I decided to take a little break and launched SSTV (Slow Scan TeleVision) reception via WebSDR in Linux Mint.

SSTV is an analog standard for transmitting color images over the air for radio amateurs.
HamDRM is a digital standard based on OFDM (16QAM) with the ability to transfer links to FTP for displaying pictures. Files can also be transferred using HamDRM.

Both standards use a standard audio bandwidth of 2.5 kHz, which allows data to be transmitted using a sound card and an SSB transceiver on both HF and VHF in SSB or FM mode. While the SSTV standard has been around since the 1970s and is widely popular, HamDRM is a recent development thanks to Cesco ham, HB9TLK, who adapted the Digital Radio MondiaLe audio and video standard for amateur radio communications.

The advantages of the SSTV and EasyDRF standards are that they do not use any network resources for transmission. In addition, SSTV can be launched on any frequency range of 80, 40, 20 or 10 meters - "calling frequencies" are provided. SSTV can even be received on CB in the 11 meter range.

The article will only consider the option of receiving images and links (URL) in the QSSTV program, without using a transceiver through a WebSDR receiver. The fact is that currently I do not have HF antennas in the city where I live. SSTV activity occurs in the 14 MHz band.

Beyond the scope of this post is the Easy Digital Radio Files transfer mode (EasyDRF). Therefore, not all transmissions on DSSTV calling frequencies can be decoded using QSSTV.

Preparation

In a system for use with a transceiver, i.e. To work on air, a sound card must be installed:

aplay -l

**** list of PLAYBACK devices **** 
Card 1: Audio [USB Audio], Device 0: USB Audio [USB Audio]
Subdevices: 0/1
Subdevice #0: subdevice#0

If the sound card is connected but is not detected, check the device using the program hwinfo.

sudo apt install hwinfo
hwinfo

51: USB 00.3: 0000 Unclassified device
[Created at usb.122]
Unique ID: wFvx.Npxn1JxF5yB
Parent ID: pBe4.v2Pu0CqjCO5
SysFS ID: /devices/pci0000:00/0000:00:1e.0/0000:01:09.2/usb2/2-4/2-4:1.3
SysFS BusID: 2-4:1.3
Hardware Class: unknown
Model: "Realtek USB Audio" 
Hotplug: USB
Vendor: usb 0x0bda "Realtek Semiconductor Corp." 
Device: usb 0x4e16 "USB Audio" 
Revision: "0.03" 
Driver: "usbhid" 
Driver Modules: "usbhid" 
Device File: /dev/input/event3
Device Files: /dev/input/event3, /dev/input/by-id/usb-Generic_USB_Audio-event-if03
Device Number: char 13:67
Speed: 480 Mbps
Module Alias: "usb:v0BDAp4E16d0003dc00dsc00dp00ic03isc00ip00in03" 
Driver Info#0:
    Driver Status: usbhid is active
    Driver Activation Cmd: "modprobe usbhid" 
Config Status: cfg=new, avail=yes, need=no, active=unknown
Attached to:#61 (Hub)

One day I encountered a problem - the sound card was not detected. Restarting the computer, pressing F2 and changing the sound card mode in BIOS Setup from HD Audio to AC97 helped (but I don’t remember exactly).

dconf write /org/gnome/desktop/sound/input-feedback-sound "false" 
dconf write /org/gnome/desktop/sound/event-sounds "false" 

and reboot the PC.

Without using a transceiver, only for SSTV reception via WebSDR,no sound card needed. Since the sound input is made through a software loopback device to the input of the QSSTV program, which “listens” to the sound from the PulseAudio server, connected to a web browser that receives sound from WebSDR.

To redirect sound from a web browser to the input of the receiving program - QSSTV, a regular or virtual audio cable is not needed, just make a small adjustment to the sound mixer pavucontrol, as previously described in the article Redirecting WebSDR audio to FLdigi input in Linux using the standard Pavucontrol volume control.

1) Installing an application for receiving SSTV and HamDRM

For Linux there is a package qsstv containing a program of the same name. It can receive and transmit slow TV pictures - SSRV and digital HamDRM.

sudo apt install qsstv

After installing the qsstv program, I launched it from the command line

qsstv

2) Dutch WebSDR will be used as a source http://websdr.ewi.utwente.nl:8901/, at which you need to set the frequency to 14230 kHz.

3) In the QSSTV program, you must click the "Receive" button. - Far left "Start receiver"
. The audio device specified in the Config menu will begin receiving information. You should use the PulseAudio method in Options - Config (default):

In the QSSTV program, reception and transmission are carried out on a standard (default) audio device in the system. If there is a sound card, then from it. For me this is an external USB sound card, but there may also be a sound controller built into the motherboard.

For the mode with WebSDR, you need the audio input to go into the QSSTV program from a virtual device, you need to install and configure the volume mixer.

sudo apt install pavucontrol

4) Setting up the pavucontrol volume mixer

With the qsstv receiver running, run pavucontrol in another window. It can also be found in the "Start" menu - "Audio and Video" -"PulseAudio volume control".

In the mixer we find a bookmark "Record". On it we change the sound source - either a USB card (for real reception and transmission using the radio station) or “Monitor of USB Audio” (for WebSDR):

For redirected audio source (WebSDR):

Real sound source (sound card) for transmission through the transceiver:

If a WebSDR receiver is launched in parallel in the browser, the qsstv program will immediately begin receiving images.
But first you need to set your web browser to a frequency of 14230 or 14233 kHz (depending on the “waterfall”).

Daytime SSTV frequencies:

• 14230 kHz - for analogue SSTV
• 14233 kHz - calling HamDRM

SSTV Night Frequency:

• 3730 kHz

On waterfall, SSTV and HamDRM broadcasts look different. SSTV is a pronounced tone mode, and HamDRM spectrum is similar to uniform “white noise”.

HamDRM reception (hybrid mode)

Reception frequency - 14233 kHz.

In QSSTV, I clicked on the second DRM tab.
Reception is fully automatic. But the success of reception depends, as usual, on the signal-to-noise ratio.
In case of poor S/N ratio, the “constellation” is blurry and decoding does not occur:

Good S/N ratio and correct frequency - correct constellation and successful transmission decoding:

An image downloaded from FTP (hybrid mode) is of very high quality, but it is downloaded via the Internet, not via radio waves:

There is also a long mode - transferring files in another EasyDRF program - I do not touch on it.

Receiving SSTV without a transceiver

Previously, I worked on air in this mode; the series contains, as I understand it, 4 or 5 programs:

1. First correspondent CQ (general call)
2. response to the second correspondent's CQ (transmission of both call signs and report 595)
3. response to the response from the first correspondent (transmission of call signs and report)
4. 73 from the second correspondent (who works for the search)
5. there may be a farewell broadcast from the first correspondent

Transfer modes are usually -Martin 2 or Scottie 1(equal in duration). Speed mode Robot36 less common.
The QSSTV program independently recognizes the transmission mode.

Received SSTV pictures are saved as files with a .PNG extension in a folder /home/user/qsstv/rx_sstv.

Gallery of SSTV and HamDRM images received from WebSDR in the QSSTV program

SSTV frequencies

Range 20 meters

• 14227 kHz USB - spare daytime calling frequency SSTV on HF
• 14230 kHz USB - main daytime calling frequency of SSTV on HF

• 14233 kHz USB - HamDRM daytime calling frequency, also used by SSTV (QRM interference)

  Range 80 meters

• 3730 kHz LSB - SSTV night calling frequency
• 3733 kHz LSB - HamDRM night calling frequency