sdrangel/plugins/channel/demoddsd

DSD (Digital Speech Decoder) demodulator and decoder plugin

Introduction

This plugin uses the DSDcc library that has been rewritten from the original 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 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

DV serial device support

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 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.

Note also that this is not supported in Windows.

Mbelib support

DSDcc itself can use 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

Interface

1: Frequency shift from center frequency of reception

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.

2: Symbol (Baud) rate

Here you can specify which symbol rate or Baud rate is expected. Choices are:

• 2.4k: 2400 S/s used for dPMR and 4800 b/s NXDN
• 4.8k: 4800 S/s used for 9600 b/s NXDN, DMR, D-Star and YSF.

3: Type of frame detected

This can be one of the following:

• +DMRd: non-inverted DMR data frame
• -DMRd: inverted DMR data frame
• +DMRv: non-inverted DMR voice frame
• -DMRv: inverted DMR voice frame
• +D-STAR: non-inverted D-Star frame
• -D-STAR: inverted D-Star frame
• +D-STAR_HD: non-inverted D-Star header frame encountered
• -D-STAR_HD: inverted D-Star header frame encountered
• +dPMR: non-inverted dPMR non-packet frame
• -dPMR: inverted dPMR non-packet frame
• +NXDN: non-inverted NXDN frame (detection only)
• -NXDN: inverted NXDN frame (detection only)
• +YSF: non-inverted Yaesu System Fusion frame (detection only)
• -YSF: inverted Yaesu System Fusion frame (detection only)

4: Symbol synchronization zero crossing hits in %

This is the percentage per symbols for which a valid zero crossing has been detected. This can display 101% because somtimes there are two crossings per symbol period at the start of a sequence. The more the better the symbol synchronization is tracked however the zero crossing shifts much not deviate too much from 0 (see next).

5: Zero crossing shift

This is the current (at display polling time) zero crosing shift. It should be the closest to 0 as possible. However some jitter is acceptable for good symbol synchronization:

• 2400 S/s: +/- 5 inclusive
• 4800 S/s: +/- 2 inclusive

6: Matched filter toggle

Normally you would always want to have a matched filter however on some strong D-Star signals more synchronization points could be obtained without. When engaged the background of the button is lit in orange.

7: Transition constellation or symbol synchronization signal toggle

Using this button you can either:

• show the transitions constellation
• show a indicative signal about symbol synchronization
  • when a zero crossing is detected the signal is set to estimated input discriminator signal maximum value
  • when the symbol clock is 0 (start of symbol period) the signal is set to the estimated median point of the input discriminator signal

8: Discriminator input signal median level in %

This is the estimated median level (center) of the discriminator input signal in percentage of half the total range. When the signal is correctly aligned in the input range it should be 0

9: Discriminator input signal level range in %

This is the estimated discriminator input signal level range (max - min) in percentage of half the total range. For optimal decoding it should be maintained close to 100.

10: Channel power

Total power in dB relative to a +/- 1.0 amplitude signal received in the pass band.

11: Channel bandwidth before discriminator

This is the bandwidth of the pre-discriminator filter

12: Gain after discriminator

This is the gain applied to the output of the discriminator before the decoder

13: Audio volume

This is the audio volume for positive values. A value of zero triggers the auto volume (audio AGC).

14: Maximum expected FM deviation

This is the deviation in kHz leading to maximum (100%) deviation. You should aim for 30 to 50% (+/-300 to +/-500m) deviation on the scope display.

15: Squelch level

The level corresponds to the channel power above which the squelch gate opens.

16: Squelch time gate

Number of milliseconds following squelch gate opening after which the signal is actually fed to the decoder. 0 means no delay i.e. immediate feed.

17: Audio mute and squelch indicator

Audio mute toggle button. This button lights in green when the squelch opens.

18: Format specific status display

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.

18.1: D-Star status display

These is the standard D-Star embedded information that is read from the header frame.

18.1.1: Repeater 1 callsign

18.1.2: Repeater 2 callsign

18.1.3: Destination (your) callsign

18.1.4: Origin (my) callsign

18.2: DMR status display

18.2.1: Station role

• BS: base station
• MS: mobile station
• NA: not applicable or could not be determined

18.2.2: TDMA slot #0 status

• slot0: nothing received in slot #0
• [slot0]: data frame received for slot #0
• [SLOT0]: voice frame received for slot #0

18.2.3: TDMA slot #1 status

• slot1: nothing received in slot #1
• [slot1]: data frame received for slot #1
• [SLOT1]: voice frame received for slot #1

18.2.4: Color Code

This is the color code in use (0 to 15). It may briefly change value to a incorrect one. Take into account the value shown most of the time.

18.3: dPMR status display

18.3.1: dPMR frame tyoe

• --: undetermined
• HD: Header of FS1 type
• PY: Payload frame of a sitll undetermined type
• VO: Voice frame
• VD: Voice and data frame
• D1: Data without FEC frame
• D2: Data with FEC frame
• XS: Extended search: looking for a new payload frame when out of sequence
• EN: End frame

18.3.2: Colour code

Colour code in hexadecimal (12 bits)

18.3.3: Own ID

Sender's identification code in hexadecimal (24 bits)

18.3.4: Called ID

Called party's identification code in hexadecimal (24 bits)

19: Discriminator output scope display

19.1 Transitions constellation display

This is selected by the transition constellation or symbol synchronization signal toggle (see 7)

The discriminator signal at 48 kS/s is routed to the scope display with the following connections:

• I signal: the discriminator samples
• Q signal: the discriminator samples delayed by the baud rate i.e. one symbol delay:
  • 2400 baud: 20 samples
  • 4800 baud: 10 samples
  • 9600 baud: 5 samples

This allows the visualization of symbol transitions which depend on the type of modulation.

19.1.1: Setting the display

• On the combo box you should choose IQ (lin) for the primary display and IQ (pol) for secondary display
• On the display buttons you should choose the side by side display

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.

19.1.2: IQ linear display

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.

19.1.3: IQ polar display

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.

2-FSK or 2-GFSK

This concerns the following formats:

• D-Star

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)
• (-1, 1): lower right corner. The pointer can move to (1, -1) or (1, 1)
• (-1, -1): lower left corner. The pointer can stay there or move to (-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.

4-FSK or 4-GFSK

This concerns the following formats:

• DMR
• YSF
• dPMR
• NXDN

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.

19.1.4: I gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset. This corresponds to full range in optimal conditions (100%). You can set the slider fully to the left (2) for a +/- 1.0 spn if you don't exactly match these conditions.

19.1.5: Q gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset. This corresponds to full range in optimal conditions (100%). You can set the slider fully to the left (2) for a +/- 1.0 spn if you don't exactly match these conditions.

19.1.6: Trigger settings

You can leave the trigger free running or set it to I linear with a 0 threshold.

19.2 Symbol synchronization display

This is selected by the transition constellation or symbol synchronization signal toggle (see 7)

19.2.1 IQ linear display

The I trace (yellow) is the discriminator signal and the Q trace (blue) is the symbol synchronization monitor trace that goes to the estimated maximum discriminator signal level when a zero crossing in the symbol synchronization control signal is detected and goes to mid position ((max - min) / 2) of the discriminator signal when a symbol period starts.

The symbol synchronization control signal is obtained by squaring the discriminator signal and passing it through a narrow second order bandpass filter centered on the symbol rate. Its zero crossing should occur close to the first fourth of a symbol period therefore when synchronization is ideal the Q trace (blue) should go down to mid position in the first fourth of the symbol period.

19.2.2: Setting the display

• On the combo box you should choose IQ (lin) for the primary display and IQ (pol) for secondary display
• On the display buttons you should choose the first display (1)

19.2.3: Timing settings

You can choose any trace length with the third slider from the left however 100 ms will give you the best view. You may stretch further the display by reducing the full length to 20 ms or less using the first slider. You can move this 20 ms window across the 100 ms trace with the middle slider.

19.2.4: I gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset. This corresponds to full range in optimal conditions (100%). You can set the slider fully to the left (2) for a +/- 1.0 spn if you don't exactly match these conditions.

19.2.5: Q gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset. This corresponds to full range in optimal conditions (100%). You can set the slider fully to the left (2) for a +/- 1.0 spn if you don't exactly match these conditions.

19.2.6: Trigger settings

You can leave the trigger free running or set it to I linear with a 0 threshold.