Digital SSTV (DRM)
What it is
HAMDRM, also called DSSTV or just DRM, sends a picture as compressed data using many carriers in a narrow 2.2 or 2.5 kHz channel, with error correction. Where analog SSTV degrades gradually with noise (noisy lines, color errors), DRM delivers a clean copy if the signal is good enough and loses data in whole segments if it is not. The HAMDRM software in this program is based on RX/TXAMADRM by PA0MBO. DRM and analog SSTV use the same SSTV/DRM settings tab on the Receive and Transmit pages; DRM is the DRM tab.
Transmitting a DRM picture
- On the Transmit tab choose the DRM settings tab.
- Load a picture (Transmitting) and optionally set Framing and Grid.
- Pick a Profile or set Mode, QAM, BW, Interleave, Protection and Rs by hand.
- Use the size slider to choose how much the picture is compressed. The box beside it shows the size in KB and the transmit time. Smaller means a faster but lower-quality picture.
- Press Start transmit.
Parameters
These come from the tooltips in the program.
| Setting | Choices | Meaning |
|---|---|---|
| Mode | A, B, E | Robustness mode. Higher mode letters are more robust but slower. |
| QAM | 4, 16, 64 | Bits per carrier. 4 is the most robust; 64 is the fastest but needs a clean signal. |
| BW | 2.2 kHz, 2.5 kHz | Occupied bandwidth. Wider is faster. |
| Interl. | Short, Long | Time interleaving. Long survives longer fades but adds delay. |
| Prot. | High, Low | Error-correction strength. High is more robust but slower. |
| Profile | Profile 1 to 3 | A saved parameter preset; edit them in Configuration > DRM. |
| Rs | None, RS1 to RS4 | Extra Reed-Solomon coding on file data. RS1 to RS4 add more redundancy: slower, more robust. |
A good starting point is the default profile. If the other station reports errors, move to a more robust setting (lower QAM, higher protection, Long interleave, or a higher Rs).
Large pictures at 4-QAM can take many minutes. The command line option --drm-size (see
command line) and the size slider control the picture size.
Receiving
Select the DRM settings tab on the Receive tab and leave the receiver running. The status indicators TIME, FRAME, FAC and MSC turn on as the signal is found, and the picture is assembled segment by segment. The indicators and their meanings are described under Receiving. When the picture is complete it is saved in the RX DRM Images directory and shows in the gallery's RX DRM tab. Check the MER numbers: they tell you how clean the signal is.
Fixing an incomplete picture
If some segments were lost, press Fix File on the DRM receive tab. A dialog titled Send BSR (latest on top) lists the incomplete pictures. Choose one and press Compatible or Easypal to match the style the other station's software understands. The program then transmits a BSR (block status report) asking the sender to resend the missing segments. If no incomplete DRM picture has been received, you are told there is nothing to request. Fix File is only active in DRM mode when the program is not busy transmitting or receiving.
Binary files
The Send binary file toolbar button on the Transmit tab sends an arbitrary file as DRM data. If hybrid mode is on, the file is uploaded first and you confirm with Start Transmitting.
Hybrid mode
With Hybrid ticked on the Transmit tab's DRM mode, the picture goes to an internet server as well as over the air. The setup is on the Hybrid configuration page. When it is enabled, the Upload Now button uploads the current picture without transmitting. On the receiving side, the Receive tab's DRM page has the notification box and the Who is on? button. The Operator page's online status option controls whether other users can see you in their Who is on? list; it uses the same server, is published once when the program starts in DRM mode, and stays listed for an hour (details in Operator).