equalizer for audio input

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equalizer for audio input

k9dwr
One of the issues I'm dealing with right now is an audio hiss at around
5kHz coming in with the RF audio from my repeater.  I had the idea of
using alsaequal to take this out, but I don't know if there's a way to
apply it to the input side (to be honest, I don't even know if I can get
it to work on the output side, either).  

Has anyone tried pre-processing input audio?

--
David, K9DWR
[hidden email]

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Re: equalizer for audio input

Rob Janssen
David Rock wrote:
> One of the issues I'm dealing with right now is an audio hiss at around
> 5kHz coming in with the RF audio from my repeater.  I had the idea of
> using alsaequal to take this out, but I don't know if there's a way to
> apply it to the input side (to be honest, I don't even know if I can get
> it to work on the output side, either).
>

Sure you can, assuming that you know how to configure it.   I would recommend putting
the proper definitions in /etc/asound.conf instead of in ~/.asoundrc
Otherwise there might be confusion about what is the home directory :-)

We use Alsa plugins to solve certain problems, but not audio processing.  As we use
SIGLEV=NOISE it probably would not work.  But when you have a hardware squelch
indicator it may work for you.

When you have defined a pcm with a name like pcm.alsaequal you can configure it
in svxlink using:

AUDIO_DEV=alsa:alsaequal

(instead of the usual alsa:plughw:0,0)

Rob

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Re: equalizer for audio input

Mischa van Santen
Usually, 5kHz is excluded from the LF passband of the repeater. It should therefore not influence the performance too much under normal circumstances.

However, if you apply the Siglev detector in one or other way you have to take into account that LF audio between 5....5.5kHz is used to determine Signal-To-Noise behavior of the received signal. The Siglev detector is used for the Voter or the noise squelch. 

In the case that a 'hiss' is present in the RX LF, the siglev measurement is influenced in a negative way and could disrupt the SQ or voter behavior. In the case that the hiss is equalized out, the dynamic range of the Siglev detection most probably decreases in a significant matter due to the fact that the required noise is attenuated as well, leaving you with limited performance.

It would be better in such case to eliminate the issue at the source, rather then filtering it out.

73,

Mischa

2015-04-24 11:06 GMT+02:00 Rob Janssen <[hidden email]>:
David Rock wrote:
> One of the issues I'm dealing with right now is an audio hiss at around
> 5kHz coming in with the RF audio from my repeater.  I had the idea of
> using alsaequal to take this out, but I don't know if there's a way to
> apply it to the input side (to be honest, I don't even know if I can get
> it to work on the output side, either).
>

Sure you can, assuming that you know how to configure it.   I would recommend putting
the proper definitions in /etc/asound.conf instead of in ~/.asoundrc
Otherwise there might be confusion about what is the home directory :-)

We use Alsa plugins to solve certain problems, but not audio processing.  As we use
SIGLEV=NOISE it probably would not work.  But when you have a hardware squelch
indicator it may work for you.

When you have defined a pcm with a name like pcm.alsaequal you can configure it
in svxlink using:

AUDIO_DEV=alsa:alsaequal

(instead of the usual alsa:plughw:0,0)

Rob

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--
Met vriendelijke groet,

Mischa van Santen

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Re: equalizer for audio input

k9dwr
In reply to this post by Rob Janssen
* Rob Janssen <[hidden email]> [2015-04-24 11:06]:
> David Rock wrote:
> > One of the issues I'm dealing with right now is an audio hiss at around
> > 5kHz coming in with the RF audio from my repeater.  I had the idea of
> > using alsaequal to take this out, but I don't know if there's a way to
> > apply it to the input side (to be honest, I don't even know if I can get
> > it to work on the output side, either).
> >
>
> Sure you can, assuming that you know how to configure it.  

Yeah, that's the real trick, isn't it?  

> I would recommend putting the proper definitions in /etc/asound.conf
> instead of in ~/.asoundrc Otherwise there might be confusion about
> what is the home directory :-)

Seems like that has come up before somewhere. :-)

> We use Alsa plugins to solve certain problems, but not audio processing.  As we use
> SIGLEV=NOISE it probably would not work.  But when you have a hardware squelch
> indicator it may work for you.

I'm currently using SQL_DET=GPIO, so that shouldn't be a problem.

> When you have defined a pcm with a name like pcm.alsaequal you can configure it
> in svxlink using:
>
> AUDIO_DEV=alsa:alsaequal
>
> (instead of the usual alsa:plughw:0,0)

I'll play around with it and see how lucky I get.  Wrapping my head
around alsa is proving to be irritating at best.

Thanks for the hints!

--
David, K9DWR
[hidden email]

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Re: equalizer for audio input

k9dwr
In reply to this post by Mischa van Santen
* Mischa van Santen <[hidden email]> [2015-04-24 14:13]:

> Usually, 5kHz is excluded from the LF passband of the repeater. It should
> therefore not influence the performance too much under normal circumstances.
>
> However, if you apply the Siglev detector in one or other way you have to
> take into account that LF audio between 5....5.5kHz is used to determine
> Signal-To-Noise behavior of the received signal. The Siglev detector is
> used for the Voter or the noise squelch.
>
> In the case that a 'hiss' is present in the RX LF, the siglev measurement
> is influenced in a negative way and could disrupt the SQ or voter behavior.
> In the case that the hiss is equalized out, the dynamic range of the Siglev
> detection most probably decreases in a significant matter due to the fact
> that the required noise is attenuated as well, leaving you with limited
> performance.
>
> It would be better in such case to eliminate the issue at the source,
> rather then filtering it out.

In my case, the issue _is_ the source.  I'm not using svxlink to
hardwire control a repeater, I'm using a mobile radio as a repeater
link.  The hiss is in the received audio at the mobile.  Filtering
the audio coming into Rx1 in my case shouldn't be any different than
eliminating it.

--
David, K9DWR
[hidden email]

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Re: equalizer for audio input

Rob Janssen
In reply to this post by k9dwr
David Rock wrote:
* Rob Janssen [hidden email] [2015-04-24 11:06]:
David Rock wrote:
One of the issues I'm dealing with right now is an audio hiss at around
5kHz coming in with the RF audio from my repeater.  I had the idea of
using alsaequal to take this out, but I don't know if there's a way to
apply it to the input side (to be honest, I don't even know if I can get
it to work on the output side, either).

Sure you can, assuming that you know how to configure it.   
Yeah, that's the real trick, isn't it?  

Well, I presumed you knew how to install that alsaequal plugin as you mentioned it.

What I see on the internet is that you should setup something like this:

ctl.equal {
  type equal;
}

pcm.plugequal {
  type equal;
  # Modify the line below if you don't
  # want to use sound card 0.
  slave.pcm "plughw:0,0";
}

pcm.equal {
  type plug;
  slave.pcm plugequal;
}
With this config you would use the device alsa:equal in svxlink.

I'll play around with it and see how lucky I get.  Wrapping my head
around alsa is proving to be irritating at best.


Alsa is extremely powerful and versatile.   But the documentation is sorely lacking, and you
need to google a lot to find examples from other people, who often also do not know exactly
what they are doing.   And then you are only trying configuration.  When writing the co-channel
plugin I often encountered the problem that the entire programmer's documentation is automatically
generated from the sourcecode.  There are lots of nice looking pages with long lists of functions
and the type of their parameters and return value, but not what they are doing and how they
are to be combined.   But after a lot of tinkering and sometimes looking in the source, I was able
to get a working program that achieves microsecond-level timing.

Rob

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Re: equalizer for audio input

k9dwr
* Rob Janssen <[hidden email]> [2015-04-24 18:04]:

> David Rock wrote:
> > * Rob Janssen <[hidden email]> [2015-04-24 11:06]:
> >> David Rock wrote:
> >>> One of the issues I'm dealing with right now is an audio hiss at around
> >>> 5kHz coming in with the RF audio from my repeater.  I had the idea of
> >>> using alsaequal to take this out, but I don't know if there's a way to
> >>> apply it to the input side (to be honest, I don't even know if I can get
> >>> it to work on the output side, either).
> >>>
> >> Sure you can, assuming that you know how to configure it.
> > Yeah, that's the real trick, isn't it?
>
> Well, I presumed you knew how to install that alsaequal plugin as you mentioned it.

Installing it and getting it to do what you want are two completely
different things. :-)

> What I see on the internet is that you should setup something like this:

That's pretty much what I've done as well so far (my card is 1, not 0):

        ctl.equal {
          type equal;
        }

        pcm.plugequal {
          type equal;
          slave.pcm "plughw:1,0";
        }

        pcm.equal {
          type plug;
          slave.pcm plugequal;
        }


At least I can get svxlink to recognize the device now.  I was having
issues with understanding the syntax of how to refer to the device (I
was doing alsa:plughw:equal).  What I may be getting bit by is I think
alsaequal is meant to be applied to audio out, not audio in so when it
translates to hw:1,0, I think it's using the output device.  

What I'm seeing in the log output now is this:

        SvxLink v1.4.99.13 (Apr 19 2015) Copyright (C) 2003-2015 Tobias Blomberg / SM0SVX

        SvxLink comes with ABSOLUTELY NO WARRANTY. This is free software, and you are
        welcome to redistribute it in accordance with the terms and conditions in the
        GNU GPL (General Public License) version 2 or later.

        Using configuration file: /etc/svxlink/svxlink.conf
        --- Using sample rate 48000Hz

        Starting logic: SimplexLogic
        Loading RX: Rx1
        SimplexLogic: Activating QSO recorder
        Loading TX: Tx1
        Loading module "ModuleEchoLink" into logic "SimplexLogic"
                Module EchoLink v1.3.0.99.0 starting...
        SimplexLogic: Event handler script successfully loaded.
        EchoLink directory status changed to ON
        --- EchoLink directory server message: ---
        EchoLink Server v2.5.9997

        ECHOEC2-1: Herndon, VA USA

        Rx1: The squelch is OPEN (0)

        SIGINT received. Shutting down application...


When a signal comes in, it does (sort of) open the squelch, but then it
stays open forever, even though the voltage on the GPIO pin drops to
zero like it's supposed to (ie, the code is not responding properly).
I'm obviously missing something about how the plugin is supposed to
work that is causing things to hang.

I'm also investigating hardware fixes for the audio before it gets to
the system, but this would be a neat little tool if I could get it to
work.

--
David, K9DWR
[hidden email]

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Re: equalizer for audio input

Rob Janssen
David Rock wrote:

> * Rob Janssen <[hidden email]> [2015-04-24 18:04]:
>> David Rock wrote:
>>> * Rob Janssen <[hidden email]> [2015-04-24 11:06]:
>>>> David Rock wrote:
>>>>> One of the issues I'm dealing with right now is an audio hiss at around
>>>>> 5kHz coming in with the RF audio from my repeater.  I had the idea of
>>>>> using alsaequal to take this out, but I don't know if there's a way to
>>>>> apply it to the input side (to be honest, I don't even know if I can get
>>>>> it to work on the output side, either).
>>>>>
>>>> Sure you can, assuming that you know how to configure it.
>>> Yeah, that's the real trick, isn't it?
>> Well, I presumed you knew how to install that alsaequal plugin as you mentioned it.
> Installing it and getting it to do what you want are two completely
> different things. :-)

Well, rather than just installing the binary on the system, I was referring to the installation
(configuration, rather) of the plugin in the stack of Alsa plugins.

>
> At least I can get svxlink to recognize the device now.  I was having
> issues with understanding the syntax of how to refer to the device (I
> was doing alsa:plughw:equal).  What I may be getting bit by is I think
> alsaequal is meant to be applied to audio out, not audio in so when it
> translates to hw:1,0, I think it's using the output device.

Yes it looks like it is an output device.  I have not studied that plugin.  When it works for
input it should do that in svxlink as well, I think.

>
> When a signal comes in, it does (sort of) open the squelch, but then it
> stays open forever, even though the voltage on the GPIO pin drops to
> zero like it's supposed to (ie, the code is not responding properly).
> I'm obviously missing something about how the plugin is supposed to
> work that is causing things to hang.
>
> I'm also investigating hardware fixes for the audio before it gets to
> the system, but this would be a neat little tool if I could get it to
> work.
>
It could be that the program is somehow stuck reading the data via that plugin.
It is usually possible to find that using strace.

Rob

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Re: equalizer for audio input

Nils Prause
In reply to this post by k9dwr
Hi David!

David Rock wrote:

>> It would be better in such case to eliminate the issue at the source,
>> rather then filtering it out.
Ack.


> In my case, the issue _is_ the source.  I'm not using svxlink to
> hardwire control a repeater, I'm using a mobile radio as a repeater
> link.  The hiss is in the received audio at the mobile.  Filtering
> the audio coming into Rx1 in my case shouldn't be any different than
> eliminating it.

You might want to take a look into the world of ladspa plug-ins.
The several ladspa packages contain some equalizers which might fit your
needs. Most Linux distributions alread contain one or more packages.

Some months ago I played around with ladspa thinking of creating a
better TX audio by applying some limiting and compressing. But as we all
need analogue audio is full of noise and noise is a ***** for each audio
processor. I don't wanna just maximize noise so I gave up. Maybe a
special algorithm for detecting noise is neede to get better results.




--
73s

Nils

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Re: equalizer for audio input

Tony Langdon
On 26/04/2015 1:37 AM, Nils Prause wrote:
> You might want to take a look into the world of ladspa plug-ins.
> The several ladspa packages contain some equalizers which might fit your
> needs. Most Linux distributions alread contain one or more packages.
>
> Some months ago I played around with ladspa thinking of creating a
> better TX audio by applying some limiting and compressing. But as we all
> need analogue audio is full of noise and noise is a ***** for each audio
> processor. I don't wanna just maximize noise so I gave up. Maybe a
> special algorithm for detecting noise is neede to get better results.
An expensive, but effective solution is a DSP noise reduction unit from
Michels Engineering.  These are particularly effective at reducing noise
on FM channels, to the point that they can almost be used as a very
sensitive squelch, except for a noise burst when the signal first drops,
and on FM, they can make a barely audible (and totally unreadable)
signal quite readable.

I have one of these units, which I have used as part of a SSB squelch
(which was used on Echolink for a while), in conjunction with a PIC
based VOX unit that I designed to ignore noise pulses and act as a
multiport link controller.

--
73 de Tony VK3JED/VK3IRL
http://vkradio.com


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Re: equalizer for audio input

Rob Janssen
Tony Langdon wrote:

> On 26/04/2015 1:37 AM, Nils Prause wrote:
>> You might want to take a look into the world of ladspa plug-ins.
>> The several ladspa packages contain some equalizers which might fit your
>> needs. Most Linux distributions alread contain one or more packages.
>>
>> Some months ago I played around with ladspa thinking of creating a
>> better TX audio by applying some limiting and compressing. But as we all
>> need analogue audio is full of noise and noise is a ***** for each audio
>> processor. I don't wanna just maximize noise so I gave up. Maybe a
>> special algorithm for detecting noise is neede to get better results.
> An expensive, but effective solution is a DSP noise reduction unit from
> Michels Engineering.  These are particularly effective at reducing noise
> on FM channels, to the point that they can almost be used as a very
> sensitive squelch, except for a noise burst when the signal first drops,
> and on FM, they can make a barely audible (and totally unreadable)
> signal quite readable.
>
> I have one of these units, which I have used as part of a SSB squelch
> (which was used on Echolink for a while), in conjunction with a PIC
> based VOX unit that I designed to ignore noise pulses and act as a
> multiport link controller.
>

When DSP first became accessible to radio amateurs (early 90's) there was a lot of
fuzz around those "noise eliminators" that used the LMS algorithm. There were
different methods, one to reduce coherent signals (the automatic notch filter that
reduces beatnotes on SSB) and the one to reduce non-coherent signals (to reduce
noise).   Of course there are some parameters to tune, and judging by the comments
this unit had a favourable set of parameters to make it work nicely.

But of course a big part of the cost in those days was in the mere hardware, that is
the A/D, DSP, D/A and of course a cabinet etc.   We don't need those, when it can be
made into an Alsa plugin.  The CPU power in a modern machine exceeds the power
of the DSP of those days.

In those days I built the "Alef Null DSP CARD 4", a card with the Motorola 56001 DSP,
and had a lot of fun with it.  But today it would not be the preferred way of adding such
functionality to an amateur radio station anymore...

Rob

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Re: equalizer for audio input

Tobias Blomberg
In reply to this post by k9dwr
On Friday 24 April 2015 13:13:42 David Rock wrote:

> * Rob Janssen <[hidden email]> [2015-04-24 18:04]:
> > David Rock wrote:
> > > * Rob Janssen <[hidden email]> [2015-04-24 11:06]:
> > >> David Rock wrote:
> > >>> One of the issues I'm dealing with right now is an audio hiss at
> > >>> around
> > >>> 5kHz coming in with the RF audio from my repeater.  I had the idea of
> > >>> using alsaequal to take this out, but I don't know if there's a way to
> > >>> apply it to the input side (to be honest, I don't even know if I can
> > >>> get
> > >>> it to work on the output side, either).
> > >>
> > >> Sure you can, assuming that you know how to configure it.
> > >
> > > Yeah, that's the real trick, isn't it?
> >
> > Well, I presumed you knew how to install that alsaequal plugin as you
> > mentioned it.
> Installing it and getting it to do what you want are two completely
> different things. :-)
>
> > What I see on the internet is that you should setup something like this:
> That's pretty much what I've done as well so far (my card is 1, not 0):
>
> ctl.equal {
>  type equal;
> }
>
> pcm.plugequal {
>  type equal;
>  slave.pcm "plughw:1,0";
> }
>
> pcm.equal {
>  type plug;
>  slave.pcm plugequal;
> }
>
>
> At least I can get svxlink to recognize the device now.  I was having
> issues with understanding the syntax of how to refer to the device (I
> was doing alsa:plughw:equal).  What I may be getting bit by is I think
> alsaequal is meant to be applied to audio out, not audio in so when it
> translates to hw:1,0, I think it's using the output device.
>
> What I'm seeing in the log output now is this:
>
> SvxLink v1.4.99.13 (Apr 19 2015) Copyright (C) 2003-2015 Tobias Blomberg
/

> SM0SVX
>
> SvxLink comes with ABSOLUTELY NO WARRANTY. This is free software, and you
> are welcome to redistribute it in accordance with the terms and conditions
> in the GNU GPL (General Public License) version 2 or later.
>
> Using configuration file: /etc/svxlink/svxlink.conf
> --- Using sample rate 48000Hz
>
> Starting logic: SimplexLogic
> Loading RX: Rx1
> SimplexLogic: Activating QSO recorder
> Loading TX: Tx1
> Loading module "ModuleEchoLink" into logic "SimplexLogic"
> Module EchoLink v1.3.0.99.0 starting...
> SimplexLogic: Event handler script successfully loaded.
> EchoLink directory status changed to ON
> --- EchoLink directory server message: ---
> EchoLink Server v2.5.9997
>
> ECHOEC2-1: Herndon, VA USA
>
> Rx1: The squelch is OPEN (0)
>
> SIGINT received. Shutting down application...
>
>
> When a signal comes in, it does (sort of) open the squelch, but then it
> stays open forever, even though the voltage on the GPIO pin drops to
> zero like it's supposed to (ie, the code is not responding properly).
> I'm obviously missing something about how the plugin is supposed to
> work that is causing things to hang.

SvxLink is probably not getting any samples if the plugin you have used indeed
is not meant for input use. Some timing in SvxLink is done by counting
samples, like SQL_HANGTIME. If the squelch processor does not get any samples,
the squelch will never close.

73's de SM0SVX / Tobias


>
> I'm also investigating hardware fixes for the audio before it gets to
> the system, but this would be a neat little tool if I could get it to
> work.


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Re: equalizer for audio input

k9dwr
In reply to this post by Rob Janssen
* Rob Janssen <[hidden email]> [2015-04-26 08:50]:
>
> But of course a big part of the cost in those days was in the mere hardware, that is
> the A/D, DSP, D/A and of course a cabinet etc.   We don't need those, when it can be
> made into an Alsa plugin.  The CPU power in a modern machine exceeds the power
> of the DSP of those days.

Still looking.  As I've researched this, it seems that something like
JACK or PA might give me a way to build a processing workflow, but I
really don't want that much bloat.  I also considered an alsa loopback
device as a possibility.  I'm definitely in the deep end of linux sound
configuration right now (much more than I ever intended).  It's
interesting, but more than a little frustrating.  A (logical?)
brute-force method would seem to be creating something in alsa that
virtually creates something like:

audio in->virt input-to-output-> alsaequal->virt output-to-input-> svxlink

That feels like a loopback device to me.

I started looking at the ladspa plugin arch, but that's a morass of
disconnected/hard to list plugins that make it difficult to tell if
something would be useful or not.

--
David, K9DWR
[hidden email]

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