<h1>DSD (Digital Speech Decoder) demodulator and decoder plugin</h1>
<h2>Introduction</h2>
This plugin uses the [DSDcc](https://github.com/f4exb/dsdcc) library that has been rewritten from the original [DSD](https://github.com/szechyjs/dsd) program to decode several digital speech formats. At present it covers the following:
- DMR/MOTOTRBO: European two slot TDMA standard. MOTOTRBO is a popular implementation of this standard.
- D-Star: developed and promoted by Icom for Amateur Radio customers.
The modulation and format is automatically detected and switched.
To enable this plugin at compile time you will need to have DSDcc installed in your system. Please follow instructions in [DSDcc readme](https://github.com/f4exb/dsdcc/blob/master/Readme.md) to build and install DSDcc. If you install it in a custom location say `/opt/install/dsdcc` you will need to add these defines to the cmake command: `-DLIBDSDCC_INCLUDE_DIR=/opt/install/dsdcc/include/dsdcc -DLIBDSDCC_LIBRARIES=/opt/install/dsdcc/lib/libdsdcc.so`
<h2>DV serial device support</h2>
You can use a serial device connected to your system that implements and exposes the packet interface of the AMBE3000 chip. This can be for example a ThumbDV USB dongle. In order to support DV serial devices in your system you will need two things:
- Compile with [SerialDV](https://github.com/f4exb/serialDV) support Please refer to this project Readme.md to compile and install SerialDV. If you install it in a custom location say `/opt/install/serialdv` you will need to add these defines to the cmake command: `-DLIBSERIALDV_INCLUDE_DIR=/opt/install/serialdv/include/serialdv -DLIBSERIALDV_LIBRARY=/opt/install/serialdv/lib/libserialdv.so`
- Enable DV serial devices in your system by checking the option in the Preferences menu. YOu will need to enable the DV serial devices each time you start SDRangel.
Please note that such serial devices work with a serial interface at 400 kb. While this seems large for a serial interface this limits the throughput and hence the capability of decoding several channels in parallel. The software can enqueue requests over many devices so the more you have the more channels you can decode in parallel. Note also that a channel that does not output voice frames will not require a device for decoding so only channels that receive voice frames at any one time will require a DV serial device.
DSDcc itself can use [mbelib](https://github.com/szechyjs/mbelib) to decode AMBE frames. While DSDcc is intended to be patent-free, `mbelib` that it uses describes functions that may be covered by one or more U.S. patents owned by DVSI Inc. The source code itself should not be infringing as it merely describes possible methods of implementation. Compiling or using `mbelib` may infringe on patents rights in your jurisdiction and/or require licensing. It is unknown if DVSI will sell licenses for software that uses `mbelib`.
If you are not comfortable with this just do not install DSDcc and/or mbelib and the plugin will not be compiled and added to SDRangel. For packaged distributions just remove:
- For Linux distributions: `plugins/channel/libdemoddsd.so`
- For Windows distributions: `dsdcc.dll`, `mbelib.dll`, `plugins\channel\demoddsd.dll`
For software built fron source if you choose to have `mbelib` support you will need to have DSDcc compiled with `mbelib` support. You will also need to have defines for it on the cmake command. If you have mbelib installed in a custom location, say `/opt/install/mbelib` you will need to add these defines to the cmake command: `-DLIBMBE_INCLUDE_DIR=/opt/install/mbelib/include -DLIBMBE_LIBRARY=/opt/install/mbelib/lib/libmbe.so`
<h3>1: Frequency shift from center frequency of reception</h3>
Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Use the "+/-" button on the left side to toggle between positive and negative shift.
<h3>2: Modulation detected</h3>
This is the type of modulation detected by DSDcc. It can be the following:
-`C4FM`: 4-FSK modulation as implemented for example by Yaesu. It is also the default display. Sometimes 4-GFSK signals can be detected as C4FM if the signal is relatively noisy. This should not affect decoding provided noise is not too strong.
-`GFSK`: Gaussian mean frequency shift keying. it is either 2 or 4-FSK (2-GFSK or 4-GFSK). DMR transmissions will typically be identified as GFSK as it is 4-GFSK. D-Star will also be detected as GFSK as it is 2-GFSK.
-`QPSK`: At present no decodable transmissions use QPSK.
When the display is active the background turns from the surrounding gray color to dark green. It shows informatory or status messages that are particular to each format.
On the same line you can choose any trace length. If it is too short the constellation points will not appear clearly and if it is too long the polar figure will be too dense. Usually 100ms give good results.
The yellow trace (I) is the direct trace and the blue trace (Q) is the delayed trace. This can show how symbols differentiate between each other in a sort of eye diagram.
This shows the constellation of transition points. You should adjust the frequency shift to center the figure and the maximum deviation and/or discriminator gain to contain the figure within the +/-0.4 square. +/- 0.1 to +/- 0.3 usually give the best results.
There are 4 possible points corresponding to the 4 possible transitions. x represents the current symbol and y the previous symbol. The 4 points given by their (y,x) coordinates correspond to the following:
- (1, 1): upper right corner. The pointer can stay there or move to (1, -1)
- (1, -1): upper left corner. The pointer can move to (-1, -1) or (-1, 1)
As you can see the pointer can make all moves except between (-1, -1) and (1,1) hence all vertices between the 4 points can appear except the one between the lower left corner and the upper right corner.
There are 16 possible points corresponding to the 16 possible transitions between the 4 dibits. The 4 dibits are equally spaced at relative positions of -3, -1, 1, 3 hence the 16 points are also equally spaced between each other on the IQ or (x,y) plane.
Because not all transitions are possible similarly to the 2-FSK case pointer moves from the lower left side of the diagonal to the upper right side are not possible.
