Unit IV – Speech Coding

Unit IV – Speech Coding

                                                                        PART - A

1.Define Encoder.

The analog-to-digital converter, located on the  transmitter, is also known as the encoder or simply coder.

2. What is Decoder?

The digital-to-analog converter, located in the receiver is known as the decoder.

3. Define CODEC.

The word CODEC is derived from coder/decoder.

Coder (or) encoder = analog-to-digital converter

Decoder = digital –to-analog converter

Simply it is the combination of coder and decoder.

4. What are the major classifications of speech coders?

Based on compression, speech coders are broadly classified into two categories

a)  Waveform coders

b)  Vocoders(Voice Coder)

5. Define waveform coders.

Waveform coders essentially used to reproduce the time waveform of the speech signal as closely as possible. They are designed to be source independent and can hence code equally well a variety of signal.

6. Write the examples of waveform coders.

a)  Pulse code modulation (PCM)

b)  Differential pulse code modulation(DPCM)

c)  Adaptive differential pulse code modulation(ADPCM)

d)  Delta modulation(DM)

e)  Adaptive delta modulation(ADM)

f)   Continuously variable slope delta modulation(CVSDM) AND

g)  Adaptive Predictive Coding(APC)

7. Define vocoders.

Vocoders is a circuit used for digitizing voice at a low data rate by using knowledge of  the  way  in  which  voice  sounds  are  produced.  A  vocoder  is  an  example  of  lossy compression applied to human speech.

8. What are the types of speech signals available?

The speech signal is assumed to be of two types. a)   Voiced

b)  Unvoiced

Voiced sound: (“m”,”n”,”v” pronunciations) are a result of quasiperiodic vibrations of

The vocal chord.

Unvoiced: (“f”,”s”,”sh” pronunciations) are fricatives produced by turbulent air flow

Through a constriction.

9. Write the vocoders parameters.

The parameters associated with vocoders are the voice pitch, the pole frequencies of the modulating filter, and the corresponding amplitude parameters.

10. Give the advantages of vocoders.

a)  It achieves very high economy in transmission bit rate. b)  Less robust.

11. What are the types of vocoders available?

a)  Linear predictive coder (LPC)

b)  Channel vocoders c)   Formant vocoders

d)  Cepstrum vocoders and e)   Voice-excided vocoders.

12. What is LPC vocoder?

Linear predictive coders (LPCs) are belongs to the time domain class of vocoders. This vocoders attempt to extract the significant features of speech from the time wave form, this is a low bit rate vocoders.

13. Write the transfer function of LPC.

The linear predictive coding system models the vocal tract as an all pole linear filter with transfer function described by

    b     

f

η  = 1 −  OH   ×100%

     bT     

Where, G is a gain of the filter and

Z-1 represents a unit delay function.

14. What are the types of linear predictive coders available?

(i) LPC vocoders

(ii) Multipulse excited LPC.

(iii) Code excited LPC and

(iv) Residual excited LPC.

15. What is MPE-LPC?

By using, more than one pulse typically eight per second, and adjusting the individual pulse positions and amplitudes sequentially to minimize a spectrally weighted mean square error. This technique is called the multiple excited LPC (MPC-LPC).

16. Write the applications of CELP.

(i)                 Advanced  DSP  and  VLSI  technology,  real-time  implementation  of CELP codec’s are possible

(ii)               The CDMA digital cellular standard (IS-95) proposed by QUALCOMM uses a variable rate CELP codec at 1.2 to 14.4 kbps.

17. Mention the advantages of CELP?

(i)                 CELP can provide high quality even when the excitation is coded at only 0.25 bits per sample.

(ii)               These coders can achieve transmission bit rates as low as 4.8kbps.

18. What are the factors, we are considering to select speech codec’s for mobile communications?

Factors must be considered are

(a) Compression

(b) Overall system cost

(c) Capacity

(d) End-to-end delay

(e) The algorithmic complexity of the coder

(f) The dc power requirements

(g) Compatibility with existing standards and

(h) Robustness of the encoded speech to transmission errors.

19. What are the blocks are available in GSM encoders?

The GSM codec consists of four blocks. They are

(i)        Pre-processing

(ii)       Short-term prediction(STP)

(iii)      Long-term prediction(LTP) and

(iv)      Regular pulse excited(RPE)

20. What are the blocks are available in GSM  decoder?

                    (i)RPE decoding

                    (ii)LTP synthesis

                    (iii)STP synthesis

  (iv)Post-processing

21. What are the advantages of frequency domain channel sounding method?

This techniques work well and indirectly provides amplitude and phase information in the time domain.

The system requires careful calibration and hardwired synchronization between the transmitter and receiver making it useful only for very close measurements.

22. Write the drawbacks of frequency domain channel sounding method.

(i)        This system is the non-real time nature of the measurement.

(ii)       For time varying channels, the channel frequency response can change rapidly, giving an erroneous impulse response measurement.

23.  Define mean excess delay.

This is the average delay measured with respect to the first (arrival) moment of the power delay profile

24.  What is Doppler spread?

Doppler spread BD  is defined as the range of frequencies over which the received

Doppler spectrum is essentially non-zero.

Doppler spread is also defined as the spectral width of a received carrier when a single sinusoidal carrier is transmitted through the multipath channel.

The maximum Doppler frequency fd  is given by

f  = Vf

d            c

Where , C – Velocity of light

λ – Wavelength of the radio signal in meters f – Radio frequency

25. What is flat fading?

If the mobile radio channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of the transmitted signal, then the received signal will undergo flat fading.

26. Write the conditions for flat fading.

The condition for flat fading is

BW of signal < < BW of channel

And    symbol period > > delay spread

Ts > >στ

Where, TS – Reciprocal bandwidth (i.e, symbol period)

BS – Signal bandwidth

στ – rms delay spread and

BC – Coherence bandwidth

27. What is frequency-selective fading?

If the channel possesses a constant-gain and linear phase response over a bandwidth that is, smaller than the bandwidth of transmitted signal, then the channel creates frequency selective fading on the received signal.

 28. Define fast fading channel.

The channel impulse response changes rapidly within the symbol duration. This type of channel is called fast fading channel.

29.  Write the conditions for fast fading channel.

A signal undergoes fast fading if

Symbol period > coherence time

and      BW of signal < BW of Doppler spread

30. Define slow fading channel.

The channel impulse response changes at a rate much slower than the transmitted base band signal. This type is channel is called slow fading channel.

31. Write the conditions for slow fading channel.

A signal undergoes slow fading if

Symbol period < < Coherence time

TS < < TC

And, BW of signal > > BW of Doppler spread,

BS >> BD

32. What is the need of statistical model for multipath fading channels?

Several multipath models are used to explain the observed statistical nature of a mobile channel.

33. Mention some of the statistical modes?

The statistical modes are

a)   Ossana model

b)  Clarke’s model for flat fading

c)  Simulation of Clarke and Gans fading model

d)   Two-ray rayleigh fading model

e)  Saleh and Valenzuela indoor and outdoor statistical models

f)   SIRCIM and SMRCIM indoor and outdoor statistical models.

34. Define level crossing rate(LCR)

The level crossing rate is defined as the expected rate at which the Rayleigh fading envelope,  normalized to the total rms signal level, crosses a specified level in a positive going direction.

35. What is the need of Rayleigh fading simulators?

Several Rayleigh fading simulators may be used in conjunction with variable gains and time delays to produce frequency selective fading effect.

36. Write the disadvantages of hata model.

(i)               Hata’s model does not have any of the path specific corrections which  

                                                are available in Okumura’s model.

(ii)       This model is not suitable for PCS which have cells on the order of 1km radius.

37. What is Okumura model?

Okumura model is one of the most widely used models for signal prediction in urban areas.

This model is applicable for frequencies in the range of 150MKz to 1920MHz and

distances of 1km to 100km.

38. Define primary ray tracing.

The method of drawing a single ray between the transmitter and receiver is said to be primary ray tracing.

39. Define process gain.

The processing gain represents the gain achieved by processing a spread spectrum signal over an unspread signal.

Processing gain = BW of spread spectrum signal/BW of unspread signal

PG = (S / N                                                          )out

(S / N )in

40. What do you meant by slide factor?

The slide factor is defined as the ratio between the transmitter chip clock rate and the difference between the transmitter and receiver chip clock rates.

γ = α / (α –β)

Where, α – Transmitter chip clock rate (Hz)

β – Receiver chip clock rate (Hz)

41. Mention the drawbacks of spread spectrum method.

(i)        These measurements are not made in real time.

(ii)       Depending on system parameters and measurement objectives, the time required to make power delay profile measurements may be excessive.

(iii)       Non-coherent detector is used, so that phases of individual multipath components can not be measured.

42. Write the types of trunked system.

           There are two types of trunked systems which are commonly used,

                                (i) Blocked calls cleared and

            (ii)Blocked calls delayed

43. Define trunking efficiency

Trunking efficiency is a measure of the number of users which can be offered a particular GOS with a particular configuration of fixed channels.

44. What are the techniques are used to expand the capacity of cellular systems?

Three techniques are used to expand the capacity of cellular systems

(i)        Cell splitting

(ii)       Sectoring

(iii)      Coverage zone approaches(A Microcell zone concept)

45. Define sector and sectoring.

Sector: The area covered by each directional antenna is called as section.

Sectoring:  The technique for decreasing co-channel interference and thus increasing system performance by using directional antennas is called sectoring.

46. Write the drawbacks of sectoring.

(i)        Each sector is nothing but a new cell with a different shape, because channels have to be partitioned between the different sectors of a cell.

(ii)       The network load is substantially increased because a handoff has to be made each time a mobile terminal moves from one sector of a cell to another.

47.  Write the advantages of micro cell concept.

(i)        The co-channel interference in the cellular system is reduced since a large central base station is replaced by several lower powered transmitters (zone transmitters) on the edge of the cell.

(ii)       Decreased co-channel interference improves the signal quality.

(ii)               It  will  increase  the  channel  capacity  without  the  degradation  in  trunking efficiency caused by sectoring.

PART B

1. Discuss briefly about frequency domain coding of speech.

2. Discuss briefly about vocoders.

3. Discuss briefly about the types of equalization techniques, and explain any two.

4. Draw the block diagram of LPC coding system and explain the different types of LPC used for wireless systems?

5. Explain LMS algorithm for adaptive equalization, in detail.

6. Explain RLS algorithm for adaptive equalization, in detail.

7. Explain in detail about selection of speech codecs for mobile communication.

8. With neat block diagram explain in detail about GSM codec.

9. Explain in detail about USDC codec.

10. Explain about ADPCM

11. Differentaite between waveform coders and vocoders in detail

12. Explain about quantization techniques