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From donald@knudsenengineering.com Fri Dec 21 07:10:48 2001
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Date: Fri, 21 Dec 2001 10:09:40 -0500
To: henkart@sioseis.ucsd.edu
From: Don Knudsen 
Subject: Re: take a peek please
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Paul,

I hate to have to tell you this, but the data may be lost forever. Donna 
has provided a technical explanation which is attached, but the end result 
is that if the window was larger than 1000m or so the bottom (deepest) 
section of the record may not have been recorded. The problem is related to 
data rates and buffer size limitations.

I hope this isn't going to cause major problems at your end.

Regards,
Don



From Donna:

The following numbers are all based on the assumption of V4.30 firmware 
code or equivalent.  The basic concept still remains the same.

The echosounder acquires the raw carrier data recorded to the SEG-Y and the 
envelope data recorded to the binary files at different effective sample 
rates.  It is designed to have an envelope sampling rate that allows full 
acquisition of any data window size without truncation of the data record. 
The carrier sampling has limited data sampling rates and is limited by the 
size of the acquisition buffer.

The envelope sample buffer is 16256.  For a 5000m window at 1500m/s, the 
effective sample rate (Fe)  needs to be 2438 HZ or less. To support full 
autophase search windows, the lowest Fe available is actually well below 
this. Thus, the envelope data can always track the bottom through the 
entire 5000m window without any loss of data.

The carrier data buffer is actually larger at 32505 but the min Fe is only 
14836 for 3.5kHz or 16025 for 12kHz.  At 1500m/s, this limits the maximum 
recordable window size to 1643m for 3.5kHz or 1521m for 12kHz.  If a window 
size larger than this is used, all data beyond those listed is lost; it is 
simply thrown away to avoid buffer overflow.

The unfortunate effect of the factors is that the user looking at the 
envelope presentation in a 2000m or 5000m window will believe he is 
tracking and recording the data correctly, when in reality he may be losing 
critical data due to the buffer overflow truncation.  If he was doing the 
recording with the 1000m window, the carrier data would be correctly 
recorded as seen on the envelope presentation.

The latest version of EchoControl being prepared for our new upcoming 
release has a warning message about the potential for data truncation and 
offers controls changes to perform to avoid the truncation.


Donald Knudsen - President
Knudsen Engineering Limited
10 Industrial Rd. Perth, Ontario, Canada, K7H 3P2
Phone: (613) 267-1165 Fax: (613) 267-7085
http://knudsenengineering.com

Manufacturers of High Performance Rugged Marine Instrumentation

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