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13 Cards in this Set

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- To convert an analog signal to a digital signal, what are the 4 steps needed?

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- 1 Sample the analog signal regularly; sampling rate must be twice the frequency
- 2 Quantize the sample
- 3 Encode the value into 8 bit digital form
- 4 Compress the samples to reduce (or save) bandwidth.


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- What is the most commonly used method of converting analog to digital signal?

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- PCM – Pulse code modulation

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- To convert a digital signal to an analog signal, what are the 3 steps needed?

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- Decompress the samples (if compressed)
- Decode the samples into voltage amplitudes, rebuilding the PAM signal
- Filter the signal to remove any noise.


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- Describe the Nyquist Theorem which is the basis for digital signal technology;

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- If you sample an analog signal at twice the rate of the highest frequency of interest, you can accurately reconstruct that signal back into its analog form.

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- What is quantization?

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- It involves dividing the range of amplitude values that are present in an analog signal sample into a set of discrete steps that are closest in value to the original analog signal.

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- Name 2 methods of quantization

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- a-law (most countries)
- u-law (Canada, U.S and Japan)


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- The a-law and the u-law “compand” the signal; explain what this means.

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- They both compress the signal for transmission and then expand the signal back to its original form at the other end.

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- Name two types of voice-compression schemes;

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- Waveform coding; (PCM,ADPCM)
- Source coding;(LDCELP, CS-ACELP)


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- What is a vocoder?

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- A vocoder (or voice coder) is a device that converts analog speech into digital speech, using a specific compression scheme that is optimized for coding human speech.

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- List 2 ITU standards (compression protocols-codec) for waveform compression scheme;

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- G.711 rate 64 Kbps
- G.726 rate can be 16,24 or 32 Kbps


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- Name the compression algorithm that Cisco uses for high-quality 8 kbps voice;

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- G.729

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- List the 3 common voice compression techniques standardized by ITU-T;

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- PCM; Amplitude and voice signal is sampled and quantized at 8000 times per second.
- ADPCM; Quantize the difference between the speech signal and a prediction that has been made of the speech signal
- CELP;


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- List 2 methods used to measure the quality of voice transported on a telephone line.

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- Mean Opinion Score (M.O.S)- graded by humans and very subjective
- Perceptual Speech Quality Measurement (P.S.Q.M) – works by comparing the transmitted speech to the original input and yields a score.
- Note: Cisco voice equipment does not perfome voice quality measurements.


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