Thank you Ron! This does look like exactly what I need. I almost reached out to you directly, seeing all the DrMpeg commits related to gnuradio DTV blocks.
Thanks for your help and all the contributions you've made to the community.
Matt
On Thu, Apr 18, 2024 at 6:33 PM Ron Economos <w6rz@comcast.net> wrote:
Matt,
Take a look at my DVB-S2 OOT. This is a different version of the DVB-S2
transmitter that implements VCM (as opposed to the regular CCM version
included in GNU Radio).
https://github.com/drmpeg/gr-dvbs2
It implements exactly what you're talking about to achieve VCM (Variable
Coding and Modulation). In VCM, the coding and modulation parameters can
change on a frame by frame basis. To accomplish this, the parameters are
sent as stream tags.
To get you started, here's where the stream tag is first created. It bit
packs a 64-bit PMT with all the parameters.
https://github.com/drmpeg/gr-dvbs2/blob/master/lib/bbheader_bb_impl.cc#L496
Then it's sent downstream and all the subsequent blocks use it for
configuration (and also pass it downstream to the next block).
Ron
On 4/18/24 09:39, Matt Clemons wrote:
> Hello,
>
> I've been using the dtv/dvbt blocks in GnuRadio Companion to decode
> dvbt signal recordings. I noticed a lot of the blocks in the RX chain
> require the user to select the modulation parameters (Constellation,
> Hierarchy, Code Rate, etc.). My goal is to remove a lot of these user
> parameters with the use of Transmission Parameter Signals (TPS) data
> called out in the DVB-T spec. This would make the DVBT receiver more
> autonomous, and be better than brute forcing all the modulation
> settings until you get a recovered transport stream in the cases where
> you don't know the transmitter settings. This solution seems to be a
> worthwhile improvement to the current DVBT blocks by utilizing the TPS
> data called out in the spec. Implementing this in the c++ code is
> where I'm running into issues and where I'm looking for help/advice.
>
> In the DVB-T spec the Transmission Parameter Signals (TPS) data can be
> recovered which will tell you all the modulation parameters of the
> main signal. As long as you know the TX mode and Guard Time, you can
> configure the OFDM Symbol Acquisition block and the FFT block, which
> are the only 2 required blocks before the Demod Reference Signals
> (DRS) block. The DRS code already decodes the TPS data but is only
> looking at 2 of the total 68 bits to determine what the frame index
> is, and then clears the std::deque of TPS data.
>
> My goal is to use all of the TPS data and either pass it in a tag or
> pass through a new port so that all the remaining downstream blocks
> can get the modulation parameters from that data rather than a user.
> I've dug into the tags and the PMT types required to pass data, but I
> can't wrap my head around how I would configure the modulation
> parameters of the downstream blocks from the port or tag TPS data
> passed to it.
>
> For example, a lot of the DRS modulation parameters are not needed for
> the DRS block to work, and when you recover Constellation Type,
> Hierarchy Type, and Tx Mode you could then pass these parameters to
> the DVB-T Demap block instead of having to create the block with those
> parameters.
>
> If there are any examples of other blocks using tags or ports to
> change the functionality of another block, that would be useful. Since
> I am fairly new to GnuRadio and C++ these modifications seem difficult
> to me, hoping someone with more experience in both could point me in
> the right direction.
>
> thanks!
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