The Audio App is the main way that signals can be heard and seen in detail. Three types of decoders are provided for audio modulated signals and a spectrum view of the signals. The user interface has the ability to view and change:
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SPEC: Display a Spectrum of the received signal and allow viewing of 10MHz of RF Spectrum, centered on a configurable frequency, with 5MHz above the frequency and 5MHz below.
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AM: Demodulate and Record RF Signals modulated using the Amplitude Modulation scheme. It can demodulate Double-Sideband AM (ITU Designation: A3E) and both Lower-Sideband and Upper-Sideband Single-Sideband AM (ITU Classification: R2E, H3E, J3E) signals.
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NFM: The Narrow Band Frequency Modulation decoding ITU Classification: FM3
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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.
The Key Items on the App that can be seen or selected with the cursor and changed with the encoder knob are:
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Title bar: The usual Items may be changed and displayed.
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Mode: On the line below title bar is the demodulation mode AM, NFM, WFM, SPEC. When either of these are selected it will bring up a secondary set of relevant items on the line below. These are discussed in secondary items below.
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Frequency: The Centre frequency of the demodulation band.
Note 1 : From Frequency field, you can move down to the below Step field , to adjust your best suitable Freq-step, according to your needs.
Note 2 : If you have load/save App Settings from SD card enabled, it will continue to use what it was last set at. And you can always edit the file in the SD card, /SETTINGS/rx_audio .ini and edit the step size there, according to your needs in that App.
Note 3 : Additionally , to be more user friendly , from version 1.7.4+ holding in the Select button on the Frequency field for a second until a digit turns blue, then you can use Left/Right select which digit you'd like to adjust, and then you can use the Encoder Dial to adjust any digit up/down by 1 to tune more precisely. (Press Select again to exit this tuning mode.
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Gain: Settings are shown in order of LNA(IF) (0-40) and VGA (Baseband Gain) (0-62). When either of these are selected in the secondary line the AMP setting is shown and can be as either set to 0=0db or 1=14dB.
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Signal Display: The three coloured displays are top to bottom RSSI(Red/Blue) with an average marker in the line. Next is the Baseband signal and last the Audio level.
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Volume: The Last item on this line is the audio volume control (O-99) that is used with either headphone or speaker if fitted.
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Secondary Information: This line provides associated information for the following items these are:
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AM: Bandwidth settings of DSB 9k, DSB 6k, USB+3k, LSB-3k, CW. The Spectrum view is +/-20k.
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NFM: Bandwidth Settings of 16k,11k,8k5. Note there is no setting for the more common 6k5 used in European Spectrum plans. Next item is SQ: which is shown in the format of 40/99 allow the noise squelch point to be set Between 0-99. Typically, around 40-50 is a good threshold.
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Gain: The RF Amp settings. The Spectrum view is +/-20k.
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WFM: There are three option filters in that Secondary settings :
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(1) 200k ,the original filter for commercial FM stations with soft transition,
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(2) 180k with sharp transition, for also commercial FM broadcast station, specially useful to improve demodulated S/N in around 6 to 8 dB‘s in weak signals .
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(3) 40k with also sharp transition, for supporting NOAA APT weather satellite reception in 137 MHz . (that filter is too narrow for WFM with 75khz delta deviation and can produce audio distortion in the demodulated sound). The Spectrum view is +/-100k with a marker That may be changed (though seem the incorrect value).
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Just below REC Icon we have a marker Frequency field input (0 Hz) , that we can adjust from 0..48kHz range with the knob rotary encoder, with 200 Hz step jumps. It will move a short vertical red line pointer cursor across the Multiplex FM demodulated baseband FFT spectrum graphic. It could be useful to see and confirm the exact frequency of those spectrum peaks, like the below picture , showing 19Khz pilot carrier tone from a usual Stereo FM Broadcasting.
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* **SPEC.** The spectrum Secondary Items allows the view of the RF spectrum with different setting for maximum bandwidth shown:
20M with markers at +/- 5M
10M with markers at +/- 3M
5M with markers at +/- 2M
2M with markers at +/- 500k
1M with markers at +/- 300k
500k with markers at +/-200k
The next item is the setting of the bin sizes used for the waterfall (0-63) with “0” being the minimum information being the fastest display and “63” the maximum information collected the slowest display. Adjust to give a balance of speed and information seen.
And next right to that field ,while you are still in SPEC mode, you will see the Rx_IQ_CAL field. It actually can improve (8 to 10 dB's from worse CAL point to the best one ) the receiver Image Reject Ratio (IRR) only in the Receiver Applications that are using Zero IF-frequency tuning ,like this SPECTRUM mode. This calibrated value will be stored in the SD card , /settings/rx_audio.ini .
To calibrate it , you will need to try to find some clean isolated continuous air carrier signal (please make sure that it is a real air signal, not internal harmonic residual interference clock beats) , and try to tune it , (for example, to the right of the positive frequencies ). Select properly the SPEC BW to try to isolate that air carrier signal (in below example , I selected BW = 2 Mhz). Then, to make slightly more visible the bare Image mirror frequency -that appears in the other symetric left part screen.-, you will need to start set up CAL to 0 (min). Then , you will need to adjust AMP , LNA and GAIN to receive strong enough signal to the right part (but at the same time , not so strong to not produce intermodulations , otherwise re-adjust again decreasing AMP, LNA, GAIN) , and not so weak , till just getting the carrier +freq. clean (desired carrier signal) , and its bare visible Image mirror Signal (in the other symetrical -xx mirror freq . (unwanted Image signal to be minimized) . Then, when you got similar situation as left below picture, you can start adjusting Rx IQ phase CAL till minimizing the received Image "mirror" signal (as in the middle below picture).
That below example is calibrating H2+ old r6 version (max2837) that has 31 steps ,(Note , using Hackrf r9 (max2839 ,you will have 64 steps)
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CTCSS: This Continuous Tone Coded Squelch System is a display at the end of the secondary information line. It is used by many systems and standardised by EIA/TIA, with a description here. In the NFM mode in Audio app, the tone field is increased from 11 to 14 characters. Now , the CTCSS tone frequency and CTCSS code numbers are displayed at once (if a matching freq was found in the tone_key table). See the Annex to this document below. It should be noted that most of the time the display is jumping around and only clearly displays the received tone when there is a gap in the voice and the Signal is of good quality.
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Record: The record button if selected will show the record file name, % of the SD Card used, and at the end of the line is the total recording time available left on the SD card and this decrements when recording. The file name of the recorded 16-bit mono WAV file includes the date, time, and radio tuning frequency. The WAV file sampling rate depends on the receiver mode; 48KHz for WFM, 24KHz for NFM, or 12KHz for AM.
CTCSS Tone List
None 0.0
0 XZ 67.000
1 WZ 69.400
2 XA 71.900
3 WA 74.400
4 XB 77.000
5 WB 79.700
6 YZ 82.500
7 YA 85.400
8 YB 88.500
9 ZZ 91.500
10 ZA 94.800
11 1ZB 97.400
12 21Z 100.000
13 1A 103.500
14 1B 107.200
15 2Z 110.900
16 2Z 114.800
17 2B 118.800
18 3Z 123.000
19 3A 127.300
20 3B 131.800
21 4Z 136.500
22 4A 141.300
23 4B 146.200
24 5Z 151.400
25 5A 156.700
40 -- 159.800
26 5B 162.200
41 -- 165.500
27 6Z 167.900
42 -- 171.300
28 6A 173.800
43 -- 177.300
29 6B 179.900
44 -- 183.500
30 7Z 186.200
45 -- 189.900
31 7A 192.800
46 -- 196.600
47 -- 199.500
32 M1 203.500
48 8Z 206.500
33 M2 210.700
34 M3 218.100
35 M4 225.700
49 9Z 229.100
36 -- 233.600
37 -- 241.800
38 -- 250.300
50 0Z 254.100
Axient 28kHz 28000.0
Senn. 32.768k 32768.0
Senn. 32.000k 32000.0
Sony 32.382k 32382.0
Shure 19kHz 19000.0