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.

DSDcc itself uses 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

If you choose to compile and install this plugin 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

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: Modulation detected

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.

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
• (+DMR): non-inverted isolated DMR voice frame
• (-DMR): inverted isolated 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

4: Decoder input level in %

Most satisfactory decodes for values 50~70%. When no signal is detected it defaults to 91%.

5: Channel power

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

6: Channel bandwidth before discriminator

This is the bandwidth of the pre-discriminator filter

7: Gain after discriminator

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

8: Audio volume

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

9: 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.

10: Squelch level

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

11: 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.

12: Audio mute and squelch indicator

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

13: 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.

13.1: D-Star status display

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

13.1.1: Repeater 1 callsign

13.1.2: Repeater 2 callsign

13.1.3: Destination (your) callsign

13.1.4: Origin (my) callsign

13.2: DMR status display

13.2.1: Station role

• BS: base station
• MS: mobile station

13.2.2: TDMA slot #0 status

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

13.2.3: TDMA slot #1 status

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

14: Discriminator output scope display

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.

14.1: 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.

14.2: 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.

14.3: 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

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.

14.4: I gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset.

14.5: Q gain

You should set the slider to a unity (1) span (+/- 0.5) with no offset.

14.6: Trigger settings

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