Re: USB sound card dongle with stereo INPUT
I don't know what to tell you. Upon reading this I double checked using first my internal card and then my iMic USB dongle.
Here is my procedure.... maybe you can tell me where I am making a wrong assumption.
1) Calibrate HDSDR using a -73 dBm source to read "S9".
2) Disconnect the calibration signal and read the noise floor on HDSDR using 2.5 KHz filter, expanding for 1 dB resolution vertically, and averaging 128 samples to get a smooth baseline.
3) Added 54 dB of calibrated attenuation to the calibrator signal and flipped the "1 dB" switch on/off to check the display is registering a 1 dB change at -127 dBm.
4) Verified a high level signal on the spectrum analyzer for level at -4 dBm.
5) Connected the high level signal through a 21 dB step attenuator to the receiver.
6) Reduced the attenuation one dB at a time until I reached a point where 1 dB in reduction no longer resulted in 1 dB of increased level indicated on HDSDR.
On the internal card the noise floor read as above was at -131 dBm and the max level for still seeing 1 dB increments was -18 dBm
On the iMic dongle the noise floor was -127 dBm and the compression point was -17 dBm.
Any one of my numbers could easily be + or - 2-3 dB. But no way they are 20 dB off.
My first thought was perhaps my internal card was actually 24 bits.... I do not know what is in there. However the iMic is clearly 16 bits.
I think my measurements are clearly demonstrating 110 dB of dynamic range for both devices.
Any further thoughts?
Warren Allgyer - W8TOD
--- In firstname.lastname@example.org, "victor" wrote:
> Warren, a 16 bit card would have a theoretical dynamic range of 96dB and in practice even less (~90dB would be a good number). In your example you talk about -12 -(-125)= 113dB. You just can't get it with a 16 bit card.
> Victor - 4Z4ME
> --- In email@example.com, "warrenallgyer" wrote:
> > What you say Shirley is absolutely correct. IF you could reduce the input gain of the radio to the point where the atmospheric noise was only a dB or so above the card noise, then this would maximize the dynamic range of the radio and allow reception in the presence of very strong signals.
> > There are two practical issues with this which I explored in an earlier thread on the subject:
> > 1) Even by reducing the Windows audio input gain to its' lowest possible level you cannot reduce the received atmospheric noise to the internal noise level of a 16 bit card, let alone a 24 bit card.
> > So
> > 2) To accomplish what you suggest you would need to resort to external attenuators for your radio. External attenuators for an RXTX are problematic because you would want to switch them out of the circuit on transmit.
> > In my earlier thread I strongly recommended that input audio gains be reduced to minimum and that by doing so you could raise the received RF level at which most PC audio cards go into overload to about -12 dBm..... which is 61 dB over S9 on a properly calibrated meter. Even at this setting however my 16 bit card exhibits an internal noise floor at -125 dBm with the atmospheric noise floor at -115. So moving to 24 bits is a complete waste of money, even for strong signal handling, in my case. I suspect this would be the case in most installations.
> > Warren Allgyer - W8TOD
> > --- In firstname.lastname@example.org, Shirley Márquez Dúlcey wrote:
> > >
> > > With proper gain distribution the lower noise floor of a good sound card
> > > will improve the total dynamic range of your Softrock and thus STRONG
> > > signal handling will be better. The ideal arrangement to maximize dynamic
> > > range is for the band noise floor to be just barely above the noise floor
> > > of the sound card; this can be achieved by adjusting the input gain of the
> > > sound interface (if such a control is available) and/or using attenuation
> > > between the Softrock and the sound card.
> > >
> > > A really low noise floor will also help if you are using an inefficient
> > > receive antenna such as a Beverage.
> > >
I am on the road until Saturday and cannot do any more testing until then. I have taken the opportunity on the planes to read up on the definition of noise floor. The methodology in the ARRL handbook requires measuring the RMS value of the noise at the speaker terminals of the receiver as a reference. Then a signal generator is tuned to the receive frequency and the signal attenuated until the measured audio power is exactly 3 dB above the reference level. This is the "noise floor" and the MDS by the ARRL definition.
I suspect the HDSDR baseline noise floor may turn out to be the same but I want to confirm that when I return.
Note this measurement is highly dependent upon the filter bandwidth selected, as is the HDSDR baseline noise. I have tried to stay with a filter bandwidth of 2.5 KHz because that is the reference used by Joe Taylor for his SNR measurements.
I further suspect, in the WSPR example for instance, a measured SNR of "-20 dB" may well be measured on a signal that is visible above the HDSDR baseline because it would take significant power in a single frequency spike to double the noise power in 2.5 KHz..... but again, all I can do is speculate until I get back on the ground.
Finally, I have observed that I cannot attenuate the signal generator completely at -130 dB and I have always put this down to signal generator leakage. Now, with Victor's input, I am wondering if this is indeed leakage or if it is LSB quantization error.
So many things to learn..... and so little time. This is really starting to impinge on my night life! :-)
Warren Allgyer - W8TOD
--- In email@example.com, "Alan" wrote:
> ----- Original Message -----
> From: "victor"
> Subject: [softrock40] Re: USB sound card dongle with stereo INPUT
> > Warren, the 96 dB is indeed the dynamic range in which an ideal ADC will convert a signal with error smaller than 1/2 LSB. Any
> > larger signal will be positively clipped.
> I tried to duplicate your tests using your method. The generator I used did not have a sufficiently high output but with HDSDR I
> seemed to get a range of at least 109dB.
> Delta 44 with 16 bit MME driver.
> With the levels I used the noise floor remained steady.
> But using Rocky, SDR# and SpecLab I could not get results anywhere near this. I think more tests will be needed. HDSDR seemed to be
> responding linearly but I should see the same with other software.
> > I certainly felt an improvement of less spurs and better clean spectrum when you receive a dense spectrum with many strong signals
> > after changing the audio card to 24 bit.
> Sorry, I am reluctant to accept subjective assessments. In the absence of proper tests I'd at least like to see simultaneous
> spectra. One of 16 bit and one of 24 bit taken at the same time. Preferably on identical cards but even two cards in the same
> computer can have different levels of spurious pickup.
> What SDR were you using for this check?
> 73 Alan G4ZFQ