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

[Phil Frost]

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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Re: [Freetel-codec2] 700C mode development?

[Phil Frost]

On 09/20/2016 12:39 AM, David Rowe wrote:
> General C-style programming skills required with a willingness to learn 
> how to at least read Octave.  I'll give you plenty of help.
> 
> Just contact me off list if you are interested.

I can probably handle that. Where should I start?

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[Freetel-codec2] 700C mode development?

[Phil Frost]

I've heard there's a 700C mode being developed. I can't promise to be
useful, but I'd like to help. I can't find much information on it though,
besides a blog post[1] that mentions it indirectly. Is there some code I
should check out?

And more generally, what kind of help would be most useful? Ample
programming experience here, but nothing specific to radio. Hoping to learn
though. And I have a humble HF station for on-air testing if that's of any
use.

[1]: http://www.rowetel.com/?p=4291
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Re: [Freetel-codec2] Symbol rate of central sync carrier

[Phil Frost]

Thanks David, that explains so much. I was really scratching my head for a
while trying to figure out why it wouldn't sync :)

If I wanted to update the description after I hash out the details and get
my implementation working, what would be the best way to do that?

On Wed, Aug 17, 2016 at 8:25 PM David Rowe <da...@rowetel.com> wrote:

> Hello Phil,
>
> Good question, and actually took me a while to remember why the two sync
> carrier lines are just 25 Hz apart.
>
> FreeDV 1600 uses a symbol rate of Rs=50 symbols/s for each of the 17
> carriers, including the central DBPSK sync carrier.  Note that we are
> using Differential PSK (DPSK) - not PSK.
>
> The start of a 40ms codec frame is indicated by a change in phase in the
> DPSK sync carrier, so for example this sequence of phases (in radians)
> every 20ms:
>
> ... pi pi -pi -pi pi pi -pi -pi .
>
> Note the _change_ in phase every 2 symbols (40ms), indicating codec
> frame sync.
>
> The cosine of the sequence of phases above is 1 1 -1 -1 1 1 -1 -1
>  which (when up sampled and filtered) approximates a sampled sine
> wave of frequency Rs/4 (takes 4 symbols to repeat).  So using this Rs/4
> frequency sequence to modulate the center carrier at frequency fc we get
>
>s(t) = cos(2*pi*t*fc)*cos(2*pi*t*Rs/4)
>s(t) = 0.5*cos(2*pi*t*(fc+Rs/4) + 0.5*cos(2*pi*t(fc-Rs/4)
>
> which is two sine waves Rs/2 = 25Hz apart.
>
> Cheers,
>
> David
>
>
>
>
> The BPSK sync carrier sends alternating ...101010, this a modulating
> signal that takes two symbol periods to repeat.  So it's frequency is
> Rs/2 or 25Hz.
>
> So when you multiply a Rs/2 signal by the central carrier:
>
>   cos(2*pi*fcentre*t)*cos(2*pi*t*Rs/2)
>   = 0.5*cos(2*pi*t(fcentre+Rs/2) + 0.5*cos(2*pi*t(fcentre-Rs/2)
>
> which should look like two sine waves Rs apart
>
> On 18/08/16 00:00, Phil Frost wrote:
> > I've been working on an independent implementation of the FDMDV modem as
> > a personal exercise. As I understood the description*, the central BPSK
> > subcarrier should be at a symbol rate of 50 per second, but looking at
> > the tone spacing it's too narrow: but a 25 symbols per second seems to
> > line up. Am I overlooking something or is the sync subcarrier really at
> > 25 sym/sec?
> >
> > *:
> > http://freedv.org/tiki-index.php?page=FreeDV+Specification
> > http://n1su.com/fdmdv/FDMDV_Docs_Rel_1_4b.pdf
> > http://www.arrl.org/files/file/FDMDV.pdf
> >
> >
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[Freetel-codec2] Symbol rate of central sync carrier

[Phil Frost]

I've been working on an independent implementation of the FDMDV modem as
a personal exercise. As I understood the description*, the central BPSK
subcarrier should be at a symbol rate of 50 per second, but looking at
the tone spacing it's too narrow: but a 25 symbols per second seems to
line up. Am I overlooking something or is the sync subcarrier really at
25 sym/sec?

*:
http://freedv.org/tiki-index.php?page=FreeDV+Specification
http://n1su.com/fdmdv/FDMDV_Docs_Rel_1_4b.pdf
http://www.arrl.org/files/file/FDMDV.pdf

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