stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?

Leif Asbrink leif at sm5bsz.com
Mon Jul 9 21:26:36 UTC 2012


Hello Jay,

> Well, this is interesting. Think out of the box. Ask yourself if a 8 bit NFM
> is high fidelity. 
You have to specify what you mean by this.
The NFM signal carries information by the instantaneous frequeny
of the signal. When sampled with 8 bit frequently enough the
frequency can be determined with great precision.

> I do high fidelity; 6 channel, 24 bit Wav files, that is
> high fidelity. 
This statement is basically false. There is no way that you could have
24 bit accuracy in a .wav file (unless the waveforms are synthetized
in the computer from mathematical functions.)

A good 24 bit soundcard like the Delta 44 provides 18 bit data.
Bits 19 to 24 contain only noise and can be truncated without
any effect on the recorded information. The best modern A/D
converters may provide nearly 20 bits, but providing 24 bit is
not possible. The noise floor is set by the thermal noise
of any resistor at room temperature.

The 8 bit data in RTL2832 is from two separate A/D converters
and together they provide 8 bit at 4MHz sampling rate. That is 
the same as 9 bit at 1MHz or 10 bit at 250 kHz or 11 bit at 62.5 kHz.

I was thinking you were referring to FM broadcast in which case
the FM detector would improve S/N by about 20 dB. (3 bit)

> When you have high conversion rates and high bit depth
> signals you get high fidelity. Audio NFM signals are only meant to satisfy
> walky-talky radio communication (Trunk Radio). 
Hmmm, "Audio NFM"? do you mean the audio signal delivered by an FM detector?
They would have about 13 bit accuracy when produced from the 8 bit 2*2 MHz
data stream from a rtl2832. In real life the S/N would limit performance
and it would be identical to what you observe on a better radio unless you
have really strong interferring signals.

> The high value targets in
> Radio technology today is Data Services. Look in the (Data Stream) Mine for
> the Gold, Data is Gold. Data is wideband, multiple channel, and frequency
> diverse (not NFM), then there are all those wideband RADAR Signals. With
> high fidelity conversion you do not need anti-aliasing. Anti-Aliasing is a
> technique to filter, the very word means to take something away. Data
> services count on the fact that the Data is difficult to intercept.
It does not matter at all whether we do anti-aliasing in analog hardware
or whether we sample at a higher rate and do it by a digital filter.
The result is identical.

The effective bandwidth is what matters.

> The object of any great effort is to rise above all the rest. To get the
> most out of the Tuner the ADC must be improved. All you guys have so much to
> offer and you have spent so much of your time and money to get the most out
> of this $20 Dongle, all because the Manufacturer refuses to open Source the
> Chip. Jump ahead and make better use of it. Go to the next level and hack
> the circuit, not just the Operating Code. 
Well, I did that and came to the conclusion it would be a good idea
until the AGC problem was solved. Now, without the AGC, the RTL2832
chip matches the performance of the E4000 tuner pretty well. 
Adding a much better A/D converter will not improve performance
significantly. We would need a better tuner to motivate a better
A/D converter.

> The most extensive effort should be in acquiring intelligence from the
> Either, not listening to high power squawking Police channels. This requires
> very high granularity, the RTL2832 is low granularity. 
Are you sure you know what you are saying here?

The RTL2832/E4000 dongle provides a dynamic range of 80 dB.
You might tell us what the received power levels are from
the stations you want to hear - and what is the signal level
of the stations that might interfere (high power squawking 
Police channels...)

> If you want to drop
> in on intelligence from over the horizon, very good selectivity is required
> and the RTL2832 is just not that; highly selective, intentionally by design.
The RTL2832 is just a dual A/D converter. (As operated for SDR)
Replacing it with something 2 or 3 times faster would allow
us to remove the alias signals that we see in the DVB dongles.
It would not improve performance much in other respects however
because the minimum noise floor from the E4000 is about 1 bit
on the RTL2832 and running the E4000 at higher output levels
would make the harmonic distortion unacceptable.

Surely performance of the DVB dongle is limited, but it is not 
bad. Quite respectable actually. (But you need the latest 
software - or maybe even a little later than what you can 
download today. I have not evaluated the latest changes so 
I do not know.)

Regards 

Leif / SM5BSZ





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