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DSP C5000

DSP C5000. Chapter 20 Polyphase FIR Filter Implementation for Communication Systems. Multirate Processing 1 of 2. Multirate processing arises in many fields of digital signal processing: Digital audio: sampling frequency conversion (32 kHz, 44.1kHz, 48kHz), sharp cut-off of FIR filter, …

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DSP C5000

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  1. DSP C5000 Chapter 20 Polyphase FIR Filter Implementation for Communication Systems

  2. Multirate Processing 1 of 2 • Multirate processing arises in many fields of digital signal processing: • Digital audio: sampling frequency conversion (32 kHz, 44.1kHz, 48kHz), sharp cut-off of FIR filter, … • Signal processing for digital communications: symbol rate processing, bit rate processing, sample rate processing, … • Speech processing: 3G speech codec (Adaptive Multi Rate), fractionnal pitch estimation, ... • …

  3. Fe Fe/M M Fe LFe L Multirate Processing 2 of 2 • Involves two actions on the digital signal: • Downsampling: resampling downwards the digital signal in the digital domain. • Upsampling: resampling upwards the digital signal in the digital domain. Retain one sample over M and discard the M-1 others, every M samples. Insert L-1 zeros between each sample

  4. x(n) y(m) M Folding term Downsampling 1 of 2

  5. fc : (Fe/M)/2 x(n) y(m) H(z) M Fe Fe/M H(z) H(zM) M M Downsampling 2 of 2 • Anti-aliasing Filter • Noble identity for decimation

  6. Upsampling 1 of 2 x(m) y(n) L

  7. fC : (Fe/L)/2 x(m) y(n) H(z) Fe LFe L L L H(z) H(zL) Upsampling 2 of 2 • Interpolating Filter • Noble identity for upsampling

  8. H(z) E(zM) M M Polyphase Implementation of FIR FiltersDecimation Case 1 of 4 Let n=lM+k with

  9. Processing load (MAC/s) E1(zM) E0(zM) N z-1 z-1 EM-1(zM) M MTe Time Fe Fe/M H(z) M Polyphase Implementation of FIR FiltersDecimation Case 2 of 4 • M-1 filter evaluation over M • are discarded. • N filter length

  10. Processing load (MAC/s) M E0(z) N z-1 M E1(z) z-1 M EM-1(z) MTe Time Fe Fe/M Polyphase Implementation of FIR FiltersDecimation Case 3 of 4 • Using noble identity • No more useless computations, but one sampling period over M, CPU is burdned with N MAC/s.

  11. Polyphase Implementation of FIR FiltersDecimation Case 4 of 4 • Equivalent commutator model Processing load (MAC/s) E0(z) E1(z) N/M EM-1(z) MTe Time Fe Fe/M • Commutator runs at Fe,. At each input sample only one component is computed and accu- • mulated with the result of the previous one. The result is output when the last component • is reached and accumulator is reset. This spreads the processing load over MTe.

  12. R(zL) H(z) L L Polyphase Implementation of FIR FiltersInterpolation Case 1 of 5 Let n=mL+L-1-k with

  13. R1(zL) H(z) R0(zL) z-1 L L z-1 RM-1(zL) Polyphase Implementation of FIR FiltersInterpolation Case 2 of 5 Processing load (MAC/s) N Te/L Time • L-1 multiplications by 0 over L • For each filter evaluation. • N filter length. Fe LFe

  14. R0(z) z-1 L L L R1(z) z-1 RM-1(z) Polyphase Implementation of FIR FiltersInterpolation Case 3 of 5 • Using noble identity • At each output sampling instant, • only one component is non zero Fe LFe

  15. R0(z) R1(z) RM-1(z) Polyphase Implementation of FIR FiltersInterpolation Case 4 of 5 • Equivalent commutator model Processing load (MAC/s) N/L Te/L Time Fe LFe • For each output sampling instant one polyphase component is computed. • When we reach again the first component (M-1) a new input sample is inputed • in the delay line of each polyphase component.

  16. h0 z-1 z-1 z-1 z-1 h1 hL-1 h0 h2L hL h2L hL hL+1 h2L+1 h2L-1 h3L-1 Polyphase Implementation of FIR FiltersInterpolation Case 5 of 5 • Linear Periodically Varying Time system z-1 z-1 hL-1 h3L-1 h2L-1 z-1 z-1 h1 h2L+1 hL+1

  17. Case Study • Shaping filters for a QPSK modem : • Emitter: interpolation case. • Receiver: decimation case • Efficient Algorithm Implementation : • Good ordering of computations, • Efficient memory organization and management.

  18. Emitter 1 of 4 s(t)=1/2[cos(2pfot).cos(f(Ak,Bk))-sin(2pfot).sin(f(Ak,Bk))] QPSK modulator Cos() RCF DAC Ak fk fk: Phase computation bits Bk Sin() RCF DAC Fb Fs Fe Bit frequency Symbol frequency Sample frequency

  19. H(z) 16 Emitter 2 of 4 • Let Fe=16Fs (16 sample / symbol) • Define a raised cosine filter with: • 6 symbols length. • Roll_off : 0.5 • Matlab command • h=RCOSFIR(0.5,3,16,1); Equivalent system • In red: ideal interpolating filter • In blue: actual RC filter

  20. Emitter 3 of 4 • The 16 Polyphase filters are defined by : Filter length is 97, impulse response is padded with 0 to reach 112=7*16 With N=112 and L=16

  21. Emitter 4 of 4 Coefficients Symbols Shuffle coefficients 1st sample R=flipud(reshape(h,8,12)); R=round(R*2^15); fid=fopen('coef.inc','wt'); for p=1:8 fprintf(fid,'\t.word\t%d\n,R(p,:)) end fclose(fid); 2nd sample 15th sample When coefficient pointer reaches this address a new symbol will be input at the next output sample period

  22. Emitter (C callable) .sect "coefs2" Ncomp .set 16 ;number of polyphase component coefs2 .include "coefpoly2.inc" coefsfin: coefsize .set coefsfin-coefs2 Lfil .set coefsize/Ncomp ;polyphase component length filbufQ .usect "filtre2",Lfil ;data buffer .text _firinit: ST #coefs2,*(adbufQ) ;pointer to current coefs pointer STM #filbufQ,AR2 ;zeroed initial buffer condition RPT #Lfil-1 STL A,*AR2+ RET

  23. _firTxQ:… ;context save LD #var,DP STM #coefsize,BK MVDM adbufQ,AR2 ;current coefs pointer STM #1,AR0 STM #filbufQ,AR3 ;symbol buffer STL A,*AR3 ;new sample (guess hold during 16 samples) RPTZ A,#Lfil-1 ;compute one polyphase component MAC *AR2+0%,*AR3+,A MVMD AR2,adbufQ ;save new current coefs pointer SFTA A,-16 SFTA A,-1 ;output of RCF can be greater than 1 ! CMPM @adbufQ,#coefs2 ;test if delay symbols is needed BC endTxQ,NTC ;jump if not necessary MAR *+AR3(-2) RPT #Lfil-2 DELAY *AR3- endTxQ: … ;context restore RET

  24. Symbol vs Sample Output Symbol output Sample output f= Fs/8=125 Hz Fe: 16 khz Fs: 1 khz Df : p/4 constant for each symbol

  25. Receiver 1 of 2 Bit processing Symbol processing ADC RCF ADC RCF Fe Fb Fs

  26. Receiver 2 of 2 • Receiver structure is quite similar, except that: • Each polyphase component has its own delay tap • Each polyphase output has to be accumulated for M polyphase computations and accumulator is outputed every M input sample and reset. E0(z) E1(z) EM-1(z)

  27. Follow on Activities • Laboratory 10 for the TMS320C5416 DSK • Illustrates the effects of decimation and anti-aliasing filters. • Laboratory 11 for the TMS320C5416 DSK • Illustrates the effects of interpolation and anti-imaging filters. • Application 9 for the TMS320C5416 DSK • Uses interpolation and decimation to produce sharper cut-offs FIRs than would be obtained otherwise.

  28. Reference • Digital Signal Processing a Practical Approach by Emmanuel C. Ifeachor and Barrie W. Jervis. Chapter 9. Multirate digital signal processing.

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