This plugin can be used to view amateur analog television transmissions a.k.a ATV. The transmitted video signal can be black and white or color (PAL, NTSC) but only the black and white level (luminance) is retained and hence image is black and white. There is no provision to demodulate the audio subcarrier either. The modulation can be either AM or FM (SSB with carrier is only experimental). A plugin supporting audio can be used in the same baseband to demodulate an audio carrier but this does not work for a subcarrier which excludes FM.
Experimental modes with smaller number of lines and FPS values can be used in conjunction with the [ATV Modulator plugin](https://github.com/f4exb/sdrangel/tree/master/plugins/channeltx/modatv) to reduce sample rate and occupied bandwidth. Very low line frequencies and thus small bandwidths are reachable with degraded image quality. This is known as NBTV see: [Wikipedia article](https://en.wikipedia.org/wiki/Narrow-bandwidth_television) and [NBTV.org](http://www.nbtv.org/)
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. Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows.Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.
This is the fractional part of Sb ÷ (l × F). The demodulator can accomodate a non integral value of points per horizontal line. This value represents the fraction of a point needed to complete the real number of points per line.
With the previous example this value is 0.333... rounded to 0.33 in the display. Thus a line contains effectively 333.333... points.
This allows adjustment of BFO frequency in 1 Hz steps from -2 to +2 kHz. You will have to look for the right value to lock to the carrier. See (6) for the lock indicator. This will work only at relatively low sample rates say below 1 MS/s.
⚠ in AM modes (also USB and LSB) there is an automatic amplitude scaling so that the video signal fits in the 0.0 to 1.0 range. For synchronziation standards other than HSkip (see B.3) it may be difficult for the system to find the appropriate level when the standard changes or the signal power changes abruptly. In this case you have to select HSkip (B.3) until the video signal reaches the 0.0 to 1.0 range approximately (use scope tab see C) before returning to the desired standard.
⚠ USB and LSB modes are experimental and do not show good results for sample rates greater than 1 MS/s. Adjusting the BFO can be picky and unstable.
Using this button you can adjust the nominal FM deviation as a percentage of the channel bandwidth that is displayed on the right of the button. When a signal with this deviation is received the demodulated signal is in the range -0.5/+0.5 which is shifted to a 0/1 range video signal.
☞ The value is accurate only with the atan2 differential demodulator i.e. FM3. With FM1 and FM2 you will have to adjust it for best image results. You can use the scope as an aid to try to fit the video signal in the 0/1 range.
This is a factor in % applied to the detected AM signal range. Because of possible overshoots the detected range may be artificially reduced from the original causing possible errors on the different detection lavels on the video signal. This control lets you correct this. Watch the video signal in the scope tab for fine tuning. It affects only AM signals.
This is a factor in % applied to the signal resulting from the previous correction and that shifts its relative position. Using this control you can position the video signal correctly so that the black level is around 0.3V. Again watch the video signal in the scope tab for fine tuning. It affects only AM signals.
Use this button to enable/disable the FFT asymmetrical filter. Use this filter when you want to optimize the reception of vestigial sideband AM signals.
Choice is between 819, 640, 625, 525, 480, 405, 360, 343, 240, 180, 120, 90, 60 and 32 lines. The actual number of image lines depends on the synchronization scheme.
This combo lets you chose between a 30, 25, 20, 16, 12, 10, 8, 5, 2 and 1 FPS. This is the resulting FPS. In interleaved modes the half frame rate is doubled.
☞ Perception of continuous motion is said to be acceptable down to 16 FPS. Down to 8 FPS fluidity is still acceptable. The 5 to 1 FPS modes can be used for transmission of images with only few movements or where motion is not important such as fixed webcams. Low FPS will allow for more lines and therefore definition in the same bandwidth.
This combo lets you set the TV standard type. This sets the number of lines per complete image, frame synchronization parameters and number of blank (black) lines. Choice is between:
- **PAL625**: this is based on the classical 625 lines PAL system. It uses 7 or 8 synchronization lines depending on the half frame (field). It has also 17 black lines on the top of each half frame.
- **PAL525**: the only difference with PAL625 is the number of black lines which is down to 15
- **819L**: this is the 819 lines system F (Belgium).
- **ShortI**: this is an experimental mode that uses the least possible vertical sync lines as possible. That is one line for a long synchronization pulse and one line equalizing (short) pulses level to reset the vertical sync condition. Thus only 2 lines are consumed for vertical sync and the rest is left to the image. In this mode the frames are interleaved and an odd number of lines should be used.
- **ShortNI**: this is the same as above but with non interleaved frames.
- **HSkip**: this is the horizontal sync skip technique for vertical synchronization. This has been in use in the first TV experiments with a small number of lines. This method just skips one horizontal synchronization pulse to mark the last or the first line (here it is the last). This method does not use any full line for vertical sync and all lines can be used for the image thus it suits the modes with a small number of lines. With more lines however the risk of missing pulses gets higher in adverse conditions because the pulses get shorter and may get swallowed by a stray pulse or a stray pulse can be taken for a valid one. In this case two images might get out of sync instead of just two lines. In practice this is suitable up to 90~120 lines.
☞ Interleaved mode requires an odd number of lines because the system recognizes the even and odd frames depending on a odd or even number of lines respectively for the half images
☞ For non interleaved mode all standards are supposed to work for any number of lines. You may experiment with any and see if it fits your purpose. However it will be easier to obtain good or optimal results in general with the recommendations in the next table provided the transmitter is set to the same standard. Official standards are only 819L (Belgian), PAL625 and PAL525. In this case one should stick to their respective number of lines:
Use this slider to adjust the trigger level of the synchronization pulse on a 0 to 1V scale. The value in mV appears on the right of the slider. Nominal value: 150 mV.
This is the line length in time units. The value depends on the line frequency. For example with 625 lines and 12 FPS The line frequency is 625 × 12 = 7500 Hz thus the nominal line time is the inverse of this value that is ≈133.33 μs
Use this slider to adjust the black level of the video signal on a 0 to 1V scale. This also triggers the end of horizontal synchronization pulse. The value in mV appears on the right of the slider. Nominal value: 300 mV.
This is the length in time units of a horizontal or line synchronization pulse. The value depends on the nominal line length as described above. The pulse length is derived from the 4.7 μs pulse of a 625 lines and 25 FPS standard system with a 64 μs line length. For example with a 625 lines &#times; 12 FPS transmission that has a line length of ≈133.33 μs this is (4.7 ÷ 64) × 133.33 ≈ 9.79 μs.
This is where the TV image appears. Yes on the screenshot this is the famous [Lenna](https://en.wikipedia.org/wiki/Lenna). The original image is 512 × 512 pixels so it has been cropped to fit the 4:3 format. The screen geometry ratio is fixed to 4:3 format. You will have to choose the standard (B.3) matching the transmission to ensure that the transmitted image fits perfectly.
This is a scope widget fed with the video signal. Controls of the scope are the same as with the ChannelAnalyzerNG plugin. Please refer to [this plugin](https://github.com/f4exb/sdrangel/tree/master/plugins/channelrx/chanalyzerng) for more details.
Note that the video signal is a real signal so the imaginary part is always null. Therefore only the "Real" mode for both the trace and the trigger is interesting.
