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1993-09-16
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Subject: AT&T TrueVoice DSP Analysis
Date: Fri, 3 Sep 93 13:39:17 -0700
From: malcolm@apple.com
I've just finished an analysis of the AT&T TrueVoice demo that has
been advertised by AT&T and discussed in the TELECOM Digest. Here are
the results of the analysis.
First, there is a reason that the TrueVoice version sounds louder.
Above 300Hz there is a pretty uniform gain of 4dB. Between 100-300 Hz
there is a 14 dB gain peak (with a notch at 180 Hz to kill the third
harmonic of the power line hum.) Here are the details:
0-100 Hz Not much change.
100-300Hz A broad gain bump of about 14dB
180Hz Deep 12.5dB notch (back to 1.5 dB)
300-4kHz 4dB extra gain with a .5dB ripple
Method:
The AT&T TrueVoice demo (1 800 932-2000) was digitized with the
Telecom pod on the Macintosh Quadra 840AV (using the AT&T 3210 DSP
chip). Samples were converted with 16 bits of linear resolution,
sampled at 8kHz, and stored in an AIFF file. All data was collected
from a single phone call using an analog phone line.
The results were loaded into Matlab on a Macintosh for analysis. The
TrueVoice demo starts with 30 seconds of normal speech and then
automatically switches on the TrueVoice modification and replays the
same speech. The beginning of each demo sample was aligned by eye,
and then a cross correlation was used to refine the relative starting
positions. All further analysis was done on these aligned samples of
modified and unmodified speech.
Note, the TrueVoice modification is turned on over a second or two.
(I don't know why AT&T does this. Perhaps they didn't want the change
to seem too jarring.) The cross correlation analysis was done using
these first couple of seconds: this should make the correlation more
accurate. The spectral analysis that followed skipped the first three
seconds.
A periodogram analysis was performed using a window of data of length
2048 and skipping 16 samples between windows. A Hamming window was
used before FFT'ing and the power spectral density was computed. The
power spectral density for all windows was summed (averaged) to arrive
at a robust estimate of the power spectral density of the entire
sample. A total of 170,000 data samples was used from each example.
The power spectral density for each example (unmodified and modified
speech) was converted to dB. The two PSDs are then subtracted to
estimate the difference in spectral energy due to the TrueVoice
filtering.
Results:
The broad filter characteristics are shown below.
0-100 Hz Not much change.
100-300Hz A broad gain of 13.7 dB, 3dB Q is 1.5
120Hz About a 1dB dip in the gain
180Hz Narrow band reject down to 1.5 dB, 3dB Q is 18
300-4kHz 4dB extra gain with a .5dB ripple
Note, there is a deep notch at the 3rd harmonic of the power line
frequency and a extra small dip at the second harmonic. I don't have
enough information to tell if they are doing anything different at
60Hz.
The filtering appears to be stationary. The demo over the phone turns
the filtering on gradually, but after a second there appears to be no
change in the filter's characteristics. The analysis described above
was performed over several subsections of the data and there was no
visible change in the filter's characteristics.
Acknowledgements:
Many thanks to Stuart Davidson for setting up the hardware and
software to digitize the demo. Thanks also to Michele Covell
(Interval Research) for help with the filter analysis. The
periodogram is discussed in Larry Marple's book "Digital Spectral
Analysis with Applications."
Disclaimer:
I have no affiliation with AT&T except as a mostly satisfied customer.
This work is not an official Apple project. TrueVoice and AT&T are
undoubtably trademarks of AT&T.
Malcolm Slaney malcolm@apple.com
=================== Matlab Periodogram Code ============================
% function y=periodogram(x,width,skip)
% Compute the periodogram (an estimate of the power spectral
% density) of an array. The width of each analysis defaults to
% 256 points, and the default skip between windows is half the
% window size.
function y=periodogram(x,width,skip)
if (nargin < 2) width = 256; end
if (nargin < 3) skip = width/2; end
nwinds = fix((length(x)-width)/skip);
x = reshape(x,length(x),1);
wind = hamming(width);
y = zeros(width,1);
for i=1:nwinds
firstPoint = (i-1)*skip + 1;
d = [x(firstPoint:firstPoint+width-1).*wind;zeros(width,1)];
f = fft(d);
f = f(1:width);
y = y + f .* conj(f);
end
y = y/nwinds;
=============== Matlab TrueVoice Analysis Code =========================
% Analyze a recorded sample of the AT&T TrueVoice demo.
% This script assumes that the entire demo is stored in
% an array called tv. The start of the first and second
% version of the demo are indicated by the first and second
% pointers. (These will be different for anybody else's
% recording of the phone call.)
first=116000;
second=395500+41;
firstEnd = 355000;
secondEnd = 610000;
sampleRate = 8000;
%segment = 170000;
segment = 32768;
offset = 30000;
if (second + offset + segment > secondEnd)
disp('Segment is past end of second example.');
return;
end
s1=tv(first+offset:first+offset+segment-1);
s2=tv(second+offset:second+offset+segment-1);
% Optionally compute the optimum cross correlation.
% If the alignment is done right, then the maximum
% value of the cross correlation should be in the
% middle of the cross correlation or sample number
% equal to segment size .
% cor = xcorr(s1,s2);
% plot(cor)
% [m,i] = max(cor);
% i
r1 = 10*log10(periodogram(s1, 512, 128));
r2 = 10*log10(periodogram(s2, 512, 128));
f = (0:length(r1)-1)/length(r1)*sampleRate/2;
=============== Spectral Analysis of TrueVoice =========================
% Spectral analysis (in dB) of the AT&T TrueVoice demo.
% The results are shown below at 1.9531Hz intervals from
% 0Hz to nearly 4kHz.
-0.0156 -0.0156 -0.0154 -0.0138
2.8762 1.5637 1.3204 0.2571
0.6921 0.9584 0.7187 -0.7438
-1.7550 -2.0471 -2.0754 -1.9768
-1.6302 -0.7998 0.7231 0.9736
0.9899 0.9882 0.9827 0.9418
0.2723 -0.3172 -0.4245 0.0216
-0.1162 -0.1018 -0.0875 -0.0747
-0.0601 -0.0360 0.1168 0.9137
2.4710 2.5066 3.5501 2.9577
3.3023 3.0256 3.8413 4.5084
6.3061 6.4052 7.6116 7.6745
8.5071 8.4967 9.0233 9.0818
9.5900 9.7797 10.1627 10.3327
10.6086 10.7367 10.9011 10.8782
10.5145 10.0140 10.3593 11.2615
11.8157 11.9453 12.0295 12.0388
12.0603 12.1355 12.3434 12.4985
12.6426 12.7032 12.8563 12.9541
13.0520 13.0989 13.2080 13.2716
13.3565 13.3807 13.4805 13.5283
13.5936 13.6157 13.6703 13.6777
13.6661 12.1029 5.5750 2.3909
1.6613 2.2414 4.9604 11.2490
13.7420 13.7310 13.7230 13.6977
13.6595 13.5861 13.5226 13.4833
13.4672 13.4355 13.3998 13.3534
13.3033 13.2455 13.1833 13.1151
13.0436 12.9571 12.8699 12.7882
12.6880 12.5852 12.5082 12.4377
12.3462 12.2276 12.1066 11.9765
11.8630 11.7580 11.6211 11.4772
11.3559 11.2418 11.0955 10.9580
10.8388 10.6905 10.5217 10.3533
10.2052 10.0873 9.9788 9.8533
9.7000 9.5401 9.3863 9.2267
9.0551 8.8746 8.7166 8.5856
8.4393 8.2574 8.0735 7.8216
7.2432 6.4982 6.2854 6.7300
7.0660 7.0194 6.8627 6.6837
6.4944 6.3324 6.1924 6.0580
5.9400 5.8304 5.6939 5.5120
5.3422 5.2091 5.0764 4.9489
4.8327 4.7075 4.5998 4.4920
4.3769 4.2938 4.2189 4.1290
4.0337 3.9499 3.8720 3.7967
3.7345 3.6756 3.6097 3.5443
3.4910 3.4470 3.4026 3.3690
3.3538 3.3306 3.2824 3.2459
3.2375 3.2455 3.2562 3.2654
3.2769 3.2867 3.2941 3.3026
3.3110 3.3201 3.3315 3.3440
3.3697 3.4085 3.4461 3.4825
3.5228 3.5549 3.5623 3.5717
3.6202 3.6865 3.7361 3.7728
3.7982 3.8225 3.8617 3.8934
3.9120 3.9411 3.9805 4.0103
4.0313 4.0423 4.0499 4.0719
4.1125 4.1634 4.1984 4.2129
4.2306 4.2533 4.2706 4.2897
4.3161 4.3271 4.3166 4.3097
4.3212 4.3222 4.3041 4.3011
4.3121 4.3238 4.3343 4.3339
4.3267 4.3170 4.2965 4.2704
4.2473 4.2342 4.2294 4.2322
4.2381 4.2338 4.2211 4.2096
4.2020 4.1905 4.1708 4.1571
4.1513 4.1370 4.1146 4.0957
4.0799 4.0686 4.0589 4.0159
3.9430 3.9157 3.9485 3.9754
3.9758 3.9720 3.9625 3.9376
3.9162 3.9101 3.9068 3.9034
3.9000 3.8920 3.8796 3.8663
3.8585 3.8588 3.8562 3.8374
3.8205 3.8194 3.8240 3.8239
3.8113 3.8002 3.8025 3.8091
3.8143 3.8212 3.8278 3.8367
3.8441 3.8418 3.8338 3.8264
3.8311 3.8473 3.8638 3.8787
3.8898 3.8965 3.8994 3.8999
3.9121 3.9445 3.9715 3.9803
3.9848 3.9975 4.0300 4.0597
4.0501 4.0212 4.0095 4.0264
4.0668 4.1108 4.1344 4.1324
4.1079 4.0761 4.0591 4.0615
4.0805 4.1049 4.1293 4.1566
4.1745 4.1746 4.1766 4.1942
4.2163 4.2459 4.2697 4.2636
4.2516 4.2672 4.2902 4.2960
4.2897 4.2854 4.2939 4.3081
4.3115 4.3180 4.3467 4.3717
4.3674 4.3452 4.3170 4.2937
4.2886 4.2898 4.2866 4.2869
4.2744 4.2453 4.2252 4.2041
4.1907 4.1994 4.2001 4.1973
4.2036 4.2054 4.1940 4.1572
4.1137 4.1197 4.1507 4.1396
4.1036 4.0570 4.0095 3.9778
3.9774 3.9987 4.0189 4.0231
4.0033 3.9736 3.9444 3.9261
3.9289 3.9371 3.9147 3.8360
3.7424 3.7082 3.7061 3.6903
3.6718 3.6725 3.6802 3.6756
3.6680 3.6843 3.7143 3.7304
3.7240 3.7022 3.6766 3.6636
3.6838 3.7364 3.7735 3.7715
3.7745 3.8092 3.8338 3.8216
3.8117 3.8196 3.8251 3.8316
3.8483 3.8663 3.8788 3.8834
3.8788 3.8763 3.8858 3.8962
3.8959 3.8940 3.9102 3.9491
3.9801 3.9850 4.0079 4.0531
4.0789 4.0709 4.0647 4.0794
4.0913 4.0958 4.1114 4.1341
4.1524 4.1771 4.1970 4.2059
4.2127 4.2154 4.2141 4.2150
4.2213 4.2226 4.2111 4.1941
4.1933 4.2252 4.2576 4.2572
4.2150 4.1531 4.1139 4.1130
4.1501 4.1874 4.1887 4.1605
4.1311 4.1236 4.1473 4.1852
4.2052 4.2233 4.2426 4.2489
4.2538 4.2531 4.2337 4.2260
4.2384 4.2403 4.2367 4.2453
4.2668 4.2789 4.2627 4.2359
4.2013 4.1635 4.1479 4.1810
4.2237 4.2029 4.1435 4.1030
4.0890 4.0912 4.1109 4.1343
4.1405 4.1113 4.0482 3.9993
3.9791 3.9588 3.9272 3.8873
3.8515 3.8288 3.8098 3.8065
3.8195 3.8113 3.7923 3.8059
3.8273 3.8364 3.8258 3.7813
3.7552 3.7867 3.8330 3.8302
3.8012 3.7939 3.7789 3.7384
3.7168 3.7019 3.6732 3.6598
3.6861 3.7230 3.7378 3.7430
3.7643 3.7819 3.7818 3.7938
3.7803 3.7341 3.7190 3.7303
3.7457 3.7783 3.8146 3.8645
3.9324 3.9773 4.0000 4.0181
3.9884 3.9119 3.8909 3.9387
4.0071 4.0808 4.1228 4.1229
4.1029 4.0704 4.0309 4.0110
4.0390 4.0767 4.0965 4.1365
4.1915 4.2151 4.2345 4.2727
4.2879 4.2576 4.2202 4.2258
4.2612 4.2724 4.2555 4.2427
4.2471 4.2661 4.2896 4.3010
4.3005 4.2957 4.2855 4.2740
4.2666 4.2695 4.2819 4.2873
4.2675 4.2188 4.1749 4.1607
4.1578 4.1489 4.1283 4.1087
4.1226 4.1646 4.1975 4.2025
4.1791 4.1383 4.1318 4.1720
4.1981 4.1785 4.1589 4.1588
4.1699 4.1828 4.1738 4.1261
4.0842 4.0948 4.1499 4.1747
4.1536 4.1222 4.0861 4.0489
4.0195 4.0093 4.0231 4.0133
3.9700 3.9524 3.9736 4.0181
4.0664 4.1272 4.1630 4.1220
4.0943 4.0931 4.0471 3.9736
3.9507 3.9191 3.8515 3.8284
3.8700 3.9406 3.9809 3.9753
3.9601 3.9425 3.9219 3.9120
3.8903 3.8508 3.8170 3.7936
3.7934 3.8167 3.8262 3.8181
3.8137 3.7817 3.7410 3.7353
3.7615 3.8004 3.8142 3.8099
3.8200 3.8412 3.8395 3.8060
3.7680 3.7490 3.7546 3.7850
3.8190 3.8374 3.8329 3.8101
3.7999 3.8239 3.8626 3.8995
3.9293 3.9370 3.9439 3.9866
4.0243 4.0340 4.0458 4.0547
4.0425 4.0240 4.0253 4.0536
4.0953 4.1350 4.1705 4.1965
4.2225 4.2664 4.2941 4.2819
4.2690 4.2711 4.2839 4.2996
4.3138 4.3118 4.2843 4.2663
4.2734 4.2854 4.2986 4.3212
4.3424 4.3508 4.3505 4.3336
4.3145 4.3226 4.3632 4.4118
4.4020 4.3286 4.2562 4.2137
4.2224 4.2605 4.2589 4.2231
4.2219 4.2602 4.2963 4.2905
4.2564 4.2230 4.2089 4.2149
4.2181 4.2004 4.1570 4.0962
4.0639 4.0803 4.0981 4.1046
4.0952 4.0567 4.0047 3.9813
3.9860 3.9971 4.0172 4.0371
4.0283 4.0047 3.9873 3.9645
3.9413 3.9515 3.9824 4.0022
3.9996 3.9690 3.9208 3.8792
3.8483 3.8223 3.8188 3.8511
3.8860 3.8773 3.8441 3.8372
3.8458 3.8453 3.8374 3.8258
3.7941 3.7560 3.7504 3.7584
3.7720 3.8059 3.8312 3.8020
3.7679 3.7848 3.8008 3.7833
3.7682 3.7858 3.8391 3.8853
3.8705 3.8409 3.8636 3.8996
3.8944 3.8579 3.8566 3.9188
4.0062 4.0474 4.0027 3.9218
3.8811 3.9004 3.9594 4.0117
4.0281 4.0135 3.9890 3.9866
4.0231 4.0668 4.0711 4.0718
4.0907 4.1074 4.1136 4.1138
4.1124 4.1224 4.1473 4.1677
4.1812 4.1931 4.2098 4.2496
4.2805 4.2600 4.2349 4.2179
4.1955 4.1807 4.1928 4.2267
4.2628 4.2767 4.2725 4.2779
4.2896 4.3012 4.3192 4.3254
4.3072 4.2831 4.2739 4.2812
4.2903 4.2892 4.2842 4.2814
4.2716 4.2685 4.2888 4.3127
4.3113 4.2756 4.2453 4.2532
4.2708 4.2649 4.2519 4.2465
4.2432 4.2347 4.2358 4.2439
4.2367 4.2059 4.1670 4.1390
4.1169 4.0873 4.0583 4.0420
4.0230 3.9962 3.9904 3.9978
3.9931 3.9969 3.9990 3.9734
3.9391 3.9346 3.9351 3.9215
3.9048 3.8817 3.8531 3.8234
3.7945 3.7757 3.7681 3.7504
3.7561 3.8093 3.8333 3.8044
3.7842 3.7740 3.7390 3.6953
3.7004 3.7884 3.8750 3.8778
3.8252 3.7715 3.7343 3.7136
3.7297 3.7714 3.7938 3.7986
3.8132 3.8214 3.8113 3.8082
3.8193 3.8296 3.8395 3.8442
3.8188 3.8019 3.8431 3.8958
3.9200 3.9416 3.9829 4.0340
4.0593 4.0356 3.9976 3.9609
3.9166 3.8948 3.9234 3.9567
3.9652 3.9733 3.9786 3.9787
4.0049 4.0699 4.1040 4.0790
4.0643 4.0772 4.0804 4.0748
4.0894 4.1372 4.1957 4.2248
4.2129 4.1991 4.2083 4.2033
4.1846 4.1891 4.2261 4.2594
4.2756 4.2879 4.2997 4.2884
4.2581 4.2359 4.2251 4.2259
4.2409 4.2656 4.2852 4.2798
4.2515 4.2234 4.2279 4.2725
4.3213 4.3505 4.3574 4.3353
4.2998 4.2797 4.2735 4.2693
4.2616 4.2305 4.1807 4.1626
4.1935 4.2462 4.2942 4.2951
4.2192 4.1520 4.1463 4.1483
4.1176 4.0722 4.0506 4.0581
4.0586 4.0511 4.0593 4.0653
4.0676 4.0712 4.0433 3.9804
3.9430 3.9524 3.9672 3.9626
3.9507 3.9430 3.9276 3.9017
3.8604 3.8038 3.7741 3.7966
3.8357 3.8557 3.8527 3.8440
3.8220 3.7784 3.7385 3.7200
3.7338 3.7643 3.7726 3.7769
3.7994 3.8130 3.8246 3.8414
3.8317 3.8082 3.7971 3.7695
3.7312 3.7098 3.7156 3.7561
3.7995 3.8393 3.8735 3.8685
3.8533 3.8750 3.9050 3.8990
3.8768 3.8811 3.9113 3.9429
3.9672 3.9982 4.0258 4.0165
3.9839 3.9711 3.9829 4.0049
4.0441 4.0756 4.0873 4.1022
4.1212 4.1225 4.0995 4.0704
4.0589 4.0752 4.0984 4.1119
4.1343 4.1586 4.1726 4.1798
4.1820 4.1855 4.2094 4.2572
4.2993 4.3176 4.3228 4.3257
4.3240 4.3044 4.2854 4.2829
4.2876 4.2851 4.2630 4.2451
4.2550 4.2679 4.2826 4.3160
4.3282 4.2968 4.2558 4.2432
4.2685 4.3072 4.3280 4.3404
4.3346 4.2759 4.1998 4.1702
4.2124 4.2732 4.2847 4.2629
4.2425 4.2098 4.1770 4.1763
4.1911 4.1882 4.1759 4.1663
4.1330 4.0891 4.0682 4.0682
4.0624 4.0448 4.0438 4.0718
4.0979 4.0821 4.0266 3.9864
4.0021 4.0353 4.0288 4.0025
3.9863 3.9888 4.0062 4.0005
3.9590 3.9127 3.8824 3.8720
3.8572 3.8300 3.8234 3.8445
3.8487 3.8179 3.7819 3.7594
3.7596 3.7757 3.7787 3.7721
3.7664 3.7472 3.7242 3.7326
3.7628 3.7598 3.7196 3.6824
3.6889 3.7206 3.7392 3.7456
3.7603 3.7863 3.8155 3.8447
3.8480 3.8398 3.8611 3.8849
3.8789 3.8712 3.8924 3.9250
3.9419 3.9403 3.9318 3.9006
3.8682 3.9011 3.9809 4.0246
4.0353 4.0516 4.0705 4.0590
4.0341 4.0256 4.0151 4.0109
4.0392 4.0860 4.1355 4.1624
4.1581 4.1609 4.1803 4.2018
4.2184 4.2250 4.2147 4.1899
4.1642 4.1482 4.1911 4.2695
4.2826 4.2485 4.2136 4.1887
4.2074 4.2838 4.3433 4.3051
4.1978 4.1125 4.1055 4.1904
4.2885 4.3294 4.3390 4.3447
4.3437 4.3337 4.2939 4.2301
4.1919 4.2039 4.2005 4.1584
4.1648 4.2182 4.2373 4.2478
4.2490 4.2036 4.1512 4.1438
4.1641 4.1804 4.1629 4.1034
4.0591 4.0682 4.0891 4.0944
4.1156 4.1410 4.1247 4.0698
4.0288 4.0200 4.0364 4.0512
4.0237 3.9955 4.0102 4.0145
3.9760 3.9662 4.0031 3.9971
3.9255 3.8581 3.8404 3.8401
3.8177 3.7523 3.7374 3.8234
3.8702 3.8596 3.8727 3.9066
3.8972 3.8594 3.8644 3.9048
3.9325 3.9488 3.9425 3.8726
3.7838 3.7260 3.7145 3.7316
3.7265 3.7039 3.7232 3.7572
3.7269 3.6752 3.6567 3.6789
3.7518 3.8376 3.8883 3.9189
3.9443 3.9508 3.9067 3.8554
3.9047 4.0260 4.0644 4.0403
4.0125 3.9892 4.0571 4.1430
4.0919 3.9942 3.9804 4.0280
4.0635 4.1130 4.2038 4.2403
4.2103 4.1852 4.1776 4.1395
4.0567 4.0227 4.1071 4.1994
4.2576 4.3552 4.4440 4.4127
4.3346 4.3393 4.3957 4.3489
4.1848 4.0975 4.0920 4.0744
4.1138 4.2179 4.2339 4.1554
4.1073 4.1208 4.0668 3.9841
4.0039 4.0003 3.9251 3.9485
4.0722 4.1164 4.0952 4.1228
4.2088 4.2388 4.2068 4.2166
4.2125 4.1202 4.0571 4.0532
4.1558 4.3152 4.2978 4.1616
4.1443 4.1086 4.0384 4.0687
4.0723 3.9987 3.9775 3.9429
3.9011 3.8918 3.8856 3.8072
3.6220 3.5458 3.7027 3.8685
3.8723 3.8022 3.7797 3.6861
3.4658 3.3700 3.4747 3.5662
3.6115 3.6012 3.4953 3.5476
3.7015 3.6951 3.5577 3.4871
3.4478 3.3931 3.3936 3.3991
3.3322 3.1924 3.0838 3.1296
3.3243 3.5037 3.5536 3.5758
3.5586 3.4281 3.3716 3.4918
3.5988 3.6349 3.5910 3.5086
3.4555 3.4986 3.6916 3.8286
3.7717 3.6641 3.6092 3.6049
3.6598 3.7989 3.8886 3.6724
3.4683 3.5635 3.7365 3.8570
3.9400 3.9338 3.8813 3.9444
4.1208 4.2367 4.1597 4.0182
3.9569 3.9355 3.9143 3.8876
3.9056 3.9516 3.9832 4.0654
4.1691 4.2439 4.2878 4.2749
4.2742 4.3295 4.3168 4.1415
3.9372 3.9371 4.1325 4.3543
4.4586 4.4060 4.3445 4.3805
4.3726 4.2661 4.1896 4.1046
3.9667 3.8929 4.0597 4.3152
4.3867 4.3182 4.2544 4.2140
4.1667 4.1149 4.0481 4.0083
4.0227 4.0487 4.1225 4.1894
4.1586 4.1040 4.0811 4.0557
4.1132 4.2713 4.3924 4.3247
4.1275 4.0199 4.0101 4.0239
4.0590 4.0638 3.9718 3.8246
3.7107 3.7541 3.9144 3.9891
3.9794 3.8180 3.6486 3.6818
3.7822 3.8763 4.0135 4.1085
4.0391 3.9151 3.8199 3.6551
3.4665 3.4429 3.5590 3.6877
3.7650 3.7755 3.7277 3.6902
3.6983 3.7011 3.6887 3.7134
3.7730 3.7947 3.7823 3.7317
3.6262 3.5784 3.5698 3.4989
3.5104 3.6605 3.7955 3.9019
3.9897 4.0021 3.9406 3.8701
3.7931 3.6613 3.5998 3.6305
3.6473 3.7123 3.8283 3.9245
4.0329 4.0829 4.0211 4.0096
4.1040 4.1869 4.2066 4.1136
3.9039 3.7299 3.7298 3.8234
3.8948 3.9680 4.0104 3.9448
3.8663 3.8502 3.8368 3.8798
4.0532 4.2471 4.3475 4.2682
4.1016 4.1576 4.4150 4.5706
4.5509 4.4277 4.3082 4.2466
4.1368 4.0491 4.0839 4.1753
4.2691 4.2812 4.1577 4.0094
3.9138 3.9199 4.0023 4.0903
4.2195 4.3667 4.4259 4.4122
4.4122 4.3466 4.1514 3.9618
3.9475 4.0487 4.1192 4.1147
4.0907 4.1024 4.1024 4.0307
3.9274 3.8137 3.7349 3.7556
3.8267 3.8344 3.8090 3.8208
3.9034 4.0498 4.1513 4.1832
4.2183 4.1704 3.9715 3.7718
3.6723 3.5610 3.4824 3.5552
3.6635 3.6271 3.5437 3.6304
3.7303 3.7012 3.6188 3.5569
3.5710 3.6532 3.7184 3.7485
3.7741 3.7638 3.7374 3.7146
3.6672 3.6052 3.6175 3.7730
3.8550 3.6726 3.4110 3.4656
3.8464 4.1414 4.1006 3.9079
3.8500 3.9691 4.0257 3.8513
3.6397 3.6523 3.8355 3.9043
3.8691 3.9021 3.9751 4.0474
4.0936 4.1213 4.1933 4.3028
4.4050 4.3897 4.2845 4.2412
4.1160 3.9072 3.9179 4.1168
4.3097 4.3819 4.3499 4.2688
4.1972 4.1233 4.0957 4.1848
4.2591 4.2379 4.2120 4.1625
4.0897 4.0789 4.1586 4.2863
4.3210 4.2028 4.0356 3.8960
3.8457 3.9294 4.0042 3.8854
3.7650 3.7648 3.7415 3.5443
3.3887 3.3969 3.3726 3.2292
3.0580 2.9826 2.9791 2.9773
2.9272 2.7399 2.5032 2.3386
2.2621 2.1826 2.0198 1.7527
1.3994 1.0421 0.8313 0.8379
1.0071 1.1620 1.1241 0.8451
0.5447 0.3268 0.0256 -0.3426
-0.5481 -0.5391 -0.4992 -0.6288
-0.8772 -1.0927 -1.3706 -1.9960
-2.6370 -2.8954 -3.0923 -3.2882
-3.3825 -3.5081 -3.5893 -3.6076
-3.7203 -3.8884 -3.9815 -4.0522
-4.0332 -3.9273 -4.2025 -4.8333
-5.4544 -5.7822 -5.4846 -4.8133
-4.6546 -5.0683 -5.6868 -5.9887
-5.6421 -5.0674 -4.9115 -4.9862
-5.0753 -5.4126 -5.7638 -5.3747
-4.3985 -3.9502 -4.3882 -4.6642
-4.0590 -3.2857 -3.0553 -3.0171
-2.9334 -2.6387 -2.3332 -2.4326
-2.7259 -2.8058 -2.7649 -2.7696
-2.8219 -2.9168 -3.0049 -2.9978
-2.8766 -2.7459 -2.7877 -2.7952
-2.6307 -2.4433 -2.6332 -3.1869
-3.2425 -2.2819 -1.0299 -0.2065
-0.0031 -0.2092 -0.4339 -0.9713
-1.9048 -2.3555 -2.1326 -2.0028
-2.2714 -2.4559 -2.1483 -1.5499
-1.1712 -1.4571 -2.2413 -2.4907
-2.1361 -1.7915 -1.7738 -1.9718
-2.1032 -1.8157 -1.2609 -1.0888
-1.4205 -1.6319 -1.4174 -1.0870
-0.6456 0.0054 0.4237 0.3262
0.0030 -0.1614 0.0100 0.1762
0.0037 -0.4210 -0.8006 -1.0810
-1.2003 -1.0901 -0.9077 -0.6167
-0.0365 0.6056 0.6823 0.2631
0.0315 0.1720 0.5950 0.9670
1.2035 1.0868 0.7313 0.2079
-0.4428 -1.0584 -1.3108 -0.8113
-0.1252 -0.0700 -0.4471 -0.4977
-0.0009 0.4090 0.4309 0.3372
0.3134 0.3018 0.2507 -0.0140
-0.2245 -0.2516 -0.0333 0.2052
0.3167 0.3388 0.2979 0.2408
0.2438 0.1288 0.0355 -0.0726
0.1884 0.5430 0.5037 0.3928
0.6536 0.9805 1.0402 0.8571
0.8218 0.8197 0.7281 0.5522
0.6925 1.0045 1.0707 0.6734
0.6085 1.0526 1.2845 1.1089
1.2953 1.3806 1.1912 0.9422
0.8630 0.9159 1.4254 1.8088
1.7759 1.2884 0.9880 0.8059
0.6631 0.3597 0.2877 0.5101
0.9324 1.1062 1.1189 1.0873
1.2963 1.4214 1.6282 1.7053
1.4973 0.9790 1.0983 1.3852
1.5429 1.5234 1.5292 1.3104
1.2757 0.8869 0.5145 0.7113
1.3543 1.3890 0.9667 0.7196
1.4537 2.0366 2.1264 1.8825
1.6793 1.3166 1.3493 1.3008
1.2872 1.2882 1.6478 1.8070
2.1904 2.2762 2.0932 1.6617
1.7195 2.0066 2.6213 2.6506
2.5683 2.3499 2.5807 2.5199
2.4837 1.9532 1.7694 1.8284
2.2359 2.1669 2.0885 1.9388
2.2269 2.5849 3.0879 2.8728
3.2147 3.0896 2.6566 1.8906
1.8568 1.8620 1.9197 0.5811
0.0707 -0.1140 -0.2507 -0.3872
-0.5261 -0.3324 0.6124 0.6609
0.7726 0.7911 0.8297 0.4739
0.2283 0.2148 0.4259 0.5007
-------------------------------
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Subject: AT&T TrueVoice DSP Analysis
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Subject: AT&T TrueVoice DSP Analysis
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A special report for the readers, submitted by malcolm@apple.com.
I hope you find it useful and interesting.
PAT
Subject: AT&T TrueVoice DSP Analysis
Date: Fri, 3 Sep 93 13:39:17 -0700
From: malcolm@apple.com
I've just finished an analysis of the AT&T TrueVoice demo that has
been advertised by AT&T and discussed in the TELECOM Digest. Here are
the results of the analysis.
First, there is a reason that the TrueVoice version sounds louder.
Above 300Hz there is a pretty uniform gain of 4dB. Between 100-300 Hz
there is a 14 dB gain peak (with a notch at 180 Hz to kill the third
harmonic of the power line hum.) Here are the details:
0-100 Hz Not much change.
100-300Hz A broad gain bump of about 14dB
180Hz Deep 12.5dB notch (back to 1.5 dB)
300-4kHz 4dB extra gain with a .5dB ripple
Method:
The AT&T TrueVoice demo (1 800 932-2000) was digitized with the
Telecom pod on the Macintosh Quadra 840AV (using the AT&T 3210 DSP
chip). Samples were converted with 16 bits of linear resolution,
sampled at 8kHz, and stored in an AIFF file. All data was collected
from a single phone call using an analog phone line.
The results were loaded into Matlab on a Macintosh for analysis. The
TrueVoice demo starts with 30 seconds of normal speech and then
automatically switches on the TrueVoice modification and replays the
same speech. The beginning of each demo sample was aligned by eye,
and then a cross correlation was used to refine the relative starting
positions. All further analysis was done on these aligned samples of
modified and unmodified speech.
Note, the TrueVoice modification is turned on over a second or two.
(I don't know why AT&T does this. Perhaps they didn't want the change
to seem too jarring.) The cross correlation analysis was done using
these first couple of seconds: this should make the correlation more
accurate. The spectral analysis that followed skipped the first three
seconds.
A periodogram analysis was performed using a window of data of length
2048 and skipping 16 samples between windows. A Hamming window was
used before FFT'ing and the power spectral density was computed. The
power spectral density for all windows was summed (averaged) to arrive
at a robust estimate of the power spectral density of the entire
sample. A total of 170,000 data samples was used from each example.
The power spectral density for each example (unmodified and modified
speech) was converted to dB. The two PSDs are then subtracted to
estimate the difference in spectral energy due to the TrueVoice
filtering.
Results:
The broad filter characteristics are shown below.
0-100 Hz Not much change.
100-300Hz A broad gain of 13.7 dB, 3dB Q is 1.5
120Hz About a 1dB dip in the gain
180Hz Narrow band reject down to 1.5 dB, 3dB Q is 18
300-4kHz 4dB extra gain with a .5dB ripple
Note, there is a deep notch at the 3rd harmonic of the power line
frequency and a extra small dip at the second harmonic. I don't have
enough information to tell if they are doing anything different at
60Hz.
The filtering appears to be stationary. The demo over the phone turns
the filtering on gradually, but after a second there appears to be no
change in the filter's characteristics. The analysis described above
was performed over several subsections of the data and there was no
visible change in the filter's characteristics.
Acknowledgements:
Many thanks to Stuart Davidson for setting up the hardware and
software to digitize the demo. Thanks also to Michele Covell
(Interval Research) for help with the filter analysis. The
periodogram is discussed in Larry Marple's book "Digital Spectral
Analysis with Applications."
Disclaimer:
I have no affiliation with AT&T except as a mostly satisfied customer.
This work is not an official Apple project. TrueVoice and AT&T are
undoubtably trademarks of AT&T.
Malcolm Slaney malcolm@apple.com
=================== Matlab Periodogram Code ============================
% function y=periodogram(x,width,skip)
% Compute the periodogram (an estimate of the power spectral
% density) of an array. The width of each analysis defaults to
% 256 points, and the default skip between windows is half the
% window size.
function y=periodogram(x,width,skip)
if (nargin < 2) width = 256; end
if (nargin < 3) skip = width/2; end
nwinds = fix((length(x)-width)/skip);
x = reshape(x,length(x),1);
wind = hamming(width);
y = zeros(width,1);
for i=1:nwinds
firstPoint = (i-1)*skip + 1;
d = [x(firstPoint:firstPoint+width-1).*wind;zeros(width,1)];
f = fft(d);
f = f(1:width);
y = y + f .* conj(f);
end
y = y/nwinds;
=============== Matlab TrueVoice Analysis Code =========================
% Analyze a recorded sample of the AT&T TrueVoice demo.
% This script assumes that the entire demo is stored in
% an array called tv. The start of the first and second
% version of the demo are indicated by the first and second
% pointers. (These will be different for anybody else's
% recording of the phone call.)
first=116000;
second=395500+41;
firstEnd = 355000;
secondEnd = 610000;
sampleRate = 8000;
%segment = 170000;
segment = 32768;
offset = 30000;
if (second + offset + segment > secondEnd)
disp('Segment is past end of second example.');
return;
end
s1=tv(first+offset:first+offset+segment-1);
s2=tv(second+offset:second+offset+segment-1);
% Optionally compute the optimum cross correlation.
% If the alignment is done right, then the maximum
% value of the cross correlation should be in the
% middle of the cross correlation or sample number
% equal to segment size .
% cor = xcorr(s1,s2);
% plot(cor)
% [m,i] = max(cor);
% i
r1 = 10*log10(periodogram(s1, 512, 128));
r2 = 10*log10(periodogram(s2, 512, 128));
f = (0:length(r1)-1)/length(r1)*sampleRate/2;
=============== Spectral Analysis of TrueVoice =========================
% Spectral analysis (in dB) of the AT&T TrueVoice demo.
% The results are shown below at 1.9531Hz intervals from
% 0Hz to nearly 4kHz.
-0.0156 -0.0156 -0.0154 -0.0138
2.8762 1.5637 1.3204 0.2571
0.6921 0.9584 0.7187 -0.7438
-1.7550 -2.0471 -2.0754 -1.9768
-1.6302 -0.7998 0.7231 0.9736
0.9899 0.9882 0.9827 0.9418
0.2723 -0.3172 -0.4245 0.0216
-0.1162 -0.1018 -0.0875 -0.0747
-0.0601 -0.0360 0.1168 0.9137
2.4710 2.5066 3.5501 2.9577
3.3023 3.0256 3.8413 4.5084
6.3061 6.4052 7.6116 7.6745
8.5071 8.4967 9.0233 9.0818
9.5900 9.7797 10.1627 10.3327
10.6086 10.7367 10.9011 10.8782
10.5145 10.0140 10.3593 11.2615
11.8157 11.9453 12.0295 12.0388
12.0603 12.1355 12.3434 12.4985
12.6426 12.7032 12.8563 12.9541
13.0520 13.0989 13.2080 13.2716
13.3565 13.3807 13.4805 13.5283
13.5936 13.6157 13.6703 13.6777
13.6661 12.1029 5.5750 2.3909
1.6613 2.2414 4.9604 11.2490
13.7420 13.7310 13.7230 13.6977
13.6595 13.5861 13.5226 13.4833
13.4672 13.4355 13.3998 13.3534
13.3033 13.2455 13.1833 13.1151
13.0436 12.9571 12.8699 12.7882
12.6880 12.5852 12.5082 12.4377
12.3462 12.2276 12.1066 11.9765
11.8630 11.7580 11.6211 11.4772
11.3559 11.2418 11.0955 10.9580
10.8388 10.6905 10.5217 10.3533
10.2052 10.0873 9.9788 9.8533
9.7000 9.5401 9.3863 9.2267
9.0551 8.8746 8.7166 8.5856
8.4393 8.2574 8.0735 7.8216
7.2432 6.4982 6.2854 6.7300
7.0660 7.0194 6.8627 6.6837
6.4944 6.3324 6.1924 6.0580
5.9400 5.8304 5.6939 5.5120
5.3422 5.2091 5.0764 4.9489
4.8327 4.7075 4.5998 4.4920
4.3769 4.2938 4.2189 4.1290
4.0337 3.9499 3.8720 3.7967
3.7345 3.6756 3.6097 3.5443
3.4910 3.4470 3.4026 3.3690
3.3538 3.3306 3.2824 3.2459
3.2375 3.2455 3.2562 3.2654
3.2769 3.2867 3.2941 3.3026
3.3110 3.3201 3.3315 3.3440
3.3697 3.4085 3.4461 3.4825
3.5228 3.5549 3.5623 3.5717
3.6202 3.6865 3.7361 3.7728
3.7982 3.8225 3.8617 3.8934
3.9120 3.9411 3.9805 4.0103
4.0313 4.0423 4.0499 4.0719
4.1125 4.1634 4.1984 4.2129
4.2306 4.2533 4.2706 4.2897
4.3161 4.3271 4.3166 4.3097
4.3212 4.3222 4.3041 4.3011
4.3121 4.3238 4.3343 4.3339
4.3267 4.3170 4.2965 4.2704
4.2473 4.2342 4.2294 4.2322
4.2381 4.2338 4.2211 4.2096
4.2020 4.1905 4.1708 4.1571
4.1513 4.1370 4.1146 4.0957
4.0799 4.0686 4.0589 4.0159
3.9430 3.9157 3.9485 3.9754
3.9758 3.9720 3.9625 3.9376
3.9162 3.9101 3.9068 3.9034
3.9000 3.8920 3.8796 3.8663
3.8585 3.8588 3.8562 3.8374
3.8205 3.8194 3.8240 3.8239
3.8113 3.8002 3.8025 3.8091
3.8143 3.8212 3.8278 3.8367
3.8441 3.8418 3.8338 3.8264
3.8311 3.8473 3.8638 3.8787
3.8898 3.8965 3.8994 3.8999
3.9121 3.9445 3.9715 3.9803
3.9848 3.9975 4.0300 4.0597
4.0501 4.0212 4.0095 4.0264
4.0668 4.1108 4.1344 4.1324
4.1079 4.0761 4.0591 4.0615
4.0805 4.1049 4.1293 4.1566
4.1745 4.1746 4.1766 4.1942
4.2163 4.2459 4.2697 4.2636
4.2516 4.2672 4.2902 4.2960
4.2897 4.2854 4.2939 4.3081
4.3115 4.3180 4.3467 4.3717
4.3674 4.3452 4.3170 4.2937
4.2886 4.2898 4.2866 4.2869
4.2744 4.2453 4.2252 4.2041
4.1907 4.1994 4.2001 4.1973
4.2036 4.2054 4.1940 4.1572
4.1137 4.1197 4.1507 4.1396
4.1036 4.0570 4.0095 3.9778
3.9774 3.9987 4.0189 4.0231
4.0033 3.9736 3.9444 3.9261
3.9289 3.9371 3.9147 3.8360
3.7424 3.7082 3.7061 3.6903
3.6718 3.6725 3.6802 3.6756
3.6680 3.6843 3.7143 3.7304
3.7240 3.7022 3.6766 3.6636
3.6838 3.7364 3.7735 3.7715
3.7745 3.8092 3.8338 3.8216
3.8117 3.8196 3.8251 3.8316
3.8483 3.8663 3.8788 3.8834
3.8788 3.8763 3.8858 3.8962
3.8959 3.8940 3.9102 3.9491
3.9801 3.9850 4.0079 4.0531
4.0789 4.0709 4.0647 4.0794
4.0913 4.0958 4.1114 4.1341
4.1524 4.1771 4.1970 4.2059
4.2127 4.2154 4.2141 4.2150
4.2213 4.2226 4.2111 4.1941
4.1933 4.2252 4.2576 4.2572
4.2150 4.1531 4.1139 4.1130
4.1501 4.1874 4.1887 4.1605
4.1311 4.1236 4.1473 4.1852
4.2052 4.2233 4.2426 4.2489
4.2538 4.2531 4.2337 4.2260
4.2384 4.2403 4.2367 4.2453
4.2668 4.2789 4.2627 4.2359
4.2013 4.1635 4.1479 4.1810
4.2237 4.2029 4.1435 4.1030
4.0890 4.0912 4.1109 4.1343
4.1405 4.1113 4.0482 3.9993
3.9791 3.9588 3.9272 3.8873
3.8515 3.8288 3.8098 3.8065
3.8195 3.8113 3.7923 3.8059
3.8273 3.8364 3.8258 3.7813
3.7552 3.7867 3.8330 3.8302
3.8012 3.7939 3.7789 3.7384
3.7168 3.7019 3.6732 3.6598
3.6861 3.7230 3.7378 3.7430
3.7643 3.7819 3.7818 3.7938
3.7803 3.7341 3.7190 3.7303
3.7457 3.7783 3.8146 3.8645
3.9324 3.9773 4.0000 4.0181
3.9884 3.9119 3.8909 3.9387
4.0071 4.0808 4.1228 4.1229
4.1029 4.0704 4.0309 4.0110
4.0390 4.0767 4.0965 4.1365
4.1915 4.2151 4.2345 4.2727
4.2879 4.2576 4.2202 4.2258
4.2612 4.2724 4.2555 4.2427
4.2471 4.2661 4.2896 4.3010
4.3005 4.2957 4.2855 4.2740
4.2666 4.2695 4.2819 4.2873
4.2675 4.2188 4.1749 4.1607
4.1578 4.1489 4.1283 4.1087
4.1226 4.1646 4.1975 4.2025
4.1791 4.1383 4.1318 4.1720
4.1981 4.1785 4.1589 4.1588
4.1699 4.1828 4.1738 4.1261
4.0842 4.0948 4.1499 4.1747
4.1536 4.1222 4.0861 4.0489
4.0195 4.0093 4.0231 4.0133
3.9700 3.9524 3.9736 4.0181
4.0664 4.1272 4.1630 4.1220
4.0943 4.0931 4.0471 3.9736
3.9507 3.9191 3.8515 3.8284
3.8700 3.9406 3.9809 3.9753
3.9601 3.9425 3.9219 3.9120
3.8903 3.8508 3.8170 3.7936
3.7934 3.8167 3.8262 3.8181
3.8137 3.7817 3.7410 3.7353
3.7615 3.8004 3.8142 3.8099
3.8200 3.8412 3.8395 3.8060
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-------------------------------
