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[linrad] Presonus Firebox Help Needed!!
- Subject: [linrad] Presonus Firebox Help Needed!!
- From: w3sz <comcast.net; w3sz@xxxxxxxxxxxxxxxx>
- Date: Sun, 28 May 2006 14:09:39 -0400
This is a long post and so if you are not interested in helping me with a
problem I am having with the Presonus Firebox, delete NOW.
This is another post regarding problems I am seeing with the Presonus
Firebox. I am hoping for one of the I/Q gurus to help me figure out what
is going on. My questions are basically:
1. What is happening?
2. Is it a hardware or software problem?
3. Is this behaviour present in all Fireboxes, in which case I will keep
it, or is it unique to mine in which case I will send it to the
manufacturer under warranty.
I am seeing MAJOR problems with the I/Q signals sent by the Firebox over
the Firewire to the computer when it is operated at 96kHz sampling rate.
These problems are not present at 48 kHz sampling rate, so I do not think
they are a just a result of improper soundcard connections by me. As I
will show below, they are NOT the expected 'image' problems seen when one
of the I/Q signals is not present, or is out of phase or of inappropriate
I am hoping that someone on one of these lists can explain to me what
might be going on.
I would also be interested in whether anyone else is using the FIrebox at
96 kHz with Linrad or Winrad [or other software that attempts to see an
approximately 990 KHz slice of the RF spectrum]. If so I would be
interested to know if when they have systematically looked for this
problem they have seen similar things.
The problem [since it is not an image problem] is of course not corrected
using the Channel Skew correction available in Winrad.
The problem is present in both Winrad and Linrad when using the Presonus
Firebox as the input soundcard when sampling at 96 kHz, I/Q mode receive,
with the SoftRock v6 for 80/40 meters being used for input to the
Firebox. It does not occur at all with the FIrebox run at 48 kHz, and
NEVER occurs when the Delta44 is used as input soundcard at 48 or 96 kHz
with the SoftRock as input. I am quite confident that the SoftRock is
working appropriately, as I have had no problems with it when using the
Delta44 with it.
Here is the data:
If I disconnect one of the I/Q signals coming into the Firebox from the
SoftRock I get just what one would expect: The signals are mirrored about
the center frequency and sidebands are inverted on the image signal. This
is normal to see. This is NOT what I see when both I and Q inputs are
connected to the Firebox [one to Main-In 3 and one to Main-In 4, each
using a TS 1/4 inch Phone Plug]. I did this experiment just to prove that
the problem I am describing is NOT an image problem. I knew this, but was
certain that some reading this wouldn't be so sure.
The malfunction is this:
There are identical spectra displayed above and below the center
frequency. I used the signal from CHU at 7.335 MHz as my 'signal
generator' for these tests, as all of my test equipment is at a remote
site. The CHU signal can be detected between the center frequency and up
to 23000 Hz above or below center frequency, but not beyond [i.e., the
bandwidth over which an input signal can be detected is about 48 kHz or
half the sampling rate, just as if the card were running at 48 kHz or if
I/Q sampling were NOT being performed]. But there is noisefloor displayed
over the full 96000 bandwidth of Linrad or Winrad, and the CHU signal will
be displayed over the full 96000 bandwidth as one varies the LO frequency
due to the 'extra' signal appearing on the other side of the spectrum from
the real signal, as is shown below. The displayed frequency for signal
appearing on the Linrad display within 23000 Hz of center frequency is
'appropriate'. The frequency of the 'extra' signal is of course not
appropriate but varies in a predictable way. The table below gives the LO
frequency and then the frequency of the 'correct' Linrad CHU signal and of
the 'extra' Linrad CHU signal appearing on the opposite side of center
frequency in the Linrad display. The behaviour is the same with Winrad
although the frequencies are labeled differently in that program. Note
that the spectra are NOT mirror images of each other and there is NO
inversion of sidebands [if you can't see the latter point, take my word
for it. I have confirmed it both visually and audibly]. This problem is
NOT an due to an absence of one of the two I/Q signals. I think that the
Firebox Driver must be handling the I/Q signals incorrectly at 96 KHz.
But the explanation is what I want an opinion on. Here is the data table:
LO Relative Freq as
FREQ Displayed by Linrad
IN Hz With Firebox input
[Correct / Extra]
29250 No CHU signal
29250 22500 74500
29260 20000 72000
29270 17500 69500
29280 15000 67000
29290 12500 64500
29300 10000 62000
29310 07500 59500
29320 05000 57000
29330 02500 54500
29340 00000 52000
29350 97500 45500
29360 95000 43000
29370 92500 40550
29380 90000 38000
29390 87500 35500
29400 85000 33000
29410 82500 30500
29420 80000 28000
29430 77500 25500
29440 75000 23000
29450 No CHU signal
WIth proper function, there would be CHU signal down to LO of 29160 and up
to LO of 29530. This is the case when the Delta44 is used as input.
The Linrad display is set up so that the bottom frequency is approximately
52500 and the frequency then increases to 99999 just below center
frequency, which is 00000. The frequency just above center frequency is
00001 and frequency continues to increase up to a maximum of about 45500.
So frequencies between 52500 and 99999 are below center frequency, and
frequencies from 00001 thru 45500 are above center frequency. So the CHU
frequency starts out near the middle of the upper [right] side of the
spectrum and gradually decreases or moves to the left until it disappears
at about the middle of the lower [left] half of the Linrad display. Doing
the experiment in Winrad gives the same result.
I have placed gif pictures of the Linrad display with the FIrebox
operating in this way and the LO at 29440 Hz at
You can see that the two sides of the spectrum appear identical [except
for the gradual shading caused by the very non-flat frequency response of
the Firebox]. You can see the 'correct' CHU signal at 75000 on the left
side of the screen, and the 'extra' CHU signal at 23000 on the right side
of the screen. Note that as I indicated above, the signals are NOT
mirrored as would occur with an imaging problem.
For comparison I placed a gif picture of the Linrad display with all
conditions identical, but with the Delta44 sampling at 96kHz used as input
instead of the FIrebox at
You can see that the CHU signal at 75000 is NOT reproduced at 23000 on the
right hand side of the spectrum. There IS a much weaker signal at 23000
to 25000 and that IS the weak image signal of CHU, and it does have
inverted sidebands as expected. This is a normal result of imperfect I/Q
phasing and does not indicate a problem at all. All of these tests were
done without I/Q correction for obvious reasons. Incidentally note the
MUCH flatter frequency response and thus flatter noise floor with the
Delta 44 as compared with the Firebox.
These experiments show that the problem is not with Linrad [or Winrad;
data not shown] or with the SoftRock.
I can change the sampling rate Linrad thinks it is seeing, and when I
change that to 48 kHz I get entirely different results. If I change the
sampling rate of the Firebox to 48 kHz I get correct function of the
FIrebox and thus different results.
I hope someone out there can explain this to me.
For the cognoscenti, I have placed Linrad "S" files and wav files of the
40 meter CW contest last night that were made using both the Firebox and
Delta44 as inputs, thinking that this might provide additional
information. I will leave those files on the server for only a couple of
days, as the space they occupy is generously loaned to me and I do not
want to abuse the gift with these large files. They are 12-15 MB size.
Firebox S file
FIrebox wav file
Delta44 S file
Delta 44 wav file
I also posted gif files of the spectral analysis done by CoolEdit96 of
these wave files. It looks to me like the Firebox spectra are mirrored
around center frequency rather than showing unique information on either
side of zero frequency. I suspect that is the root of the problem, but
don't know why it is occurring. Maybe the Firebox actually only looks at
48 kHz of bandwidth but oversamples at 96 kHz?? Would that give a useful
input bandwidth of 48 kHz but an output noise floor that covers 96 kHz
when the card is set to sample at 96 kHz?
Those files are at:
http://home.comcast.net/~w3sz/fireboxwavespectrum.gif for the Firebox wave
http://home.comcast.net/~w3sz/delta44wavespectrum.gif for the Delta44 for
the delta44 wave file
Thanks in advance, and
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