel-codec2 mailing list
>>>> [email protected]
>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>>>>
>>>
>>> ------------------------------------------------------------------------------
>>> _______________________________________________
>>> Freetel-codec2 mailing list
>>> [email protected]
>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>>>
>>
>> ------------------------------------------------------------------------------
>> _______________________________________________
>> Freetel-codec2 mailing list
>> [email protected]
>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>>
>
> ------------------------------------------------------------------------------
> _______________________________________________
> Freetel-codec2 mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>
------------------------------------------------------------------------------
_______________________________________________
Freetel-codec2 mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
