Strange siglevcal results

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Strange siglevcal results

Gullik Webjorn
I have tried to fit a BF777 as receiver for svxlink. I cannot get a
reliable squelch.  Currently I have it interfaced by tapping over the
volume pot ( for a fixed level).

I have checked the input signal with a "scope", and I see at least 40 dB
between a strong carrier and an open squelch noise.

However, siglevdetcal reports a SNR of 1-5 dB, and returns ridiculous
settings.

Running the ubuntu sound setup utility, I can see the audio from the
sound card either peak the VU meter (when receiving noise) and zero or
almost zero when receiving a stady quiet carrier.

For some strange reason this can not be seen with siglevdetcal, and
also, when running svxlink the squelch jitters,

levels received either with or without PTT are in the 80-100 range, but
there seems to no difference with or without PTT.

Quick analysis; It has nothing to do wrt hardware,checked with scope
........

Completely puzzled......Sm6FBD....


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Re: Strange siglevcal results

Scott Currie
My guess would be the point where you tapped the audio is after de-emphasis and fitered to the voice range and doesn't contain enough high frequency information. Squelch circuits typically take audio from the discriminator, before de-emphasis, and then pass it through a high pass filter to get the noise above 4kHz, otherwise you would squelch on voice peaks. Svxlink probably implements the high pass filter in software, so it is not seeing much noise from your tap. Try going with discriminator audio.

-Scott, NS7C

On Thu, Mar 2, 2017 at 3:54 PM, Gullik Webjorn <[hidden email]> wrote:
I have tried to fit a BF777 as receiver for svxlink. I cannot get a
reliable squelch.  Currently I have it interfaced by tapping over the
volume pot ( for a fixed level).

I have checked the input signal with a "scope", and I see at least 40 dB
between a strong carrier and an open squelch noise.

However, siglevdetcal reports a SNR of 1-5 dB, and returns ridiculous
settings.

Running the ubuntu sound setup utility, I can see the audio from the
sound card either peak the VU meter (when receiving noise) and zero or
almost zero when receiving a stady quiet carrier.

For some strange reason this can not be seen with siglevdetcal, and
also, when running svxlink the squelch jitters,

levels received either with or without PTT are in the 80-100 range, but
there seems to no difference with or without PTT.

Quick analysis; It has nothing to do wrt hardware,checked with scope
........

Completely puzzled......Sm6FBD....


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-Scott

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Re: Strange siglevcal results

Gunnar Widell
Just some ideas.
Have your checked that you are using the correct audio channel in svxlink Rx section?
AUDIO_DEV=alsa:plughw:0
AUDIO_CHANNEL=0

Is it possible to turn up the volume in the alsamixer enough to get the "Distortion detected" message in svxlink?
PEAK_METER=1

73 de Gunnar SM3SGP

2017-03-03 1:11 GMT+01:00 Scott Currie <[hidden email]>:
My guess would be the point where you tapped the audio is after de-emphasis and fitered to the voice range and doesn't contain enough high frequency information. Squelch circuits typically take audio from the discriminator, before de-emphasis, and then pass it through a high pass filter to get the noise above 4kHz, otherwise you would squelch on voice peaks. Svxlink probably implements the high pass filter in software, so it is not seeing much noise from your tap. Try going with discriminator audio.

-Scott, NS7C

On Thu, Mar 2, 2017 at 3:54 PM, Gullik Webjorn <[hidden email]> wrote:
I have tried to fit a BF777 as receiver for svxlink. I cannot get a
reliable squelch.  Currently I have it interfaced by tapping over the
volume pot ( for a fixed level).

I have checked the input signal with a "scope", and I see at least 40 dB
between a strong carrier and an open squelch noise.

However, siglevdetcal reports a SNR of 1-5 dB, and returns ridiculous
settings.

Running the ubuntu sound setup utility, I can see the audio from the
sound card either peak the VU meter (when receiving noise) and zero or
almost zero when receiving a stady quiet carrier.

For some strange reason this can not be seen with siglevdetcal, and
also, when running svxlink the squelch jitters,

levels received either with or without PTT are in the 80-100 range, but
there seems to no difference with or without PTT.

Quick analysis; It has nothing to do wrt hardware,checked with scope
........

Completely puzzled......Sm6FBD....


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-Scott

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Re: Strange siglevcal results

Gullik Webjorn

After contemplating different issues, I finally adapted a small 5V transceiver module to

a "naked" usb sound dongle, stealing +5V as well.

Initial results are promising, siglevdetcal gives reasonable results, with these in the

config file I have a nice working squelch, and stable operation.


So, what was wrong in the first place with the BF 777/888 xceiver?

Well, still puzzled but "worked around" the problem. SIC.

SM6FBD


On 2017-03-03 08:33, Gunnar Widell wrote:
Just some ideas.
Have your checked that you are using the correct audio channel in svxlink Rx section?
AUDIO_DEV=alsa:plughw:0
AUDIO_CHANNEL=0

Is it possible to turn up the volume in the alsamixer enough to get the "Distortion detected" message in svxlink?
PEAK_METER=1

73 de Gunnar SM3SGP

2017-03-03 1:11 GMT+01:00 Scott Currie <[hidden email]>:
My guess would be the point where you tapped the audio is after de-emphasis and fitered to the voice range and doesn't contain enough high frequency information. Squelch circuits typically take audio from the discriminator, before de-emphasis, and then pass it through a high pass filter to get the noise above 4kHz, otherwise you would squelch on voice peaks. Svxlink probably implements the high pass filter in software, so it is not seeing much noise from your tap. Try going with discriminator audio.

-Scott, NS7C

On Thu, Mar 2, 2017 at 3:54 PM, Gullik Webjorn <[hidden email]> wrote:
I have tried to fit a BF777 as receiver for svxlink. I cannot get a
reliable squelch.  Currently I have it interfaced by tapping over the
volume pot ( for a fixed level).

I have checked the input signal with a "scope", and I see at least 40 dB
between a strong carrier and an open squelch noise.

However, siglevdetcal reports a SNR of 1-5 dB, and returns ridiculous
settings.

Running the ubuntu sound setup utility, I can see the audio from the
sound card either peak the VU meter (when receiving noise) and zero or
almost zero when receiving a stady quiet carrier.

For some strange reason this can not be seen with siglevdetcal, and
also, when running svxlink the squelch jitters,

levels received either with or without PTT are in the 80-100 range, but
there seems to no difference with or without PTT.

Quick analysis; It has nothing to do wrt hardware,checked with scope
........

Completely puzzled......Sm6FBD....


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-Scott

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Re: Strange siglevcal results

Rob Janssen
Gullik Webjorn wrote:

After contemplating different issues, I finally adapted a small 5V transceiver module to

a "naked" usb sound dongle, stealing +5V as well.

Initial results are promising, siglevdetcal gives reasonable results, with these in the

config file I have a nice working squelch, and stable operation.


So, what was wrong in the first place with the BF 777/888 xceiver?

Well, still puzzled but "worked around" the problem. SIC.

SM6FBD



I think a problem with the SDR based portables will be that there is no place to tap unfiltered
(no lowpass, no de-emphasis) audio similar to "discriminator out" audio in an analog receiver.

You need a receiver that has this.  Or do the entire SDR inside svxlink (using RTL-DVB stick)
so svxlink has access to the unfiltered audio inside its software.

Rob

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Re: Strange siglevcal results

Gullik Webjorn

Current suspicion is that the audio output of the transceiver I used ( BF777) has some high frequency noise, that is not or barely visible with an old philips oscope, i.e. some "grass". I will verify this with a better scope, but this would explain why quiet, i.e. a steady unmodulated carrier would not give good SNR readings if that noise is mistaken as open squelch.

Also, a cap across the audio input close to the dongle would perhaps solve the problem, or simply

"do not use inadequate equipment".

The current module gave me 26 dB initial siglev SNR, which is a little bit marginal, but maybe

the audio high pass corner is a limiting factor in THIS radio. Anyway, now it works quite well.


SM6FBD


On 2017-03-06 10:18, Rob Janssen wrote:
Gullik Webjorn wrote:

After contemplating different issues, I finally adapted a small 5V transceiver module to

a "naked" usb sound dongle, stealing +5V as well.

Initial results are promising, siglevdetcal gives reasonable results, with these in the

config file I have a nice working squelch, and stable operation.


So, what was wrong in the first place with the BF 777/888 xceiver?

Well, still puzzled but "worked around" the problem. SIC.

SM6FBD



I think a problem with the SDR based portables will be that there is no place to tap unfiltered
(no lowpass, no de-emphasis) audio similar to "discriminator out" audio in an analog receiver.

You need a receiver that has this.  Or do the entire SDR inside svxlink (using RTL-DVB stick)
so svxlink has access to the unfiltered audio inside its software.

Rob


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