Signal synchronization in the presence of noise
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From: Vladimir Vassilevsky on 19 Apr 2010 09:42 Rune Allnor wrote: > You need the *whole* template, and the max peak only occurs if the > signal contains the the *whole* sync sequence. When you geophysicists design the sonde signals, what properties of the signal do you have in mind? I.e. what is the purpose of the waveforms commonly referred as "T-power", "dB/Octave", "dB/Hz" ? Are those intended for highly dispersive media, and if so, what kind of receive processing do you need ? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Mark on 20 Apr 2010 09:17 On Apr 20, 6:43 am, Rune Allnor <all...(a)tele.ntnu.no> wrote: > On 20 apr, 12:03, "electrical_storm" > > <gauripatil24(a)n_o_s_p_a_m.gmail.com> wrote: > > But the > > sampling rate of the input is always unknown and a variable. > > In that case you are screwed. You have to know *something* about > the data to get anything useful out of them. That additional > knowledge is the difference between 'data' and 'a sequence of > numbers'. > > Rune I was also curious about the terms dB/ocatave, dB/Hz and t-power.. after some searching, I found this: http://sepwww.stanford.edu/oldsep/cliner/files/suhelp/suvibro Mark
From: Tim Wescott on 20 Apr 2010 10:15 electrical_storm wrote: (top posting fixed) >> On 20 apr, 10:28, "electrical_storm" >> <gauripatil24(a)n_o_s_p_a_m.gmail.com> wrote: >>> Thanks for your comments. >>> >>> My problem is slightly more complicated because the input signal has a >>> different sampling rate than the what the template was recorded at. >> Then get a new template signal. Either record a new template >> at the same sample rate as it will be used, or resample the >> template you have to the sampling rate it will be used. >> >> Mind you - if your template is sampled at a lower rate than >> the signal itself, you might find that significant details >> in the pre-sampling analog template waveform are missing from >> the sampled version. >> >> Rune >> > The sampling rate of the input signal varies. So to have the same sampling > rate for the template and the input is not an option. > > On second thoughts, maybe I could vary the sampling rate of the template > according to some algorithm and then correlate it with the input. But the > sampling rate of the input is always unknown and a variable. Do you _really_ mean that after sampling you _still_ don't know when the input samples were taken? Or do you just mean that the sampling rate of the input is unknown _beforehand_. If the former, then as Rune said, your case is hopeless. If the sampling times vary within some known bounds, or if they are known after sampling, then you may still have hope -- but the problem is vastly complicated. This is getting awfully close to an "if you have to ask you can't do it" sort of question. If a potential client came and laid this out to me today I'd be delighted to take the work -- but I wouldn't be making any promises beyond looking harder at what's going on and assessing feasibility. Once I'd figured out if it's even _possible_, then I'd discuss with the client what could be done. The biggest questions are if, after sampling, does the sampled signal still contain the necessary features that can be used for correlation, and would the signal processing be able to identify them. The answers have to be 'yes' to both of these questions before you'll ever succeed, so if either one is 'no' then you have to think about changing the problem somehow. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Tim Wescott on 19 Apr 2010 10:36 Rune Allnor wrote: > On 19 apr, 13:58, "electrical_storm" > <gauripatil24(a)n_o_s_p_a_m.gmail.com> wrote: >> Hello, >> >> I am looking for ideas to synchronize an incoming signal with a template >> signal. >> >> I also want to detect if the incoming signal is valid or not. For this, I >> cross-correlate a small length of the input signal with the template. This >> seems to work fine. >> >> How can I then find out a synchronization point between the noisy input and >> the template? I have been doing this in MATLAB. My initial idea was to find >> the point of maximum correlation and use this as the sync point. But this >> does not seem to work for noisy signals. > > That's the way to do it. But the reliability of the peak depends > on signal design. If you have a well-designed sync sequence you > will get a nice, narrow peak even in low SNRs. Correlate the whole > sync template with your signal, and see if the match is better. > You need the *whole* template, and the max peak only occurs if the > signal contains the the *whole* sync sequence. Communications-type signals these days will often have a sync sequence as Rune describes, but will also have some sort of error correction coding -- either forward or backward, or both -- built in. Generally the last word in whether you're really synchronized comes from getting all the right CRC checks on your data. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Rune Allnor on 19 Apr 2010 11:16
On 19 apr, 15:42, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > Rune Allnor wrote: > > You need the *whole* template, and the max peak only occurs if the > > signal contains the the *whole* sync sequence. > > When you geophysicists design the sonde signals, what properties of the > signal do you have in mind? I.e. what is the purpose of the waveforms > commonly referred as "T-power", "dB/Octave", "dB/Hz" ? Are those > intended for highly dispersive media, and if so, what kind of receive > processing do you need ? *Geophycisists* don't design signals, as marine geophysical signal sources are 'tamed' explosives - air guns: A metal container that is filled with compressed air. The air is quickly released into the water, producing an air bubble. This oscillating air bubble is the source of the geopysical signal. It's very hard to control that kind of source - one can certainly not design any sophisticated signal. Consistency between shots (the rule-of-thumb is one shot every 4-10 seconds for weeks on end) and some directivity, using source arrays, are hard enough to achieve. In *sonar*, on the other hand, the limiting factors are 1) Available pressure amplitude. Too much pressure, and the water cavitates. 2) Bandiwdth. Water is a messy medium, so too large bandwidth, and the properties of the water change too much over the bandwidth. One common solution is to use moderate amplitude (i.e. the water's acoustic properties stay linear), moderate bandwidth (dispersive effects are negligeable) and instead use e.g. FM sweep pulses that last a long time. Designing signals is an exercise in utilizing the Time-Bandwidth product, knowing and understanding the constraints and limit factors, and coming up with useful trade-offs. Rune |