Re: [Freetel-codec2] [Discuss-gnuradio] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

2017-09-13 Thread Adrian Musceac 
On 9/12/17, Phil Frost  wrote:
> I did some experimentation with this technique on BPSK31, and I was able to
> get a little more performance in simulations with a matched filter and
> trellis decoding than fldigi and PSKCore got with their unmatched filters
> designed to minimize the ISI. Though in the case of BPSK31 the ISI isn't
> all that bad, so there less than 1 dB of a gain to be had. Perhaps with DMR
> there's more potential?
>

Hi,
On a related note, I've also done some tests between Codec2 voice
frames and Opus voice frames. For the tests, Codec2 bitrate was 1300
bits/s while Opus was 19.4 kbit/s
Results can be seen here: https://www.youtube.com/watch?v=C9107EA3vNE

As far as I'm concerned, Codec2 does amazingly well at voice frequency
reproduction (I used a synthesized voice on purpose as I believe it
makes it even more difficult for the vocoder.)
However, I can see a purpose for Opus, when combined with H264 video,
since we are already using a lot of bandwidth for the video frames and
the Opus codec does not really add much to it. (the good thing about
Opus frames is that they can be reused to multiplex exactly 8 Codec2
frames into timeslots which can be useful when going TDMA).

Cheers,
Adrian YO8RZZ

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Re: [Freetel-codec2] [Discuss-gnuradio] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

2017-09-12 Thread Adrian Musceac 
Hi Phil,

Of course, you are right, but Trellis is computationally more
expensive than a straight change of the modem parameters. The DMR
software stack is huge, compared to what a small community of largely
open source enthusiasts can come up with. (my problem with DMR as a
radio equipment standard is the use of a closed source audio codec,
not the modem)
It would be even better if someone was to write a pure SDR
implementation of DMR and pit that against David's 4FSK modem :) Real
performance measurements would convince enverybody of the merits of
it.

Cheers,
Adrian

On 9/12/17, Phil Frost  wrote:
> Wouldn't the ISI largely (theoretically, completely) be ameliorated by
> trellis decoding? And I'm not talking about FEC, but rather incorporating
> the ISI into the expected Cartesian coordinates for each symbol.
>
> I did some experimentation with this technique on BPSK31, and I was able to
> get a little more performance in simulations with a matched filter and
> trellis decoding than fldigi and PSKCore got with their unmatched filters
> designed to minimize the ISI. Though in the case of BPSK31 the ISI isn't
> all that bad, so there less than 1 dB of a gain to be had. Perhaps with DMR
> there's more potential?
>
> On Tue, Sep 12, 2017 at 6:50 AM Adrian Musceac  wrote:
>
>> Hi,
>>
>> In this post: http://www.rowetel.com/?p=4650 David was asking the
>> question why the 4FSK modem used by the DMR standard seemed to be
>> performing so poorly, especially as compared to his ideal 4FSK modem.
>> I was curious myself, so I implemented both and analysed them using my
>> modified version of Gqrx, known by some people as gqrx-digital.
>> The results can be seen in this video:
>> https://www.youtube.com/watch?v=hLeJvqrFrS8
>>
>> As you can see, the DMR modem uses a sensitivity of PI/sps, which leads
>> to
>> a lot of ISI, about 8 dB worth, compared to the optimal 4FSK modem which
>> uses a sensitivity of 2*PI/sps. The reason why the DMR modem does this is
>> twofold:
>> One is bandwidth efficiency. But the most significan one is the fact that
>> DMR radios use the same FM modulator for both digital and analog voice.
>> Using double sensitivity would lead to analog voice being too wide for
>> standard FM channels in commercial bands.
>>
>> Of course, for me as a developer, I can create two Gnuradio flowgraphs,
>> so
>> I can use one modulation type for digital, and another one (proper) for
>> analog FM transmission.
>>
>> Hope this is useful for someone.
>>
>> 73,
>> Adrian YO8RZZ
>>
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>

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