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Author Topic:   My Chirping Hi-Fi
jimh posted 05-06-2014 10:07 AM ET (US)   Profile for jimh   Send Email to jimh  
Recently I indulged in a rather inexpensive audio amplifier with some digital capability and five outputs for loudspeakers, or a so-called 5.1-Surround Sound system. This is not a high-end device, and is actually more toward the bottom rung of the price ladder. I was surprised to find that the device included its own calibration microphone and a special input for the calibration microphone. When you plug-in the calibration microphone, the amplifier detects the microphone and branches into a special mode for using it.

In the calibration mode, the device gives on-screen instructions, and then begins an automated calibration procedure. The amplifier puts out some signals, one channel at a time, the calibration microphone picks up the sound, and after a few minutes the device calculates the proper levels for the five speakers, or even a sixth speaker, if you have a sub-woofer (which I don't). This is quite an amazing amount of science and technology in a very inexpensive box. But now to the (perhaps) more interesting point.

During the calibration process, the signals created by the device for its calibration process are frequency sweeps. If this were a SONAR, you'd call these signals chirps. The signals are rapid sweeps from low frequency, about 100-Hz, to high-frequency, maybe 5,000-Hz--they're so fast it is hard to tell by ear--repeated about ten times in succession for each speaker calibration.

I believe the sweeps are used for two purposes: first, to make detection and recognition of them easier, as I assume the device is also using a matching swept filter to listen for them. This helps separate the desired signal from any room background noise. And, second, of course, to get a sense of the speaker frequency response at different parts in the audible range.

I am mentioning this for several reasons. First, to acknowledge the astonishing amount of technology that can exist today in very inexpensive equipment. I don't think you could have purchased an audio amplifier with this sort of self calibration system ten years ago at any price, and certainly not for under $300. Next, to observe how sweep frequency signals or "chirps" are not something exclusive to echosounding. Here they are apparently being used to help improve the selectivity of a detector (the calibration microphone) in what be a noisy enviroment. And, finally, because I thought it was rather cool that my new surround sound amplifier uses chirps. You see, my amplifier is not "a compressed high-intensity radar pulse" device, as some devotees of strange acronyms would suggest is meant by "chirp." It is a device using a frequency sweep signal. That is actually what the echosounders are doing, too.

jimh posted 05-06-2014 10:19 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE for HiFi and SONAR techies:

An interesting outcome of the calibration microphone process is the amplifier volume control become calibrated in terms of sound level in a cinema. Once the calibration process has been run, if the volume control is set to a particular level (in this case "64" on a scale of 0 to 100) the sound level at the position used for calibration will then match the sound level used when the film was originally mixed for cinema presentation in a certified THX cinema environment. I actually find that setting is a bit too loud in the loud parts of a film, and I suspect that my amplifier and loudspeakers can't handle the peak signals at that setting. I tend to turn the volume down about 6-dB, to about 58.

This self-alignment process would be an interesting concept to explore in echosounders. Perhaps there could be some sort of self-calibration process of echosounder sensitivity. You'd position your boat at a certain depth over a sandy sea bottom and hit a "calibrate" button. Then the echosounder would emit a series of test signals, and gauge the level of the return echoes. Once calibrated, the strength of returning echoes could be better gauged in terms of the size of the target reflecting them.

Calibration of a echo range finding system, like a RADAR, is probably common. I believe that in the good old days of RADAR, say in the 1960's on the DEW line, the RADAR systems were frequently checked to verify their detector sensitivity. I don't know the exact process that was used.

With modern electronic technology it seems like a SONAR ought to be able to include self-calibration. Heck, if a $300 Hi-Fi can have it, why not a $2,000 SONAR?

Hoosier posted 05-07-2014 08:18 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
It's hard to sort stuff out in today's marketing speak but this "sounds" like the self calibration feature you're talking about.

"Advanced Signal Processing (ASP) reduces the need to manually adjust settings to see fish, structure and bottom detail more clearly"

This is a bullet point in the description of the new, $700, Lowrance Elite 7 CHIRP sonar/chartplotter: Elite-7-CHIRP-en-us.aspx

As an aside: CHIRP is more correctly known as a linear FM pulse that has unique proportites that make it easy to process, which is why it can now be found in consumer sonars.

jimh posted 05-07-2014 10:54 AM ET (US)     Profile for jimh  Send Email to jimh     
As a result of being able to manipulate a signal, in either a sonic (i.e. aubible) or an ultrasonic (i.e. above audible) range of sound, in a digital signal processor, there is really almost no limit on what can be done. In some cases, perhaps, too many options are offered. Who knows what happens to a return echo in an echosounder that applies "advanced signal processing" to the signal.

Perhaps the "advanced signal processing" observes the depth of the bottom, and adjusts the gain to be appropriate.

Many years ago I worked for an electronics manufacturer that made ultrasonic devices, and, in particular, ultrasonic flaw detectors, which were used to inspect weldments for flaws. In c.1973 the company had a new feature that it promoted as a big advantage: it was called time-controlled gain or TCG. The gain of the ultrasonic detector that was listening for return echoes could be configured to increase with time, so that the longer the duration after the transmitted pulse the more the gain of the detector would increase. In this way there was compensation for the signal loss in a longer echo path.

In terms of marine echosounding, the paradigm would be to increase the gain of the detector automatically as the water depth increased. In this way, a return from a target at, say, 100-feet, would be processed with more gain than a return from a target higher in the water column, say at 50-feet. I suppose that now this is probably trivial, and may be done automatically, so much so that it is not even mentioned. But in the 1970's it was considered remarkable, and the company made a point of remarking about it in their products.

In the 1970's the signal processing was done with analog methods, and the gain was controlled by a DC signal that ramped up or down. The feature was further enhanced by being able to overlay on the oscilloscope display the characteristics of the gain signal. In this way the operator could have a visual indication of how the gain was being adjusted with time. There was also an offset to delay the start of the TCG function. It was really quite a brilliant idea to draw the gain curve on the display as it was extremely useful for the operator to be able see what gain change was going to be applied.

Also, in ultrasonic flaw detection there were calibration blocks which contained reflectors of known size at specific depths, and these were used to establish the sensitivity of the detector. An instrument would be tested on the calibration blocks to demonstrate that it was working properly. In that way, when you inspected a weldment that was a critical component, say the containment wall of a nuclear reactor, you could certify that the instrument you used was working at a specified sensitivity.

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