Signal synchronization in the presence of noise
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From: Vladimir Vassilevsky on 19 Apr 2010 16:02 Rune Allnor wrote: > 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. I was wondering what kind of black magic is involved in the above mentioned nonlinear sweep waveforms; and if plain simple convolution is how they are processed. I guess geophysicists try different waveforms to resolve sidelobes, dispersion and valid reflections. Fortunately, Doppler is irrelevant in geophysics (?). However I don't know what is the exact reasoning for each of those sweeps ("T-power", "dB/Oct", "dB/Hz"). Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Rune Allnor on 19 Apr 2010 17:19 On 19 apr, 22:02, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > Rune Allnor wrote: > > 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. > > I was wondering what kind of black magic is involved in the above > mentioned nonlinear sweep waveforms; and if plain simple convolution is > how they are processed. Matched filters is the usual method. An FM frequency sweep signal is fairly unique as well as simple to generate. > I guess geophysicists try different waveforms > to resolve sidelobes, dispersion and valid reflections. No. The geophys processing is labour intensive, where human operators extensively interact with, and manipulate, the processing chain. When people talk about 'seismic interpretation' they mean exactly that - interpretation. > Fortunately, > Doppler is irrelevant in geophysics (?). In seismics, yes. > However I don't know what is > the exact reasoning for each of those sweeps ("T-power", "dB/Oct", > "dB/Hz"). I don't recognize those terms off the top of my head, except the latter two might indicate absorption factors as function of bandwidth. Rune
From: Vladimir Vassilevsky on 19 Apr 2010 17:36 Rune Allnor wrote: > On 19 apr, 22:02, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > >>However I don't know what is >>the exact reasoning for each of those sweeps ("T-power", "dB/Oct", >>"dB/Hz"). > > > I don't recognize those terms off the top of my head, except > the latter two might indicate absorption factors as function > of bandwidth. Aha! Thank you for the clue. Looks like they are trying to keep SNR vs frequency while having sharp autocorrelation. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Rune Allnor on 19 Apr 2010 19:03
On 19 apr, 23:36, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > Rune Allnor wrote: > > On 19 apr, 22:02, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > > >>However I don't know what is > >>the exact reasoning for each of those sweeps ("T-power", "dB/Oct", > >>"dB/Hz"). > > > I don't recognize those terms off the top of my head, except > > the latter two might indicate absorption factors as function > > of bandwidth. > > Aha! Thank you for the clue. Looks like they are trying to keep SNR vs > frequency while having sharp autocorrelation. Coming to think of it, there might be the question of Doppler processing. In water, the sound speed c is about 1500 m/s, so Doppler shifts can become quite large with small velocities (compared to EM propagation). A source speed of 15 m/s (~30 kts) is about 0.01*c. In such circumstances estimating the Doppler shift might be important, so it is common to use a parbolic FM sweep in the sonar pulse. That way the extreme frequency, at the apex of the parabola, is easily estimated from the recieved signal, so one can easily compute the Doppler shift. If one used a naive linear sweep one would not necessarily detect the fact if the upper or lower frequencies were attenuated, and thus end up with poor Doppler shift estimates. Rune |