There is a question from the CCNA Voice Official Certification guide that I have problems answering and would appreciate advise. This is the question;
Which of the following processes samples an analog waveform at timed intervals to obtain a digital mesurement of the electrical signal?
The book says the correct answer is B. I checked with PeasonView and they contacted the author and the author says B. I think it is C.
The following is from Wikepedia for PCM;
In telephony, a standard audio signal for a single phone call is encoded as 8,000 analog samples per second, of 8 bits each, giving a 64 kbit/s digital signal known as DS0. The default signal compression encoding on a DS0 is either μ-law (mu-law) PCM (North America and Japan) or A-law PCM (Europe and most of the rest of the world). These are logarithmic compression systems where a 12 or 13-bit linear PCM sample number is mapped into an 8-bit value. This system is described by international standard G.711.
Could someone explain why the answer is pulse amplitude modulation instead of pulse code modulation? Aren't voice systems using PCM?
B is indeed the correct answer. To generate a PCM signal you need to do the following: Sample the analog waveform at fixed time intervals, quantize the samples and apply the companding function (u-law or a-law). The result is a PCM signal.
PCM = PAM + Quantization + Companding.
It is true that voice systems use PCM but the process that samples the waveform at fixed intervals is PAM.
I disagree with the books answer. PAM does not produce a "digital" measurement of the electrical signal?
While PAM samples analog signals at regular intervals, it produces "pulses" of vary magnitude that are equal to the signal amplitude at the time of sampling.
PCM is the the method used to produce "digitized" voice. It is commonly implemented on a single IC chip and is what we refer to as an analog-to-digital converter (ADC).
A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals
While it is true PCM = PAM + quantization, PAM does not produce the "digital" representation. Digitization is part of the PCM process not the PAM process.
Yes, digital voice systems use PCM.
Hope this helps.
This is a poorly worded question. I agree with you that PAM + Quantization is the digital measurement. PAM is still an analog signal.
However, PCM is incorrect because it is the result of taking the PAM pulse train, quantizing it, and then companding it. PCM doesnt sample anything, it uses the PAM samples. The PCM process encompasses all 3 processes: PAM + Quantization + Companding.
They need to reword the question, and/or the answers.
Here is the question.......
Which of the following processes samples an analog waveform at timed intervals to obtain a digital measurement of the electrical signal?
This is the definition of PCM
A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals.
PCM is a process which includes several other processes or steps. You have filtering, sampling (PAM), quantization (the actual digitization process), and companding.
I would agree the question is poorly written if the author's intent was to have you select PAM. Because the PAM process does not produce a digital signal, the PCM process does.
P.S. - Here is Cisco's own document on Waveform Coding Techniques.
Yeah, the question should ask you to pick 2 answers.
The correct answers would be: PAM and Quantization.
PAM + Quantization yields a digital representation of the signal.
I think the intent was to show that PCM is made up of several sub-processes. And thanks for reminding me about the filtering portion as that is definately required. It's been about 20yrs since I took my DSP class!
Thanks you for your excellent responses. I have learned alot. I did send your comments to the practice test creators Pearson IT Certification and asked to have the question corrected with at least an explanation added by the author.
You are welcome. It was a nice discussion. Thanks to both of you for making me think about PCM/PAM and DSP concepts!
Hopefully Pearson will update the practice exam accordingly.