[linrad] Re: Presonus Firebox Help Needed!!

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Roger and All,

> 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, 
> .
> .
> .
> 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 
> .
> .
> .
You hit the nail on the head:-)

I added a 19th figure on this page:
http://www.sm5bsz.com/linuxdsp/usage/ldfw/ldfw.htm

I had seen something similar - but four signals rather than two
when trying Windows 98 on a slow computer.
After upgrading the drive routines I save exactly what you 
describe.

The sampling happens at 48 kHz and a VERY simple routine is
used to up-sample. You can see the same it you set the Delta 44
sampling speed to 192 kHz in ALSA.....

As far as I can recall, OSS has a filter that removes most of 
the false signal if you use it for up-sampling.

> 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.
Yes. The upsampling creates a spur every 48 kHz and one of them is
always visible (including its noise floor) - but not if you use OSS;-)

I suggest you write a complaint to the manufacturer. You bought
something that was said to sample at 96 kHz - but it does not.
The hardware is most probably fully capable (at least the digital
stuff - there could be an analog low pass filter somewhere but not
very likely) Try to force the to give you new firmware/ drive 
routines. The A/D converter is presumably sampling at something like
64*96 kHz and it should just be instructed to down-sample one step
less. (Why not ask for 192 kHz if you get into a dialog with the
right people. They might get many additional customers.....)


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