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        riddles >> easy >> Real Life
        
(Message started by: maryl on Jun 16^th, 2003, 2:27pm)

Title: Real Life
Post by maryl on Jun 16^th, 2003, 2:27pm

Here are a couple of real life situations:

Microwave:
Every morning when I warm my milk for breakfast, I put one cup of milk in the microwave (which is in working order) for exactly 84 seconds. Why?

Snooze:
Why is it very common to have a 9 minute snooze interval on alarm clocks, why not 10 instead ?

Title: Re: Real Life
Post by harpanet on Jun 16^th, 2003, 2:36pm

Microwave:
[hide]Once, you heated it for 90 seconds (being an easily programmed value) and it just about boiled over. So you knocked a few seconds off.[/hide]

Snooze:
[hide]Most alarms are triggered at the start of a minute, therefore a 10 minute snooze would end up being 11 minutes from the last alarm. A 9 minute snooze will wake you up again 10 minutes after it last sounded.[/hide]

Title: Re: Real Life
Post by Leonid Broukhis on Jun 16^th, 2003, 2:46pm

Because you have an older, dumber microwave that does not
[hide]finish the complete rotation after the time is up[/hide].

Title: Re: Real Life
Post by THUDandBLUNDER on Jun 16^th, 2003, 2:49pm

Microwave: [hide]
After 84 seconds the turntable has made an integer number of revolutions,
allowing you to conveniently grasp the cup's handle to take it out. [/hide]

Title: Re: Real Life
Post by Jeremiah Smith on Jun 16^th, 2003, 2:51pm

http://www.straightdope.com/classics/a991126.html

Title: Re: Real Life
Post by otter on Jun 17^th, 2003, 8:07am

Quote:

Snooze:
Why is it very common to have a 9 minute snooze interval on alarm clocks, why not 10 instead ?

I seem to remember something about this from way back in my early computer science days about the chips commonly used for alarm clocks using EBCDIC and 9 was a magic number in that system, but it's been way too many years to remember the details. Then again, I could be wrong.

Title: Re: Real Life
Post by maryl on Jun 17^th, 2003, 11:13am

on 06/17/03 at 08:07:02, otter wrote:

I don't know about nine being a magic number but it is the last digit.

Title: Re: Real Life
Post by maryl on Jun 17^th, 2003, 11:15am

on 06/16/03 at 14:49:01, THUDandBLUNDER wrote:

Microwave: [hide]
After 84 seconds the turntable has made an integer number of revolutions,
allowing you to conveniently grasp the cup's handle to take it out. [/hide]

Yup, makes life so much easier for us modern folk.

Title: Re: Real Life
Post by Icarus on Jun 17^th, 2003, 6:57pm

on 06/17/03 at 08:07:02, otter wrote:

EBCDIC, like ASCII, is a system of representing characters as numbers within the computer. (The only difference between the two is what number corresponds to what character. All I can recall right now about EBCDIC was that for some unexplained reason, they broke the alphabet up into two parts. It has a-m as consecutive numbers, but then some other characters before it continued with n-z.)

Since most clock chips have no need to store alphabetic information, I doubt they were based on EBCDIC.

Probably what you are thinking of is BSD (if I am remembering the acronym right). This is a system of storing numbers in decimal, rather than binary form. Each decimal digit is stored in a half-byte (4 bits). When adding two such numbers together, the BSD chips add them together as if they were stored as binary numbers, then do a conversion step to change them into the correct BSD digits. If I recall correctly, DEC developed this system and used it (maybe still uses it) on their business systems. While it wastes some memory and time, it also makes it easier to manipulate decimal data without introducing unexpected round-off errors.

If the snooze time works by an actual "minute" counter, then a BSD chip would certainly explain why 9 minutes would be used, since this is the largest single digit count. But it seems more likely to me that the snooze interval would be implemented as a seconds count, or as a count of what ever fundemental time unit the clock is tracking. In that case the explanation breaks down. 540 is no more easily manipulated in BSD than 600.

Title: Re: Real Life
Post by wowbagger on Jun 18^th, 2003, 1:58am

on 06/17/03 at 18:57:18, Icarus wrote:

Probably what you are thinking of is BSD (if I am remembering the acronym right).

Well, I don't know about you, Icarus, but I'd pronounce this like "bee-as-dee", not "bsd" - it's just an abbreviation, not an acronym.

Title: Re: Real Life
Post by James Fingas on Jun 18^th, 2003, 10:28am

Icarus,

I think it's BCD (Binary Coded Decimal). But I could be wrong...

Title: Re: Real Life
Post by Sir Col on Jun 18^th, 2003, 11:36am

Most digital alarm clocks trigger when the current time string (hours:minutes) matches the alarm string. You may have witnessed this if you've ever set your alarm and if, as you're advancing the hour or minute, it coincides with the current time, the alarm will sound.

I believe the reason the snooze function is set to 9 minutes is because it's cheaper to produce chips that track the last digit. When you hit the snooze button, the clock subtracts one from the end digit and when the end digit of the current time matches it again the alarm sounds. If it did 10 minutes, it would sound straight away.

Title: Re: Real Life
Post by Jeremiah Smith on Jun 18^th, 2003, 11:44am

on 06/16/03 at 14:51:15, Jeremiah Smith wrote:

http://www.straightdope.com/classics/a991126.html

Regarding 9 minute alarm timers.

Title: Re: Real Life
Post by Sir Col on Jun 18^th, 2003, 11:52am

I thought that the article was satirical?

The comment it makes a towards the end, in which it rebuffs the end digit reason, is by asking, "Why not put the snooze counter on the tens column and advance that by one?" The reason is quite simple, digital clocks use BCD counters and you would have to incorporate a modulo counter to deal with hour cross-overs; that, in engineering terms, is more expensive.

Title: Re: Real Life
Post by Leonid Broukhis on Jun 18^th, 2003, 1:47pm

on 06/18/03 at 11:52:06, Sir Col wrote:

Incrementing ones counter by nine needs a modulo counter as well. The counters are already there, so it does not matter (resource sharing can be used, to put in electronic design terms). The reason is that if you increment the tens column to implement a 10-minute snooze, you still have to compare both tens and ones. That involves 4 more register bits and an 8-bit comparator instead of a 4-bit comparator (although 5 bits would be enough, just invert the lSB of the tens digit). When fractions of a penny count, it is not a good idea.

Title: Re: Real Life
Post by Icarus on Jun 18^th, 2003, 7:44pm

on 06/18/03 at 10:28:08, James Fingas wrote:

Icarus,

I think it's BCD (Binary Coded Decimal). But I could be wrong...

Thanks - that's what I get for being too lazy to go look it up! :)

Title: Re: Real Life
Post by Sir Col on Jun 19^th, 2003, 12:57am

on 06/18/03 at 13:47:05, Leonid Broukhis wrote:

The counters are already there, so it does not matter (resource sharing can be used, to put in electronic design terms).

Of course they are! I was so preoccupied with the more significant issue, that it would require an allocation of two memory blocks, I forgot that there is a modulo 6 shift counter in place. Having said that, I'm not sure if it'd be possible to resource share the base 6 modifier allocated to the 1st minute digit, as it would not be a standard chip feature; whereas I would have thought that a few modulo 10 shift counters are standard implementations in BCD chips and the snooze function could be allocated its own unqiue resource. It's a long time since I did microelectronic systems engineering and I've been teaching mathematics since then, so I am very much out of touch – and rusty! By your posts I'm guessing that this is your area, Leonid, so I have a question. I was thinking about this last night and I was puzzled over my own reasoning. I suggested that the snooze function operating on a 9 minute interval was a cost saving feature (only needing to store the last digit), but it would, in fact, be a waste of resources. As BCDs only use 4 bits, wouldn't it make sence to utilise the full byte and store the last two digits, that way allowing a 10 (or anything up to 59) minute interval?

Title: Re: Real Life
Post by otter on Jun 19^th, 2003, 6:09am

on 06/18/03 at 10:28:08, James Fingas wrote:

Icarus,

I think it's BCD (Binary Coded Decimal). But I could be wrong...

DOH! You are absolutely correct. I should have said BCD. :P

As an aside, the clock we use in the bedroom uses a 7 minute snooze. Go figure... It does have dual "his & hers" alarms, but I don't think that has anything to do with the 7 minute snooze feature.

Title: Re: Real Life
Post by Leonid Broukhis on Jun 19^th, 2003, 9:22am

on 06/19/03 at 00:57:22, Sir Col wrote:

I was thinking about this last night and I was puzzled over my own reasoning. I suggested that the snooze function operating on a 9 minute interval was a cost saving feature (only needing to store the last digit), but it would, in fact, be a waste of resources. As BCDs only use 4 bits, wouldn't it make sence to utilise the full byte and store the last two digits, that way allowing a 10 (or anything up to 59) minute interval?

I believe that the chips used in digital clocks are optimized to the max, there are no spare bits to utilize, and adding a few dozen transistors (4 extra register bits, a wider comparator, resource sharing muxes or an additional adder, etc.) to a circuit that only consists of a few hundred transistors would increase its cost by at least a few percent, and it does not make business sense to be less cost-competitive only to conform to someone's idea of a perfect snooze interval.

Title: Re: Real Life
Post by Martin Zauberman on Jun 24^th, 2003, 2:37pm

Quote:

Snooze:
Why is it very common to have a 9 minute snooze interval on alarm clocks, why not 10 instead ?

Most people hit the snooze button 2-3 times, meaning 18-27 minutes instead of the expected 20-30, therefore, giving the snoozer an additional 2-3 minutes to rouse him/herself. It's a nicety, really. ;)