Re: [softrock40] Re: USB sound card dongle with stereo INPUT
- On 2/9/2013 4:05 PM, Alan wrote:
> ----- Original Message -----Hi Alan,
> Subject: Re: [softrock40] Re: USB sound card dongle with stereo INPUT
>> I suspect that what you claim is true if you are receiving amplitude
>> modulated signals. However, if you are receiving a digital signal, such
>> as WSPR, won't the additional bits help in the signal processing gain?
>> Since WSPR can detect signals with more than -20 dB S/N, I would think
>> that 24 bit resolution would produce better results that 16 bit
My comments inserted below--
> The way I understand it is that 24 bits does not give better resolution.24 bits does give better resolution than 16 bits. Most modern A to D
> But it gives a greater dynamic range.
> I have not heard anyone suggest that 24 bits on it's own improves the reception of weak signals.
converters can digitize a signal whose peak to peak value is less than
the supply voltage for the ADC (typically 5V). The minimum signal that
can be quantized is 2^N times smaller than the largest signal. This is
the "resolution" of the device. For N equal to 16 bits, the smallest
signal is 65535 times smaller than the largest signal. For N equal to
24 bits (most 24 bit ADC do not have a true 24 bit range, but an
"effective number of bits" that is less--e.g. 21 bits), the smallest
signal that can be resolved is more than 16 million times smaller than
the largest signal. We typically refer to dynamic range as the ratio of
the largest signal (which is frequently independent of the number of
bits) to the smallest signal that can be quantized. Therefore the 24
bit ADC has both improved resolution, and improved dynamic range.
> My observations seem to show that the noise from the Softrock opamps is greater than the noise from a low noise soundcard. AndThis is generally true, unless you are fortunate to live in an area that
> greater than antenna noise at higher frequencies, maybe above 21MHz.
has exceptionally low environmental noise (received from the antenna).
> Others have said that reducing the opamp gain in the Softrock does enable low noise 24 bit cards to increase the dynamic range.This is not increasing the dynamic range of the ADC, but is merely
shifting the signal levels to utilize as much of the range of the ADC as
possible. You may, however, sacrifice some of the weak signal
capability of the ADC.
> Without modification a Softrock will not benefit much, if anything, from a 24 bit card.I still believe that, when operating some of the sophisticated digital
> 73 Alan G4ZFQ
modes that require substantial signal processing, there is an advantage
to using 24 bit ADCs. Even though noise is present, you need to be able
to resolve tiny changes in the combination of signal and noise. By
averaging (integrating) numerous samples of signal plus noise, you are
able to suppress the effects of the noise. This, of course, assumes
that the noise is uncorrelated with the desired signal, and that the
noise has zero average value. (I won't comment on techniques that are
used when the noise does not meet this criteria).
For the typical CW or SSB user, I would agree that most would not
experience a significant improvement when going from 16 bit to 24 bit ADCs.
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 firstname.lastname@example.org, "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