stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?
jsalsburg at bellsouth.net
Sun Jul 8 14:52:53 UTC 2012
If all you want to do is watch and listen to HAM and Police Trunk Radio, the $20 Dongle is adequate, but if you want to hack the E4000 Tuner, a better ADC is needed.
1) Yes, if all you need is to be able to detect a signal on an analog stream its Nyquist Frequency will do, (2 times the frequency being measured) a 20 megasample ADC would be adequate for the E4000 tuner's I/Q output, but there is more information in the Stream than just the presence of Radio Frequency Energy. If all you need to do is detect the presence of energy or listen to it (Audio Modulation) a 2 times Sample frequency is adequate, but if you need to decode octal phase modulated sideband signals, you need to go to at least 10 times Analog to Digital. This is the case with many signals on Satellite and Cable Streams and it is the case with many phase modulated signals over the air. Instead of just the minimum Conversion being done, you need 10 times the Conversion to reconstruct the modulation, and this may even miss subtle information. It is information we are seeking, information as intelligence modulating Electromagnetic Energy. Cable Modems, for example, Digitize the entire spectrum from 100,000 KHz to 800 MHz. This is because the TV Signals are encrypted with Octal Phase Modulation. You have to be able to digitize with wideband (multi-Hundred Megahertz Converters) to be able to Analyze, Decode, or even just to detect these modulation schemes. So even though the E4000 Tuner has only 8 MHz Bandwidth. You need at least 10 times that bandwidth to accurately reconstruct Phase Modulated Signals. In the vernacular of ADCs, anti-aliasing filters are required to remove unwanted high frequency content. Thus content is discarded, and noise is introduced. To adequately sample a broadband signal without discarding anything, radical oversampling is required. Oversampling allows Signal Processing to take place like Noise Reduction, with is very important for DXing. ADCs are cheaper than ever, driven by Scientific Instrumentation, Satellite, and Cable Industry needs.
2) Remember back a in the 1990s when USB first came to be. It was expensive and a luxury on high end computers. Now we have things like the Tuner Dongle with USB and virtually every computer with USB as a Standard; now there is USB 3; even faster. Apple came up with FireWire to solve the problem that USB had. FireWire is capable of real-time wideband serial communication. Better yet, even though it is still expensive, ThunderBolt will become the new "USB." At Multi-Gigabit Streaming rates (800 Mega Bytes per second). You will see in the near future, ADCs with USB 3 and ThunberBold built-in. There is just too much money to make. Current High Definition Cameras use SDI to stream their signals. There are already available Video equipment servicing the High Definition Television Industry with both SDI and ThunberBolt Side-by-Side. You may pooh-pooh this as too expensive, but so was USB when it first rolled onto the scene.
From: Sylvain Munaut [mailto:246tnt at gmail.com]
Sent: Saturday, July 07, 2012 3:34 AM
To: Jay Salsburg
Cc: osmocom-sdr at lists.osmocom.org
Subject: Re: stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?
> If no problems arise then addition of a dual input, 14 bit, 100
> Mega-sample, ADC would be the next step to take full advantage of the
> bandwidth and dynamic range of the tuner that the RTL2832 chip is
> obviously and purposefully limiting;
1) AFAIK, The E4000 has a limited output bandwidth, in the range of 8 MHz max usable IIRC. It's designed for DVB/TV applications and they don't need more so the IF filters available don't go above that.
2) You still need to pipe all those samples to the PC somehow, this will raises the price of all of this way above the original 20 USD and make it a much bulkier setup.
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