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13 Cards in this Set
- Front
- Back
- 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. C5 |
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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. C5 |
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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) C5 |
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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) C5 |
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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 C5 |
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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; C5 |
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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. C5 |