Electronic Design

Bob's Mailbox

Dear Bob: I designed magnetic stripe readers in the eighties and would like to comment on your recent "Bob's Mailbox."* Three tracks are defined on the standard card. Most credit, ATM, and debit cards use track two in the middle of the stripe. It has data at 75 bits per inch and a checksum. In the old days, the crummy tape oxide would lose magnetic energy. Nowadays, they have very high magnetic durability.

Unfortunately, the read heads must all be spring-loaded to follow the stripe surface. And as the stripes wear, they sometimes get little bumps and troughs that cause the heads to skip over bits and, of course, get checksum errors at the end of the stripe. The plastic bag trick and Scotch tape tricks simply allow the head to ride smoothly over the now even-more-distant magnetic stripe and read it okay. I wish this was a question on Who Wants To Be A Millionaire so I would have a shot.

Ken Mathews (via e-mail)
Pease: Hi, Ken. Thanks for the explanation. Yeah, it all makes sense. Good luck the next time you're a contestant!

Hi Bob: Thanks for your recent article on noise gain (ELECTRONIC DESIGN, Feb. 2, p. 22). Every once in a while, your column has something very useful (like some of your earlier stuff on control theory). I always try to find your column in the hopes that it might have such good stuff—in lieu of the typical rant about something non-technical, or topics such as the recent rant about Spice versus prototyping. I usually know I can blow off your column unless I see one of your little scribbly schematics in the middle of it.

Tony Alfrey (via e-mail)
Pease: Hello Tony. I don't argue about which column is IMPORTANT. But even if I don't include a cute little schematic, you can bet the column is about THINKING. Read it briefly. You may or may not like it, and you may or may not find it educational. But many people get something from my columns.

Dear Bob: I've enjoyed your columns for many years. I'm now at the stage in life where I'm into hearing aids (too much time in small airplanes over 50+ years). I put on a "behind-the-ear" hearing aid for the first time a few days ago. It's somewhat helpful but not as good as a three-band equalizer from Radio Shack (separately adjustable bands labeled 900 Hz, 3 kHz, and 6 kHz) at one-tenth the cost.

It's good to get rid of the lead from the amplifier to the earpiece, but the official "hearing aid" leaves me hungering for more of the 6-kHz stuff to really understand the consonants. I looked at the response curves for a number of digital and analog devices. They all seem to roll off at about 4 kHz, with the gain being essentially 0 dB at 6 kHz.

The PhD audiologist says this is an industry problem. (That's probably right. Part of the problem is that if you turn up the gain at 4 to 6 to 8 kHz, you get a big boost for the noise, resulting in a mediocre noise-to-signal ratio./rap) Based on my limited knowledge of amplifiers, this doesn't make sense to me. In the "good old days" of fine audio amplifiers, "flat from 20 to 20,000 Hz" was the marketing hoo-haa everywhere. (Oh, but this is for best-case listening conditions. It's nice to avoid bad rolloffs and bad phase shifts of about 3 kHz, if you can help it. Most people can't hear the stuff at 20 kHz, but they like good bandwidth past 4 and 8 kHz. But even telephones have only 3 kHz, and you don't need—or want—much more than that for good audibility. /rap)

The speakers must certainly be limiting, but I'd expect that to be a low-end problem. Is there some limiting physics about pushing airborne audio energy down small passages? A Web search on "hearing aid bandwidth" doesn't turn up much.

Dick Reilly (via e-mail)
Pease: Look up Etymotics Research (Oak Grove Village, Ill.) and Mead Killion. If you're trying to get good audibility, his company may have good advice. If he can't help you directly, ask him where to turn. He's a smart cookie and very helpful.

Comments invited!
[email protected] —or:

Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

* ELECTRONIC DESIGN, Feb. 16, p.22.

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