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 levels (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. A plugin supporting audio can be used in the same passband to demodulate an audio carrier but not a subcarrier which excludes FM.
The whole bandwidth available to the channel is used. That is it runs at the device sample rate possibly downsampled by a power of two in the source plugin. It expects an integer number of MS/s and standard image quality requires a sample rate of at least 4 MS/s. The Airspy Mini 3 MS/s mode may still be acceptable. Anything below should be considered experimental quality.
<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 3.2 MS/s this will be 3 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 3 MS/s.
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 excursion as a percentage of the channel bandwidth that is displayed on the right of the button. When a signal with this excursion is received the demodulated signal is in the range -0.5/+0.5 which is shifted to a 0/1 range video signal.
Note that 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
- 405L: this is not the British standard. It just follows the same sheme as the two above but with only 7 black lines per half frame
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 length in time units of a synchronization top. The value in microseconds appears on the right of the slider. Nominal value 3 microseconds.
<h2>C: Image</h2>
<h3>Monitor</h3>
Select monitor with the monitor tab on the left side.
![ATV Demodulator plugin GUI Video monitor](../../../doc/img/ATVDemod_pluginC_monitor.png)
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.
<h3>Scope</h3>
Select scope with the scope tab on the left side.
![ATV Demodulator plugin GUI Video scope](../../../doc/img/ATVDemod_pluginC_scope.png)
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 [readme](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. In fact only the "Real" mode for the trace and trigger is interesting.