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# PCM Codec Issue

Posted on 2008-06-24
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Beings of the planet Zamco can hear frequencies up to 30 kHz. However, they can only hear two loudness levelssoft and deafening. The Zamconian telephone system uses PCM codecs. a) How many bits per second will a Zamconian telephone call generate? Do your work in Excel, and copy and paste your results into your answer page. b) How many bytes will it take to store an hour of a Zamconian telephone conversation? (Hint: The answers to a) and b) are 60 bps and 26 MB, respectively.)

Okay I am working on this question for homework for a computer networking class. I am completely puzzled as to what in the world this even means.  If anyone can provide any suggestions/resolutions/advice at all -- I would greatly appreciate it!!!

Thank you!
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Question by:danielsd
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Expert Comment

ID: 21855791
This is a terrible question.

If you want to digitize a 30 kHz signal,  what sample rate do you need?

If there are only two amplitude levels, so how many bits do you need per sample?

I think both hints are wrong:

The answers to a) and b) are 60 kbps and 27 MB, respectively.

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Author Comment

ID: 21855879
Thanks for the comment.  This is why I'm confused -- I don't know what to calculate the 30 khz? I don't know if its wanting analog to digital?  Makes no sense to me : )

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LVL 27

Accepted Solution

d-glitch earned 500 total points
ID: 21856128
The Nyquist-Shannon Theorem says that to digitize an analog signal, you need to sample at twice the highest frequency component.

http://en.wikipedia.org/wiki/Nyquist-Shannon_sampling_theorem

So if you have a 30 kHz signal, you have to digitize at 60 ksps.

What the question neglects, avoids, or assumes is that the input is filtered.
Maybe the Zarconians can only hear up to 30 kHz, but that doesn't mean that there isn't noise on the line at higher frequencies.

After you get the 60 ksps, you have to multiply by the number of seconds in an hour, and divide by the number of bits in a byte.

Probably not the best way to present these concepts.
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