Nuts & Volts - September 2006

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ASurround sound comes in many versions, with 5.1, 6.1, and 7.1 being the most popular. The first number is the number of “audio” speakers, with the x.1 designation referring to the sole subwoofer — with a frequency range extending from 80 Hz down to 10 Hz. Typically, surround sound is an evolution of 1950’s stereo and 1970’s quad channels to create the effect of theater sound in a small space. This is done by recording six discrete channels of audio on the recording media (Figure 6) — 5.1 Surround Sound. From this, all the other versions have been derived. That is, there are only six audio channels — nothing more.

In 6.1 Surround Sound, an extra rear speaker is added. This is a mono channel created by adding the two rear left and rear right channels together and placing it behind your back where the sound resembles an echo of the Center speaker. In 7.1 Surround Sound, the back speaker is split into two. Again, the sound is derived from the two rear side channels. Sometimes the two sound sources are a combination of L-R and L+R (not unlike FM stereo) to create the background sound. Sometimes the sounds are simply a duplication of the back speakers spaced accordingly, or 6.1 decoding fed into both. Neither the 6.1 or 7.1 formats have separated channels. They are an algorithm of the original five 5.1 Surround Sound.

Given these parameters, I often question who benefits from these new formats. Unless you have a video room the size of a pool hall, most readers won’t hear the difference and should not be bilked into paying for something they can’t use. Oh, about your test disc, try these people: http://ferradoyle group.com/dolby/dol302.html. I ordered one of these discs, and found it well worth the $5 asking price.

PROGRAMMERS WANTED

QI’m looking for a circuit with which to build a six-digit clock, just as I used to do with National Semiconductor’s MM5314, the T-I MM5375, and the Mostek MK50250 (my favorite). Unfortunately, all the above are obsolete, so what I want is a number for their replacement — not those excuses you make for PICs! A chip that delivers a multiplexed six-digit output and uses the line frequency for timing — which is usually more accurate than crystal time base. I also doubt that clock radios use the esoteric circuits you described. There must be a source for simple multiplexed clock chips.

■ FIGURE 6

What chips are used in the clock radios you can find in many stores, or in the kits sold by Jameco, Ramsey, and others?

AThis is a very common request — and one I have a hard time filling. The following two places say they have at least one of these items in stock, and while they don’t require a minimum order, expect to pay premium prices.

MAILBAG

Dear TJ,

I appreciated your explanation of current mirrors in the July ‘06 issue, but saw that there was a mistake in Figure 4. The right hand side labeled “current source” must be made with

_+VPNPs, not NPNs. By the way, when working with discrete devices, ^Reone suggestion I’ve found helpful is to insert resistors in the emitter leg of each BJT ^Iloadto improve the matching between ^Rloadtransistors. This improves the accuracy of the current mirroring and stabilizes the ratio

0 mV

nt Source

against small differences in temperature between the reference device and mirror device(s). The resistors should be just large enough to produce about a 100 mV drop at the operating current — and must all be the same value.

— Jeff Berwick

Response: I like your suggestion. In fact, that’s how the Figure 4 drawing error occurred. I had 16 different versions of current mirrors on one page — way too big to publish. So I did a simple process of eliminating those I didn’t want and reconfiguring the remaining. Guess what? Yep, erased the one I wanted and ... well, you can guess the rest. Figure 7 shows the correction — plus your suggestion. Thanks a lot for your feedback! — TJ