[LeanSDR](https://github.com/pabr/leansdr) framework from F4DAV is intensively used. It has been integrated in the source tree and modified to suit SDRangel specific needs.
This plugin can be used to view digital amateur analog television transmissions a.k.a DATV. The only supported standard for now is DVB-S in various modulations. The standard modulation is QPSK but experimental configurations with other PSK modulations (BPSK, 8PSK, QAMn) can be selected.
The whole bandwidth available to the channel is used. That is it runs at the device sample rate possibly downsampled by a power of two in the source plugin.
⚠ Note that DVB-S2 support is experimental. You may need to move some settings back and forth to achieve constellation lock and decode. For exmple change mode or slightly move back and forth center frequency.
This is the constellation of the PSK or QAM synchronized signal. When the demodulation parameters are set correctly (modulation type, symbol rate and filtering) and signal is strong enough to recover symbol synchronization the purple dots appear close to the white crosses. White crosses represent the ideal symbols positions in the I/Q plane.
- DVB-S: Normally only QPSK and BPSK (later addition) are supported in the standard but amateur radio use has a little bit abused of the standard so PSK6, QAM16, QAM64 and QAM256 are also supported
LeanSDR feature: attempts to fix signal spectrum shape by eliminating peaks. This is the number of peaks to be tracked. It is safer to keep the 0 default (no notch).
In addition to the controls a MODCOD status text appears on the right of the standard selector (1) that give the mode and code rate as retrieved from MODCOD information. When the MODCOD information has triggered the automatic mode and rate selection (2) and (4) the text background turns to green.
<h5>B.2b.2 and 4: Mode and rate selection</h5>
The mode and rate selection can be done manually but if a discrepancy in the number of bits per symbol appears compared to the MODCOD information then the MODCOD information takes precedence and the selection is changed automatically and the status background (3) turns to green.
<h5>B.2b.5: DVB-S specific controls</h5>
The controls specific to DVB-S are disabled and greyed out. These are: Fast Lock, Allow Drift, Hard Metric and Viterbi.
This is for experimenters only working in Linux. It can be used to decode signals lower that ~10 db MER which is the limit of LDPC hard decoding as explained next (B.2b.7). Video degrades progressively down to about 7.5 dB MER and drops below this limit.
Runs the `ldpctool` program for soft LDPC decoding. Frames are sent on its standard input and decoded frames retrieved from its standard output. Two processes executing `ldpctool` are spawned but so far it seems that only one is effectively used.
Right clicking on this control opens a dialog where you can choose:
- The `ldpctool` executable. You have to use the `ldpctool` binary produced by the build of SDRangel.
- The maximum of retries in LDPC decoding from 1 to 8.
<h5>B.2b.7: DVB-S2 specific - LDPC maximum number of bit flips allowed</h5>
May vary between 0 and 500. On strong signals (17 dB SNR or more) you may set it to 0 thus saving processing on occasional bit flips that will not cause significant degradation. When SNR decreases you may set it to a medium value of about 200 allowing bit flips corrections to take place and decoding of signals down to about 10dB MER. A higher value towards the maximum (500) will not make a significant difference and is allowed mostly for experimentation.
If you have enough processing power it is fine to always leave it at 200.
Comparatively:
- with max = 0 you need ~15 dB MER which corresponds to ~16 dB CNR
- with max = 200 you need ~10 dB MER which corresponds to ~13 dB CNR
Show the audio volume on a 0 to 100 scale. The actual coefficient applied to the audio samples follows a logarithmic rule where 0 corresponds to 0.01, 50 to 0.1 and 100 to 1.0
