This plugin can be used to generate an analog TV signal mostly used in amateur radio. It is limited to black and white images as only the luminance (256 levels) is supported.
There is no sound either. You coud imagine using any of the plugins supporting audio to create a mixed signal. This is not working well however for various reasons. It is better to use two physical transmitters and two physical receivers.
In practice 4 MS/s is the lowest sample rate that produces a standard image quality. Lower sample rates down to 1 MS/s produce low quality images that may still be acceptable for experiments.
<h3>1: Frequency shift from center frequency of reception direction</h3>
The "+/-" button on the left side of the dial toggles between positive and negative shift.
<h3>2: Frequency shift from center frequency of reception value</h3>
Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position.
Use this button to toggle mute for this channel. The radio waves on the icon are toggled on (active) and off (muted) accordingly. Default is channel active.
- FM: Frequency modulation. Excursion is half the bandwidth available given the channel sample rate. e.g. for 4 MS/s sample rate this is ±1 MHz (2 MHz total)
This control has now a couple of sliders that do not appear on the screenshot.
In all but SSB modulations the right slider is is the bandwidth in MHz of the channel filter when the interpolator is engaged. If the sink sample rate is an integer multiple of the base rate that depends on the standard then there is no interpolation else the channel rate is set to the closest integer multiple of the base rate below the sink sample rate.
The modulating video signal sample rate is the same as the channel sample rate hence for example with a channel sample rate of 4 MS/s a video sample will last 1/4 = 0.25 μs. In PAL625L a complete horizontal line lasts 64 μs hence it will be composed of 64 × 4 = 256 samples while the useful image line that lasts about 52 μs will be composed of 52 × 4 = 204 samples or points.
When no interpolation and hence no soft RF filtering in the channel takes place then filtering relies on the hardware. The hardware filter bandwidth can be set in the sink plugin.
In USB and LSB modulations the right slider controls the sideband FFT filter cutoff. This filter includes DC to avoid the distorsion of the video signal that has a significant DC component.
In VUSB and VLSB modulations in addition the left slider controls the opposite sideband FFT filter cutoff.
- PAL625L: this is the PAL 625 lines standard with 25 FPS. Since only black and white (luminance) is supported this corresponds to any of the B,G,I or L PAL standards
- Image: still image read from the file selected with button (13). If no image is selected an uniform image is sent with the luminance adjusted with button (10)
- Video: video file read from the file selected with button (14). If no image is selected an uniform image is sent with the luminance adjusted with button (10). Buttons (15) and (16) control the play.
- Camera: video signal from a webcam or supported video source connected to the system. If no source is selected an uniform image is sent with the luminance adjusted with button (10). Button (21) selects the camera source. Button (20) plays or stops the camera on a still image.
This button toggles the display of an overlay text on a still image, or a video signal from a file or a camera. Note that for still images you will have to toggle off/on this button to take any change of text or text brightness into account.
In this textbox you can type the text that will be displayed as overlay when button (11) is engaged. The brightness level of the text is selected with the luminance adjust button (10). The number of characters is limited to 12.
<h3>13. Still picture file select</h3>
Clicking on this button will open a file dialog to let you choose an image file for still image display. When the dialog is closed and the choice is validated the name of the file will appear on the space at the right of the button.
<h3>14. Video file select</h3>
Clicking on this button will open a file dialog to let you choose a video file for video play. When the dialog is closed and the choice is validated the name of the file will appear on the space at the right of the button.
<h3>15. Play loop video</h3>
Use this button to toggle on/off playing the video file in a loop
Use this button to toggle on/off the video file play. When play stops the current image is displayed as a still image. When video is stopped the button is dark and a play (►) icon is displayed on the button. When video runs the button is lit and a pause (▋▋) icon is displayed on the button.
This slider can be used to randomly set the currennt position in the file when file play is in pause state (button 16). When video plays the slider moves according to the current position.
Use this button to toggle on/off the camera play. When play stops the current image is displayed as a still image. When camera is stopped the button is dark and a play (►) icon is displayed on the button. When camera runs the button is lit and a pause (▋▋) icon is displayed on the button.
On Linux systems when the play button is engaged for the first time the FPS of the camera is scanned which can take some time (100 frames are read). A message box appears while the operation is running.
Use this combo to select the camera source when more than one is available. the number corresponds to the index of the camera during the camera scan at the startup of the plugin instance. A maximum of 4 cameras are scanned in whichever order presented by the system.
This is the camera FPS. On Windows there is no dynamic FPS check and a fixed 5 FPS is set for each camera. On Linux 90% of the calculated FPS is divided by the number of scanned cameras to give the final FPS. This is an attempt to avoid congestion when multiple cameras are available however this was only tested with two cameras.
