Trying to configure RTL-SDR for second receiver

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Trying to configure RTL-SDR for second receiver

Matthew Miller

I'm trying to configure a RTL-SDR as a second receiver and so far it's not working (not recognizing input audio?).


I currently have an Echolink simplex node on Raspberry PI and USB sound-card/GPIO working great.  Now I'm interested in adding a second receiver with a RTL-SDR I already had so I could eventually make it a repeater "as needed".


I've started by adding a second receiver "Rx2" and changing my working Echolink SimplexLogic from Rx1 to Rx2 so I can test the new receiver on otherwise known-working configuration.  I've got it to where it "usually" shows in the log squelch opening when I key my handheld but it doesn't seem to be working -- if I try and activate a module it ignores me and doesn't even print that it got any DTMF commands.  I get no error messages.


I'm intending to run on 446.1 with CTCSS 100.0 on the TX (which I know works on my simplex-node)  441.1 with CTCSS 100.0 on the RX SDR -- and that's what my handheld radio is programmed to for my tests.


I confirmed my handheld radio is working properly, I can hear it thru a scanner very clearly on the expected frequencies and tones.  I also confirmed the SDR works -- Kalibriate-RTL successfully locks on GSM signals reporting offset PPM=46 and I can then plug this in and use SDR#, pymultimonaprs, ADS-B, etc perfectly.  This SDR was previously running an APRS iGate on this same Raspberry Pi with zero issues until I shut down that service to try this new WBRX feature.


Help?




Here's what I have added to my config file:

[Rx2]
TYPE=Ddr
SQL_DET=SIGLEV
SIGLEV_DET=DDR
SIGLEV_SLOPE=2.03
SIGLEV_OFFSET=124.16
SIGLEV_OPEN_THRESH=9
SIGLEV_CLOSE_THRESH=2
CTCSS_FQ=100.0
DTMF_DEC_TYPE=INTERNAL
DTMF_MUTING=1
DEEMPHASIS=1
FQ=441100000
WBRX=WbRx1

[WbRx1]
TYPE=RtlUsb
#DEV_MATCH=0
FQ_CORR=-46
GAIN=15
#SAMPLE_RATE=960000



Here's what I get in the logfile when I try and bring up any module (help, parrot, echolink, self-ID...anything)
Sun May  1 20:32:26 2016: SvxLink v1.5.0 Copyright (C) 2003-2015 Tobias Blomberg / SM0SVX
Sun May  1 20:32:26 2016:
Sun May  1 20:32:26 2016: SvxLink comes with ABSOLUTELY NO WARRANTY. This is free software, and you are
Sun May  1 20:32:26 2016: welcome to redistribute it in accordance with the terms and conditions in the
Sun May  1 20:32:26 2016: GNU GPL (General Public License) version 2 or later.
Sun May  1 20:32:26 2016:
Sun May  1 20:32:26 2016: Using configuration file: /etc/svxlink/svxlink.conf
Sun May  1 20:32:26 2016: --- Using sample rate 48000Hz
Sun May  1 20:32:26 2016:
Sun May  1 20:32:26 2016: Starting logic: SimplexLogic
Sun May  1 20:32:26 2016: Loading RX: Rx2
Sun May  1 20:32:26 2016: Loading TX: Tx1
Sun May  1 20:32:26 2016: Loading module "ModuleHelp" into logic "SimplexLogic"
Sun May  1 20:32:26 2016:       Found /usr/lib/arm-linux-gnueabihf/svxlink/ModuleHelp.so
Sun May  1 20:32:26 2016:       Module Help v1.0.0 starting...
Sun May  1 20:32:26 2016: Loading module "ModuleParrot" into logic "SimplexLogic"
Sun May  1 20:32:26 2016:       Found /usr/lib/arm-linux-gnueabihf/svxlink/ModuleParrot.so
Sun May  1 20:32:26 2016:       Module Parrot v1.1.1 starting...
Sun May  1 20:32:26 2016: Loading module "ModuleEchoLink" into logic "SimplexLogic"
Sun May  1 20:32:26 2016:       Found /usr/lib/arm-linux-gnueabihf/svxlink/ModuleEchoLink.so
Sun May  1 20:32:26 2016:       Module EchoLink v1.3.2 starting...
Sun May  1 20:32:26 2016: SimplexLogic: Event handler script successfully loaded.
Sun May  1 20:32:26 2016: Found Rafael Micro R820T tuner
Sun May  1 20:32:26 2016: [R82XX] PLL not locked!
Sun May  1 20:32:26 2016: WbRx1: Changed state to READY
Sun May  1 20:32:26 2016:       Using device      : [0] Realtek RTL2838UHIDIR SN:00000001
Sun May  1 20:32:26 2016:       Tuner type        : R820T
Sun May  1 20:32:26 2016:       Valid tuner gains : 0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48 49.6
Sun May  1 20:32:26 2016: Connected to EchoLink proxy (my private proxy IP/port)
Sun May  1 20:32:26 2016: EchoLink directory status changed to ON
Sun May  1 20:32:26 2016: --- EchoLink directory server message: ---
Sun May  1 20:32:26 2016: EchoLink Server v2.5.9997
Sun May  1 20:32:26 2016:
Sun May  1 20:32:26 2016: ECHOEC2-1: Herndon, VA USA
Sun May  1 20:32:26 2016:
Sun May  1 20:32:29 2016: Rx2: The squelch is OPEN (24.2424)
Sun May  1 20:32:29 2016: Rx2: The squelch is CLOSED (20.8921)
Sun May  1 20:32:29 2016: Rx2: The squelch is OPEN (21.8141)
Sun May  1 20:32:30 2016: Rx2: The squelch is CLOSED (61.6086)
Sun May  1 20:32:32 2016: Rx2: The squelch is OPEN (18.4396)
Sun May  1 20:32:32 2016: Rx2: The squelch is CLOSED (18.5428)
Sun May  1 20:32:32 2016: Rx2: The squelch is OPEN (32.2562)
Sun May  1 20:32:34 2016: Rx2: The squelch is CLOSED (64.7903)


My hope is to eventually have it switchable between "simplex node" and "repeater" but for now I just need to get the SDR working to any extent.


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Re: Trying to configure RTL-SDR for second receiver

Rob Janssen
Matthew Miller wrote:

I'm trying to configure a RTL-SDR as a second receiver and so far it's not working (not recognizing input audio?).


Strange...  it works OK here on several sites.
Here is a config that works, but it looks very similar to yours:

[DdrRx_NOS]
TYPE=Ddr
WBRX=WbRx1
FQ=430125000
SQL_DET=SIGLEV
SIGLEV_DET=DDR
SIGLEV_SLOPE=2.61
SIGLEV_OFFSET=158
SIGLEV_OPEN_THRESH=15
SIGLEV_CLOSE_THRESH=5
DEEMPHASIS=1
SQL_TAIL_ELIM=250
PREAMP=-5
PEAK_METER=0
DTMF_DEC_TYPE=INTERNAL
DTMF_MUTING=1
DTMF_HANGTIME=200
DTMF_MAX_FWD_TWIST=8
DTMF_MAX_REV_TWIST=4
1750_MUTING=1

[WbRx1]
TYPE=RtlUsb
SAMPLE_RATE=960000
FQ_CORR=97
GAIN=19.7
PEAK_METER=1

Probably the error is not in the SDR config, but in the repeater logic?
You can try putting it in a remotetrx and connect it from an svxlink instance running on a PC.
(with just a repeaterlogic, a netrx and a local tx to the soundcard)
Then you can listen in on the receiver.  I used this to configure several RTL SDR receivers we
installed on remote locations and that were not configured before deployment.

Rob

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Re: Trying to configure RTL-SDR for second receiver

Matthew Miller

Thanks, I'll try that when I get home.  I don't have other machines with svxlink (I'd have to build one that's got the dependencies) but I do have ModuleEcholink configured to work with svxlink as well as my smartphone if that would be an equivalent test.


Am I correct in believing if I enter DTMF commands I should see log entries "Recognizing digit X" the same way I normally do with a sound-card receiver input?

I know my SimplexLogic works properly with a sound-card (Rx1), I figured changing the receiver to Rx2 should not make a difference in how the logic functions, only modifying the input source?


From: Rob Janssen <[hidden email]>
Sent: Tuesday, May 3, 2016 1:01:45 PM
To: Discussions about development issues
Subject: Re: [Svxlink-devel] Trying to configure RTL-SDR for second receiver
 
Matthew Miller wrote:

I'm trying to configure a RTL-SDR as a second receiver and so far it's not working (not recognizing input audio?).


Strange...  it works OK here on several sites.
Here is a config that works, but it looks very similar to yours:

[DdrRx_NOS]
TYPE=Ddr
WBRX=WbRx1
FQ=430125000
SQL_DET=SIGLEV
SIGLEV_DET=DDR
SIGLEV_SLOPE=2.61
SIGLEV_OFFSET=158
SIGLEV_OPEN_THRESH=15
SIGLEV_CLOSE_THRESH=5
DEEMPHASIS=1
SQL_TAIL_ELIM=250
PREAMP=-5
PEAK_METER=0
DTMF_DEC_TYPE=INTERNAL
DTMF_MUTING=1
DTMF_HANGTIME=200
DTMF_MAX_FWD_TWIST=8
DTMF_MAX_REV_TWIST=4
1750_MUTING=1

[WbRx1]
TYPE=RtlUsb
SAMPLE_RATE=960000
FQ_CORR=97
GAIN=19.7
PEAK_METER=1

Probably the error is not in the SDR config, but in the repeater logic?
You can try putting it in a remotetrx and connect it from an svxlink instance running on a PC.
(with just a repeaterlogic, a netrx and a local tx to the soundcard)
Then you can listen in on the receiver.  I used this to configure several RTL SDR receivers we
installed on remote locations and that were not configured before deployment.

Rob

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Re: Trying to configure RTL-SDR for second receiver

Rob Janssen
Matthew Miller wrote:

Thanks, I'll try that when I get home.  I don't have other machines with svxlink (I'd have to build one that's got the dependencies) but I do have ModuleEcholink configured to work with svxlink as well as my smartphone if that would be an equivalent test.


Am I correct in believing if I enter DTMF commands I should see log entries "Recognizing digit X" the same way I normally do with a sound-card receiver input?

I know my SimplexLogic works properly with a sound-card (Rx1), I figured changing the receiver to Rx2 should not make a difference in how the logic functions, only modifying the input source?


I have no experience with SimplexLogic but I think you can swap any working receiver for an SDR receiver.
It does need quite some CPU though.   At sampling rate 960000 it requires 55% of 1 of the 4 cores of a Pi2
for a single receiver.
It may be that a Pi1 cannot cope.  I put the SDR receiver in a separate remotetrx to distribute the load over
the different cores.  (a single svxlink or remotetrx can use only a single core)

Rob

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Re: Trying to configure RTL-SDR for second receiver

Matthew Miller

Ok, that's very good to know.  This is on a Pi2, it's running a few other things (Python LCD status display, a couple watchdog-scripts, SSH remote-tunnel) but I think most of the resources are sitting idle.  I'm planning this to be a dedicated unit so I won't be adding anything more to it.


I do know after changing my Rx1 from VOX to GPIO the CPU utilization went down substantially.  I'll also take a close look at the usage with the SDR settings you provided to ensure I'm not maxing out and causing problems.




From: Rob Janssen <[hidden email]>
Sent: Tuesday, May 3, 2016 1:13 PM
To: Discussions about development issues
Subject: Re: [Svxlink-devel] Trying to configure RTL-SDR for second receiver
 
Matthew Miller wrote:

Thanks, I'll try that when I get home.  I don't have other machines with svxlink (I'd have to build one that's got the dependencies) but I do have ModuleEcholink configured to work with svxlink as well as my smartphone if that would be an equivalent test.


Am I correct in believing if I enter DTMF commands I should see log entries "Recognizing digit X" the same way I normally do with a sound-card receiver input?

I know my SimplexLogic works properly with a sound-card (Rx1), I figured changing the receiver to Rx2 should not make a difference in how the logic functions, only modifying the input source?


I have no experience with SimplexLogic but I think you can swap any working receiver for an SDR receiver.
It does need quite some CPU though.   At sampling rate 960000 it requires 55% of 1 of the 4 cores of a Pi2
for a single receiver.
It may be that a Pi1 cannot cope.  I put the SDR receiver in a separate remotetrx to distribute the load over
the different cores.  (a single svxlink or remotetrx can use only a single core)

Rob

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Re: Trying to configure RTL-SDR for second receiver

Rob Janssen
Matthew Miller wrote:

Ok, that's very good to know.  This is on a Pi2, it's running a few other things (Python LCD status display, a couple watchdog-scripts, SSH remote-tunnel) but I think most of the resources are sitting idle.  I'm planning this to be a dedicated unit so I won't be adding anything more to it.


I do know after changing my Rx1 from VOX to GPIO the CPU utilization went down substantially.  I'll also take a close look at the usage with the SDR settings you provided to ensure I'm not maxing out and causing problems.



Ok... when you run both svxlink and remotetrx on the same Pi and connect them via 127.0.0.1 you of course get
some overhead for the networking but the advantage is that the heavy SDR stuff can run on a separate core from the
audio processing in svxlink.  When your svxlink maxes out the processor that is a good thing to try.  Of course no
compressing CODEC like OPUS or SPEEX between them, just CODEC=S16

The problem with current computers is that they reach specified CPU performance by multiple cores (e.g. 4 cores
on a Pi2 or Pi3) and the typical application runs only on a single core.  So in your case 1 core may be lightly used
for the background stuff, 1 core for svxlink and the other two sit idle.  With a separate remotetrx at least you can
make use of 3 cores :-)

Rob

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Re: Trying to configure RTL-SDR for second receiver

Matthew Miller

WOW!  Copying in your config (and changing the PPM and frequency) seems to have done it (mostly)!  I was able to get it to say “distortion detected” when I key up and realized apparently PPM offset and FQ_CORR are the same thing (I thought it was opposite?) and corrected that, now I still get the distortion warnings but it works!

 

New questions:

*I hear myself thru Echolink but the audio volume is extremely low (clear but low) compared to what I get thru a “real” radio sound-card interface.  I don’t suppose there’s an easy way to fix that?

*Also, it says the lower the gain due to distortion but I know with APRS if I put the gain below about 30 I start losing data packets (it’s at 19.7 like you had currently).  Any tips on fine-tuning this?

*Similarly, any tips fine-tuning values to require CTCSS tone for RX?  I see many variables but don’t really understand them, or if this is used in place of the SIGLEV squelch detect to do what I want?

*CPU utilization is about 50-60% running the SDR and Echolink all as one instance of svxlink…do you think it’s still going to net me any gain running remotetrx to put the SDR in its own thread?  I’m thinking this is acceptable (40% headroom on the CPU core) I may not see any useful improvement splitting it up.

 

 

For reference, here’s the logfile I have using the SimplexLogic that I had working before – it looks more or less exactly as I expected it should now finally printing junk when I key up and also recognizing DTMF commands as expected:

Tue May  3 19:48:20 2016: DdrRx: The squelch is OPEN (45.8096)

Tue May  3 19:48:20 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:21 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:22 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:23 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:23 2016: DdrRx: The squelch is CLOSED (158.415)

Tue May  3 19:48:36 2016: DdrRx: The squelch is OPEN (56.1086)

Tue May  3 19:48:36 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:37 2016: SimplexLogic: digit=1

Tue May  3 19:48:37 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:38 2016: SimplexLogic: digit=2

Tue May  3 19:48:38 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:38 2016: SimplexLogic: digit=3

Tue May  3 19:48:39 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:40 2016: SimplexLogic: digit=4

Tue May  3 19:48:40 2016: SimplexLogic: digit=5

Tue May  3 19:48:40 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:40 2016: SimplexLogic: digit=6

Tue May  3 19:48:41 2016: SimplexLogic: digit=7

Tue May  3 19:48:41 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:42 2016: SimplexLogic: digit=8

Tue May  3 19:48:42 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:42 2016: SimplexLogic: digit=9

Tue May  3 19:48:43 2016: SimplexLogic: digit=#

Tue May  3 19:48:43 2016: *** WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain

Tue May  3 19:48:43 2016: DdrRx: The squelch is CLOSED (158.352)

Tue May  3 19:48:43 2016: DTMF command received in module EchoLink: 123456789

Tue May  3 19:48:43 2016: EchoLink ID 123456789 is not in the list. Refreshing the list...

Tue May  3 19:48:44 2016: The EchoLink ID 123456789 could not be found.

Tue May  3 19:48:44 2016: Tx1: Turning the transmitter ON

Tue May  3 19:48:50 2016: Tx1: Turning the transmitter OFF

Tue May  3 19:50:00 2016: SimplexLogic: Sending short identification...

Tue May  3 19:50:00 2016: Tx1: Turning the transmitter ON

Tue May  3 19:50:05 2016: Tx1: Turning the transmitter OFF

 

 

"The fastest thing a computer does is become outdated"

~Andy Rooney

 

From: Rob Janssen [mailto:[hidden email]]
Sent: Tuesday, May 03, 2016 1:40 PM
To: Discussions about development issues
Subject: Re: [Svxlink-devel] Trying to configure RTL-SDR for second receiver

 

Matthew Miller wrote:

Ok, that's very good to know.  This is on a Pi2, it's running a few other things (Python LCD status display, a couple watchdog-scripts, SSH remote-tunnel) but I think most of the resources are sitting idle.  I'm planning this to be a dedicated unit so I won't be adding anything more to it.

 

I do know after changing my Rx1 from VOX to GPIO the CPU utilization went down substantially.  I'll also take a close look at the usage with the SDR settings you provided to ensure I'm not maxing out and causing problems.

 


Ok... when you run both svxlink and remotetrx on the same Pi and connect them via 127.0.0.1 you of course get
some overhead for the networking but the advantage is that the heavy SDR stuff can run on a separate core from the
audio processing in svxlink.  When your svxlink maxes out the processor that is a good thing to try.  Of course no
compressing CODEC like OPUS or SPEEX between them, just CODEC=S16

The problem with current computers is that they reach specified CPU performance by multiple cores (e.g. 4 cores
on a Pi2 or Pi3) and the typical application runs only on a single core.  So in your case 1 core may be lightly used
for the background stuff, 1 core for svxlink and the other two sit idle.  With a separate remotetrx at least you can
make use of 3 cores :-)

Rob


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Re: Trying to configure RTL-SDR for second receiver

Rob Janssen
Matthew Miller wrote:

WOW!  Copying in your config (and changing the PPM and frequency) seems to have done it (mostly)!  I was able to get it to say “distortion detected” when I key up and realized apparently PPM offset and FQ_CORR are the same thing (I thought it was opposite?) and corrected that, now I still get the distortion warnings but it works!


Ok the distortion warning means you need to lower the gain.
But when you test it by keying a HT just beside the antenna of course this may not really be required, test at a practical distance.

 

New questions:

*I hear myself thru Echolink but the audio volume is extremely low (clear but low) compared to what I get thru a “real” radio sound-card interface.  I don’t suppose there’s an easy way to fix that?


The audio level of the SDR receiver can be adjusted, but in our experience PREAMP=-5 is about right.
The levels are always difficult to get "right" because there is no real defined value for "right".
You can adjust the radio soundcard (and receiver) level to get the same level as the SDR receiver, adjusting the
transmitter deviation in the process, and then check again for the Echolink level.
When it is still wrong, it may be that your transmitted Echolink level is wrong.  Connect to node 9999 (the echo server) to check that.
The new "devcal" tool can be used to calibrate the audio levels, I have no experience with it because we started
our repeaters before this tool existed so we had to settle on an audio level.

*Also, it says the lower the gain due to distortion but I know with APRS if I put the gain below about 30 I start losing data packets (it’s at 19.7 like you had currently).  Any tips on fine-tuning this?


The SDR stick gain has no influence on the audio level.  Remember, it is FM.

*Similarly, any tips fine-tuning values to require CTCSS tone for RX?  I see many variables but don’t really understand them, or if this is used in place of the SIGLEV squelch detect to do what I want?


In practice you only need CTCSS_FQ.   All other parameters can be left at default value.
When you use siglevdetcal you get another parameter that you can insert in the file.

*CPU utilization is about 50-60% running the SDR and Echolink all as one instance of svxlink…do you think it’s still going to net me any gain running remotetrx to put the SDR in its own thread?  I’m thinking this is acceptable (40% headroom on the CPU core) I may not see any useful improvement splitting it up.


No, this should be fine.  Our configs are usually very complex and we cannot use our experience to apply to the general case.
(normally our remote receivers run 2 or 3 analog receivers plus an RTL stick)
Our main repeater has 22 receiver entries now, 20 real receivers and 2 uplinks for bulletin stations...

Rob

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Re: Trying to configure RTL-SDR for second receiver

Matthew Miller

Thank you very much for your reply.  I think that helps me understand what to adjust a lot better.  For testing I have a Yaesu VX-6 on the lowest power level (0.3w I think) and a stock rubber duck.

 

Now that I understand that the PREAMP is what I needed and setting is under the [Rx] not the [WbRx] I finally was able to tweak audio levels.  I found around “0” or “1” sounded good to me.  When I say “good” I mean my received audio sounds on a par with the audio level on other nodes I connect to.  I also managed to get it set to use the CTCSS tone for squelch…and after getting rid of the “extra” CTCSS tuning variables I’d copied from examples and left only CTCSS_FQ it started working.  For some reason I thought all of them were needed to make it work (normally I use CTCSS in a physical radio and it has no settings).

 

I tried putting the “gain” as low as 0 or high as 39 and until I get down where it’s not picking anything up I still have the warning “WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain”.  Maybe this doesn’t matter?  Ultimately I put it around 20 because it seemed to have little to no effect and that’s where it works best under SDRSharp.  I don’t know if this is related to my current problem or not.

 

My only remaining problem is the range is absolutely abysmal.  I was going to test it and got in my car running 25W with my mobile rig and connected my cellphone to Echolink and found that it became choppy and unusable within 0.25 miles of my apartment!  The “real” radio would get a good clean copy thru the same antenna on the same location up to about 2 miles in every direction and much farther in some directions.  Additional testing I found if I go just outside my apartment building I can’t hit it with 0.3 watts just standing outside my own window!  I thought maybe the TX was de-sensing the SDR (I don’t have a duplexer yet) so I turned off the TX radio completely so it was RX-only Echolink and that made no difference.

 

Are the SDRs just that bad in urban areas or do I need to get some kind of fancy expensive filter for it to work right?

 

 

"The fastest thing a computer does is become outdated"

~Andy Rooney

 

From: Rob Janssen [mailto:[hidden email]]
Sent: Sunday, May 08, 2016 12:41 PM
To: Discussions about development issues
Subject: Re: [Svxlink-devel] Trying to configure RTL-SDR for second receiver

 

Matthew Miller wrote:

WOW!  Copying in your config (and changing the PPM and frequency) seems to have done it (mostly)!  I was able to get it to say “distortion detected” when I key up and realized apparently PPM offset and FQ_CORR are the same thing (I thought it was opposite?) and corrected that, now I still get the distortion warnings but it works!


Ok the distortion warning means you need to lower the gain.
But when you test it by keying a HT just beside the antenna of course this may not really be required, test at a practical distance.


 

New questions:

*I hear myself thru Echolink but the audio volume is extremely low (clear but low) compared to what I get thru a “real” radio sound-card interface.  I don’t suppose there’s an easy way to fix that?


The audio level of the SDR receiver can be adjusted, but in our experience PREAMP=-5 is about right.
The levels are always difficult to get "right" because there is no real defined value for "right".
You can adjust the radio soundcard (and receiver) level to get the same level as the SDR receiver, adjusting the
transmitter deviation in the process, and then check again for the Echolink level.
When it is still wrong, it may be that your transmitted Echolink level is wrong.  Connect to node 9999 (the echo server) to check that.
The new "devcal" tool can be used to calibrate the audio levels, I have no experience with it because we started
our repeaters before this tool existed so we had to settle on an audio level.


*Also, it says the lower the gain due to distortion but I know with APRS if I put the gain below about 30 I start losing data packets (it’s at 19.7 like you had currently).  Any tips on fine-tuning this?


The SDR stick gain has no influence on the audio level.  Remember, it is FM.


*Similarly, any tips fine-tuning values to require CTCSS tone for RX?  I see many variables but don’t really understand them, or if this is used in place of the SIGLEV squelch detect to do what I want?


In practice you only need CTCSS_FQ.   All other parameters can be left at default value.
When you use siglevdetcal you get another parameter that you can insert in the file.


*CPU utilization is about 50-60% running the SDR and Echolink all as one instance of svxlink…do you think it’s still going to net me any gain running remotetrx to put the SDR in its own thread?  I’m thinking this is acceptable (40% headroom on the CPU core) I may not see any useful improvement splitting it up.


No, this should be fine.  Our configs are usually very complex and we cannot use our experience to apply to the general case.
(normally our remote receivers run 2 or 3 analog receivers plus an RTL stick)
Our main repeater has 22 receiver entries now, 20 real receivers and 2 uplinks for bulletin stations...

Rob


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Re: Trying to configure RTL-SDR for second receiver

Rob Janssen
Matthew Miller wrote:


 

Now that I understand that the PREAMP is what I needed and setting is under the [Rx] not the [WbRx] I finally was able to tweak audio levels.  I found around “0” or “1” sounded good to me.  When I say “good” I mean my received audio sounds on a par with the audio level on other nodes I connect to.  I also managed to get it set to use the CTCSS tone for squelch…and after getting rid of the “extra” CTCSS tuning variables I’d copied from examples and left only CTCSS_FQ it started working.  For some reason I thought all of them were needed to make it work (normally I use CTCSS in a physical radio and it has no settings).


The audio levels are a somewhat arbitrary, as long as everything is at the same level and the signal is not clipped (overdriving the 16-bit audio range).
CTCSS and other (DTMF) parameters can be tweaked to get maximum performance and/or desired characteristics (like DTMF tone minimal length),
but with default values it should work.

 

I tried putting the “gain” as low as 0 or high as 39 and until I get down where it’s not picking anything up I still have the warning “WARNING: Distorsion detected on Rtl tuner [0] Realtek RTL2838UHIDIR SN:00000001. Lower the RF gain”.  Maybe this doesn’t matter?  Ultimately I put it around 20 because it seemed to have little to no effect and that’s where it works best under SDRSharp.  I don’t know if this is related to my current problem or not.


The dynamic range of those RTL sticks is quite small, it is only an 8-bit ADC.  When you set the gain by using a local transmission it will probably
always be overdriven no matter what gain you set.  Try transmitting into a (not so good) dummy load.
When the warning is not or infrequently shown when you are not transmitting locally it should be fine.

 

My only remaining problem is the range is absolutely abysmal.  I was going to test it and got in my car running 25W with my mobile rig and connected my cellphone to Echolink and found that it became choppy and unusable within 0.25 miles of my apartment!  The “real” radio would get a good clean copy thru the same antenna on the same location up to about 2 miles in every direction and much farther in some directions.  Additional testing I found if I go just outside my apartment building I can’t hit it with 0.3 watts just standing outside my own window!  I thought maybe the TX was de-sensing the SDR (I don’t have a duplexer yet) so I turned off the TX radio completely so it was RX-only Echolink and that made no difference.

 

Are the SDRs just that bad in urban areas or do I need to get some kind of fancy expensive filter for it to work right?



Well, we normally get better performance than that.
But indeed they are overdriven quite easily and then the sensitivity on other frequencies will drop.
For example, when you have some transmitter in the neighborhood that transmits within a few MHz of your receive frequency it can be a problem.
You can try with SDRSharp to see if it performs the same.

Rob

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