Audio Receivers

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:

• 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.

• 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.

• NFM: The Narrow Band Frequency Modulation decoding ITU Classification: FM3

• 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:

• Title bar: The usual Items may be changed and displayed.

• 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.

• 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.

• 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.

• 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.

• Volume: The Last item on this line is the audio volume control (O-99) that is used with either headphone or speaker if fitted.

• Secondary Information: This line provides associated information for the following items these are:

  • AM: Bandwidth settings of DSB 9k, DSB 6k, USB+3k, LSB-3k, CW. The Spectrum view is +/-20k.

  • 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.

  • Gain: The RF Amp settings. The Spectrum view is +/-20k.

  • WFM: There are three option filters in that Secondary settings :

  • (1) 200k ,the original filter for commercial FM stations with soft transition,

  • (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 .

  • (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).

  • 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.

* **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)

• 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.

• 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