net464-02 - Speech Encoding.pptx - NET464 Converged...

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09/22/2020NET464-02NET464Converged MultimediaNetworksLecture 2James T. Yu, Ph.D.[email protected]School of ComputingDePaul University
09/22/2020NET464-02OverviewCodec = Coder/DecoderTradeoffsBandwidth – lower is better, but qualitymay sufferQuality – higher is better, but that takesmore bandwidth and/or more complex(costly) codecCost – the chips and software that gointo the end stations and gatewaysMore complex, more expensiveMore widely used, les expensive
09/22/2020NET464-02Analog and DigitalSignalAnalogSignal
09/22/2020NET464-02Sampling Voice Signals125 usec.+V-VHearing Range: 20 -20 kHzTypical conversation: < 3.4 kHzIn the design of a low-pass filter, it usually uses 4 kHz.Sampling Rate = 8,000 sample/sec0
09/22/2020NET464-02Nyquist TheoremIf a signal running through a low passfilter of bandwidth H, the filtered signalcan becompletely reconstructedbymaking only 2H samples per second.If a signal consists of V discrete levels, therequired bit rate for transmitting thesignal is:bit rate = 2 × Hlog2V bits/sec(bps)Example:Voice (low pass filter): 4 kHz (3.4 kHz)Quantization (discrete) level: 256Bit rate = 2 × 4 (kHz)log2256 = 64K bps
09/22/2020NET464-02CODEC OperationFilter:filter out non-voice signalSampler: samples the analog signal at fixed intervals defined by thesampling frequency, FSAMPLEQuantizer digitizes PAM samples into discrete levels.The process results in Quantization noise introduced into signalEncoder: encodes anbit binary pattern for each quantized samplefiltersamplerquantizerencoderAnalog signalDigital signal
09/22/2020NET464-02Analog to DigitalConversion via Pulse CodeModulation (PCM)PAM4 kHz+127-1278 bit Word8000Samples / Sec+21FILTERFILTERSAMPLERSAMPLERENCODERENCODERQUANTIZERQUANTIZER0 Hz to 20 kHz*PAM: Pulse Amplitude Modulatio
09/22/2020NET464-02Step One: FilteringFilterFilterFilter out frequencies below 200 Hz and above 3400 Hz0 Hz to 20 kHz4 kHz(3200 Hz)
09/22/2020NET464-02Step Two: SamplingSamplerSamplerPAM Samples8,000 per second125 µs125 µs125 µs8,000 samples per second125 µs125 µs125 µs125 µs125 µs125 µs
09/22/2020NET464-02Step Three:Quantizing+ 127+ 127- 127- 127QuantizationQuantization125 µs125 µs00PAM Samples8,000 per second-49-49
09/22/2020NET464-02Step Four: Encoding-49PAMSample(8,000 per second)00011001MSBLSBAmplitude8 bit wordEENNCCOODDEERR
09/22/2020NET464-02More on PCMa-law: the PCM method used inEuropeμ-law – the PCM method used inNorth America.Their differences are minor, and mostdevices support both a-law and μ-law.ADPCM – Adaptive differential PCM4-bits sampling, 32K bps vs. 64K bps
What is thedifferencebetweenmodem andcodec?09/22/2020NET464-02: cell phone: does it have the functions of modem and/or cod
09/22/2020NET464-02Advantages of VoiceDigitizationNoise Resistance (reliability)Can you tell whether a phone call is local or toll by itsvoice quality?

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