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 passband to demodulate an audio carrier but this does not work for a subcarrier which excludes FM.
An optional rational downsampler with lowpass filtering can be used to set the channel sample rate to the 0.5 MS/s multiple which is the closest to the source sample rate. When the downsampler is not engaged the source feeds the channel directly and thus the source sample rate is used. A standard image quality for PAL standard modes requires a sample rate of at least 4 MS/s. The Airspy Mini 3 MS/s mode may still be acceptable.
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. Acceptable images (shown in the screenshots here) can be obtained in FM with just 1.3 MHz bandwidth.
<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.
<h3>3: Rational downsampler toggle</h3>
Use this toggle button to enable or disable the rational downsampler.
Without downsampling the sample rate given by the source plugin is directly applied to the channel.
When the downsampler is engaged the channel sample rate is the closest multiple of 0.5 MS/s below the source sample rate. e.g for a source sample rate of 1.6 MS/s this will be 1.5 MS/s. If a non null sample rate cannot be obtained the decimator is disabled and the source sample rate is used instead.
When the downsampler is engaged the signal is lowpass filtered and the cutoff frequency can be adjusted with the in band filter cutoff slider (13). This works also when the decimation ratio is 1.0 e.g source sample rate is an exact multiple of 0.5 MS/s.
This is the number of points or samples per complete line including sync and padding. This is derived from the sample rate and line frequency as the ratio of the two. For example with a 625 lines × 25 FPS signal the line frequency is 15625 Hz. If the channel sample rate is 1500 kS/s this yields 1500000/15625 = 96 points. If the ratio is not an integer then the integer part is taken.
Picture definition depends largely on this number and the larger the better but it is useless to have a much greater number than the number of points per line used in transmission.
This allows adjstment of BFO frequency in 10 Hz steps from -5 to +5 kHz. You will have to look for the right value to lock to the carrier. See (6) for the lock indicator.
The BFO base frequency in Hz appears on the right. Actual frequency may change acoording to PLL locking to the carrier.
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.
- PAL625L: 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.
- PAL525L: the only difference with PAL625L is the number of black lines which is down to 15
Use this slider to adjust the top level of the synchronization pulse on a 0 to 1V scale. The value in mV appears on the right of the slider. Nominal value: 100 mV.
Use this slider to adjust the black level of the video signal on a 0 to 1V scale. The value in mV appears on the right of the slider. Nominal value: 310 mV.
This is the line length in time units. The value appears on the right of the slider. Nominal value depends on the nominal line frequency. For example with 405 lines and 20 FPS. The line frequency is 405 × 20 = 8100 Hz thus the nominal line time is the inverse of this value that is ≈123.45 μs
The slider step is set to a sample period in order to ensure that the adjustment is done with the best possible precision. For example at 1500 kS/s sample rate this will be the inverse of this value that is ≈666.67 ns. The middle position of the slider sets the nominal value and the slider step appears in the tooltip.
This is the length in time units of a horizontal or line synchronization pulse. The value appears on the right of the slider. Nominal value depends on the nominal line length as described above. The nominal pulse length is derived from the 4.7 μs pulse of a 625 lines standard system with a 64 μs line length. For example with a 405 lines × 20 FPS transmission that has a line length of ≈123.45 μs this is (4.7 / 64) × 123.45 ≈ 9.07 μs. In practice you will adjust it to a slightly smaller value to be able to synchronize.
Similarly to the line length slider the slider step is set to a sample period in order to ensure that the adjustment is done with the best possible precision. The middle position of the slider sets the nominal value and the slider step appears in the tooltip.
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 the [source folder](https://github.com/f4exb/sdrangel/tree/master/plugins/channelrx/chanalyzerng) of this plugin 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.
