VLFRX Tool Set Notes
This page documents the software component of my VLF receiving station, along with various hints, tips and experiences I've had with different tools and hardware. You can learn about the E-field receiver I use here, and the (future) H-field receiver I'm building here.

Contents

Logging And Processing
Sound Cards


Logging And Processing. In the following description, I use the same notation Paul Nicholson uses in his documentation. Reviewing the section on streams and buffers first will help.

I currently have the vlfrx tool set installed on three Linux servers, all running Debian 6.x or 7.x. One server currently houses the sound card and does data acquisition and GPS time stamping, while the second does signal processing, streaming and storage. The third does SID recording, and whistler ('event') detection. Experience has shown that at higher sample rates, and with larger numbers of VLF stations being monitored by vtsid, it is difficult to run all of the components on a single machine (vtsid is one of the biggest consumers of CPU cycles, especially when measuring absolute phase.)

The following diagram shows the signal and data flows for my current system. The rectangles represent different vlfrx-tools components, such as vtcard, etc. The elipses represent circular buffers read/written by different components. The data rate (192kHz, 32kHz) and width (2 byte integer) are also shown.


System Diagram
Current vlfrx-tools setup. Click for larger image.


Here are the startup scripts I use to run the tools on boot. I make no claim that this is the best way to do any of this - I only claim that it works very reliably. You may notice the hostnames "vlf01-priv", "vlf02-priv" and "vlf03-priv". Those are the aliases for network interfaces on a private network that vlf01, vlf02, and vlf03 share. While not necessary, I did that to maximize performance between the three machines.

On host vlf01, my data collection box:

#!/bin/bash

# Read VLF data from sound card. VLF is on channel 2 (right). Timing is on
# channel 1 (left).
/data/bin/vtcard -Bvv -L/data/log/vtcard_efield.log -A b=262144,p=2048 -r 192000 -d hw:0,0 -b16 @efield,15,i2

# Wait for @efield to be ready
/data/bin/vtwait -t @efield

# Start vttime
/data/bin/vttime -Bvv -L/data/log/vttime_efield.log -m ppsbase+,c=1.550e-3,w=auto -c1 @efield @timed
#/data/bin/vttime -Bvv -L/data/log/vttime_efield.log -m ppsbase+,c=1.457e-3,w=auto -c1 @efield @timed

# Wait for @timed to be ready
/data/bin/vtwait -t @timed

# Start forwarding data to vlf02
/data/bin/vtcat -Bvv -L /data/log/vtcat-vlf02.log @timed:2 ++vlf02-priv,9000,i2


After starting a flow of raw samples at 192kHz from the sound card, the data is timestamped and sample rate corrected to precisely 192kHz. The e-field data in @timed1 is then sent to host vlf02. @timed2 will be vlf h-field data... if I ever get it finished. :)


On to host vlf02:

#!/bin/bash

# Start collecting data from sound cards on other machines
/data/bin/vtcat -Bvv -L/data/log/vtcat_in.log ++9000 @timed,i2

# Wait for @timed to become available
/data/bin/vtwait -t @timed

# Start forwarding data to VLF03
/data/bin/vtcat -Bvv -L /data/log/vtcat-vlf03.log @timed:1 ++vlf03-priv,9000,i2

# Create a filtered version of the stream
/data/bin/vtfilter -B -a th=6 -h hp,f=200,poles=3 -h lp,f=8000,poles=3 -g4 @timed:1 @filter2
/data/bin/vtwait -t @filter2

# Create a resampled version of the filtered stream to 32k
/data/bin/vtresample -B -r32000 @filter2 @resampled
/data/bin/vtwait -t @resampled

# Create a resampled version of the raw stream to 32k
/data/bin/vtresample -B -r32000 @timed @resamp_raw
/data/bin/vtwait -t @resamp_raw

# Start recording resampled data to the long term data store
/data/bin/vtwrite -Bvv -G86400 -L /data/log/vtwrite_32k.log @resamp_raw /data/raw

# Start recording full bw data to the short term data store
/data/bin/vtwrite -Bvv -G3600 -L /data/log/vtwrite_192k.log @timed:1 /data2/raw-fb

# Start remote stream to abelian.org
/data/bin/vtvorbis -Bvv -L /data/log/vtvorbis.log -eq0.8 -kn 46.4.26.83,7835 @resampled


Here we place the 192kHz timestamped stream from vlf02 into @timed, which is read by other processes:

- It is resampled to 32kHz and then written to disk for long term storage by vtwrite. Cron jobs use vtflac to compress data older than 1 month for long term storage.

- It is written to disk at 192kHz for short term storage. Cron jobs limit the stored data to approximately 24 hours.

- @timed is also passed through a notch filter, a high pass filter with Fc=200Hz, a low pass filter with Fc=8khz, amplified, and then is written to @filter2.

- @filter2 is fed to vtvorbis, which sends the stream to abelian.org where you can hear it as vlf35.


On to host vlf03:

#!/bin/bash

# Start collecting data from sound cards on other machines
/data/bin/vtcat -Bvv -L/data/log/vtcat_in.log ++9000 @timed,i2

# Wait for @timed to become available
/data/bin/vtwait -t @timed

# Create a filtered version of the stream
/data/bin/vtfilter -B -a th=6 -h hp,f=200,poles=3 -h lp,f=8000,poles=3 -g4 @timed:1 @filter2
/data/bin/vtwait -t @filter2

# Create a resampled version of the stream to 32k
/data/bin/vtresample -B -r32000 @filter2 @resampled
/data/bin/vtwait -t @resampled

# Start SID monitor(s)
/data/bin/vtsid -Bvv -L/data/log/vtsid.log -c /data/sid/sid.conf @timed:1

# Start event monitor
/data/bin/vtevent -Bvv -L/data/log/vtevent.log -d /data/events @resampled


Here we place the 192kHz timestamped stream from vlf02 into @timed, which is read by other processes:

- It is read by vtsid, which produces amplitude and absolute phase data for vlf stations I monitor, as well as recording noise levels in different bands and frequencies of interest.

- @timed is also passed through a notch filter, a high pass filter with Fc=200Hz, a low pass filter with Fc=8khz, amplified, and then is written to @filter2.

- @filter2 is then fed into the event detector (for automated whistler/chorus/etc detection)


Sound Cards. It happens that I had a large quantity (13) of Sound Blaster Live! cards on hand from a different project. Generally speaking, I've had acceptable results using these bargain-basement cards. I've now moved up to the ASUS Xonar D1 (see below).

Here are a few caveats:

Tip No. 1: I have had much better success using the OSS drivers for the Sound Blaster! cards.  When running using ALSA drivers, my system was plagued by vtcard dropping from run mode back into setup mode (which can happen for a number of reasons.) I tried numerous cards on 6 different hosts with varying buffer sizes, and was never able to get the problem to go away. vtcard would reset between 5 and 20 times a day, interrupting my stream and data collection constantly. By moving to OSS, and selecting a proper buffer size, I had completely eliminated the problem.

Tip No. 2: Buffer sizes matter. Notice the "-A b=262144,p=2048" in the vtcard settings I use. This parameter increases the buffer used for passing samples from the sound card. To an extent, larger is better, as it helps vtcard cope with varying system loads. Its possible, however, to set it too high. vttime expects to 'see' a GPS timing pulse within a certain window of the sample buffer. If you make the buffer too large, that pulse can walk its way outside of that window and vttime will be unable to find it. In the end, you will probably need to play with the buffering a little to get a setup that works.

Tip No. 3: Not all crystal resonators are created equal. Of the 13 Sound Blaster! cards I had, only 5 had acceptable levels for sample rate drift (where acceptable means that the drift is small enough that vtcard does not revert to setup mode, and vttime is able to properly track the changes.) All sound cards will exhibit some level of sample rate drift over time. Some are quite stable; others not so much. The only way to really know is to try them. Note that environmental shifts (breezes into the PC case) can make the problem worse, so you might want to consider creating a low air flow pocket or insulation around the sound card.

UPDATE: I really wanted to be able to see stations and signals higher than 24kHz, so I began to research sound cards with a 192kHz sample rate. After a number of experiments, I settled on the ASUS Xonar D1. Note that "D1" is important. There are a variety of Xonar D-series cards that claim 192kHz sampling rate support, but have a number of limitations that make them unsuitable. The D1 It it widely available, and can be picked up on eBay quite inexpensively. There is full ALSA support, and the sample rate for all of the D1s I've purchased so far are quite stable. The D1 comes in a PCI form factor (as opposed to PCI-e), and it works well in legacy hardware (like the PCs I put to use for VLF...)

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