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Updated Audio Receivers (markdown)

Brumi-2021
2025-04-06 17:49:17 +02:00
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Audio-Receivers.md

@@ -9,7 +9,7 @@ The Audio App is the main way that signals can be heard and seen in detail. Thre
* **WFM:** The Wide FM Receiver is a Sub-Application of the Audio Receiver Application. Its purpose is to Demodulate and Record RF Signals modulated using the Frequency Modulation scheme. It can demodulate mono and stereo Wide FM signals of 200KHz bandwidth. Such signals are commonly used for VHF FM Broadcast services.
* **AMFM:** Here in that Audio App we can tune , and RECORD the APT signal into file.wav ,mainly for investigation and check signal levels (signal strenght , saturation , filters check , debugging). But we have also another external receiver [Wefax App](https://github.com/portapack-mayhem/mayhem-firmware/wiki/WeatherFax) (based on that AMFM one) , more convenient to the user, that combines both things : (that APT demodulation and the additional display weather map to the LCD (in low resolution) able to record the received weather map into bmp file (in full resolution. -much better than the displayed in the screen)
* **AMFM:** Here in that Audio App we can tune , and RECORD the APT signal from a HF-Weather Fax map Broadcasting Station into audio_format_APT_file.wav ,mainly for investigation , debugging , and check signal levels (signal strenght , saturation , filters check ,...). But we have also another external receiver [Wefax App](https://github.com/portapack-mayhem/mayhem-firmware/wiki/WeatherFax) (based on that AMFM one) , more convenient to the user, that combines both things : (that APT demodulation and the additional display weather map to the LCD (in low resolution) able to record the received weather map into bmp file (in full resolution. -much better than the displayed in the screen)
We are combining in that demodulation , two consecutive demodulations. First RF USB audio baseband demodulation , and later demodulating the FM subcarrier audio baseband tones.That modulation , it is commonly used in Wefax (HF Weather Fax transmissions) . It demodulates APT signal of the HF weather fax maps. , and sends the demodulated low scan map APT signal in parallel to the speaker (just for our audio tuning reference ) and it can be also RECORDED to the SD card , through audio.wav file (SR =12KHZ, audio mono track).
Inside audio baseband spectrum , there is a sub-carrier audio tone , modulated in FM with the low scan APT image map signal .
(Audio Baseband with f_subcarrier fsc (1900Hz) FSK mod +-400Hz ; Shift White pixel +400 Hz = 2.300 Hz , Shift Black pixel -400 Hz = 1.500 Hz)