FFT MegaCore Function Output is not as expected

One mistake I feel is that, when we convert the integer values above 32768 to binary, they are becoming negative values since they are using signed representation.

Is this the cause of problem?

If so, then how can I give 16 bit unsigned input to FFT MegaCore?

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One mistake I feel is that, when we convert the integer values above 32768 to binary, they are becoming negative values since they are using signed representation.

Is this the cause of problem?

If so, then how can I give 16 bit unsigned input to FFT MegaCore?

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what you interpret as positive above 32767 will be viewed as negative by fft if it is set to 16 bits signed.

For a start use positive only below 32767 and see. then for negative numbers convert using the rule that a negative number is same bits as positive if you add 2^16 to it.

Now I am checking with positive numbers only..

But still the output is not as expected..

Can you please verify my calculations are correct or not//

I don't know about your fft scaling issue of exp as it is related to your ip specifics. But what do you compare with? and why do you use log.

if you are comparing with matlab fft then you don't need the log. Just compare fft outputs directly and possibly you may get scaling issues.

For example:

I am getting the following in simulation:

source_real: 2659

source_imag: 3490

source_exp: -10

So as per my calculation:

out_real = source_real * 2^(exp) = 2659 * 2^-10 = 2.596679688

out_imag = source_imag * 2^(exp) = 3490 * 2^-10 = 3.408203125

So, Output = 10*log10(sqrt(out_real^2 + out_imag^2))

= 10* log10 (sqrt(6.742745399 + 11.61584854))

= 10* log10 (4.284692981)

= 6.319197081

But the value I am expecting here is -5.42

Could you please help me//

I am calculating '10*log10()' to convert the linear value to power in 'db'.

Anyhow, can you please tell what the scaling factor I have to use? Is scaling factor '-10' as seen in 'source_exp'?

The input samples (15 bits wide) in integer are:

21577,18450,14977,11706,9150,7709,7613,8871,11289,14486,17963,21173,23610,24891,24820,23404,20867,17606,14132,10993,8680,7556,7798,9369,12021,15338,18799,21860,24041,25003,24592,22870,20114,16750,13308,10329,8282,7488,8069,9935,12796,16197,19610,22496,24401,25031,24281,22275,19322,15888,12513,9725,7963,7503,8416,10562,13601,17059,20389,23070,24683,24973,23896,21620,18501,15031,11752,9181,7722,7601,8840,11243,14433,17910,21127,23581,24882,24832,23436,20914,17660,14184,11036,8708,7564,7785,9336,11973,15284,18748,21820,24018,24999,24608,22906,20163,16804,13359,10370,8306,7490,8049,9898,12745,16144,19561,22457,24381,25031,24303,22314,19373,15942,12560,9760,7981,7500,8394,10521,13550,17005,20341,23036,24667,24978,23922,21663,18553,15083,11799,9214,7735,7594,8813,11201,14382,17858,21083,23549,24872,24843,23468,20959,17712,14237,11079,8735,7571,7772,9303,11927,15230,18695,21778,23992,24994,24623,22942,20210,16858,13410,10411,8329,7494,8032,9862,12696,16090,19510,22419,24360,25031,24324,22351,19423,15996,12610,9797,7999,7497,8370,10482,13499,16952,20293,23002,24652,24985,23947,21704,18606,15138,11844,9246,7747,7586,8785,11157,14328,17805,21038,23520,24862,24853,23496,21004,17765,14290,11124,8763,7578,7757,9270,11879,15177,18644,21736,23968,24988,24639,22976,20257,16911,13460,10450,8350,7493,8012,9823,12645,16035,19460,22381,24340,25032,24346,22390,19473,16050,12658,9834,8017,7494,8346,10440,13448,16897,20245,22968,24636,24991,23974,21747,18658,15190,11892,9279,7762,7578,8756,11112,14276,17751,20991,23488,24851,24864,23527,21049,17819,14342,11166,8790,7587,7744,9239,11834,15125,18592,21693,23940,24983,24655,23010,20306,16966,13512,10491,8375,7496,7993,9789,12597,15982,19411,22344,24320,25033,24366,22430,19524,16103,12708,9870,8034,7491,8323,10401,13396,16843,20197,22933,24621,24996,23998,21789,18709,15245,11938,9310,7775,7570,8729,11068,14223,17700,20946,23458,24839,24873,23557,21093,17872,14395,11211,8820,7596,7731,9207,11787,15071,18540,21650,23915,24976,24672,23045,20353,17019,13564,10531,8399,7500,7976,9752,12549,15929,19359,22303,24298,25032,24387,22467,19574,16159,12758,9908,8054,7489,8299,10360,13347,16789,20149,22897,24603,24999,24023,21829,18761,15297,11986,9344,7788,7562,8701,11025,14170,17646,20902,23427,24829,24885,23586,21139,17925,14447,11255,8849,7605,7719,9175,11740,15018,18489,21608,23888,24971,24686,23080,20401,17073,13615,10573,8425,7504,7959,9716,12500,15875,19309,22264,24276,25029,24407,22506,19623,16211,12807,9945,8073,7487,8277,10320,13295,16735,20100,22862,24586,25004,24050,21872,18812,15351,12032,9378,7804,7556,8674,10982,14119,17592,20856,23397,24817,24894,23617,21184,17978,14500,11299,8877,7614,7706,9142,11694,14963,18436,21566,23862,24965,24701,23113,20449,17126,13666,10613,8447,7508,7942,9680,12451,15821,19259,22224,24254,25029,24427,22543,19673,16265,12856,9982,8094,7484,8256,10282,13244,16682,20052,22826,24571,25008,24073,21912,18864,15406,12080,9412,7818,7549,8647,10940,14066,17539,20810,23365,24806,24904,23646,21229,18031,14553,11344,8907,7624,7696,9112,11649,14910,18384,21525,23835,24957,24715,23146,20497,17180,13718,10654,8472,7512,7925,9644,12403,15767,19209,22185,24231,25027,24446,22581,19722,16320,12905,10020,8113,7485,8233,10242,13193,16628,20004,22791,24553,25011,24097,21954,18916,15458,12127,9445,7833,7542,8621,10897,14015,17485,20764,23334,24793,24912,23674,21271,18083,14607,11389,8936,7633,7684,9081,11603,14858,18332,21480,23807,24949,24729,23178,20544,17233,13769,10696,8496,7516,7908,9609,12355,15713,19157,22145,24209,25025,24464,22617,19772,16373,12956,10058,8133,7483,8212,10203,13143,16574,19955,22755,24535,25014,24120,21994,18967,15513,12174,9479,7849,7537,8595,10856,13963,17433,20718,23303,24780,24921,23702,21316,18135,14659,11434,8965,7643,7673,9050,11558,14805,18280,21436,23780,24943,24744,23213,20590,17286,13820,10738,8523,7522,7892,9574,12307,15660,19107,22105,24186,25023,24484,22654,19820,16427,13007,10095,8154,7484,8191,10165,13094,16520,19906,22719,24517,25017,24146,22035,19018,15566,12223,9514,7864,7530,8567,10812,13911,17379,20671,23268,24767,24928,23732,21360,18188,14712,11479,8996,7653,7661,9019,11514,14751,18227,21393,23753,24935,24757,23245,20637,17339,13872,10780,8548,7528,7876,9540,12259,15606,19056,22066,24164,25021,24503,22692,19870,16480,13057,10135,8175,7485,8169,10125,13044,16467,19857,22682,24499,25021,24169,22076,19068,15620,12271,9548,7880,7525,8542,10769,13859,17326,20625,23237,24754,24937,23759,21404,18240,14765,11524,9027,7664,7651,8988,11468,14699,18175,21349,23724,24928,24771,23279,20684,17394,13924,10823,8574,7533,7860,9505,12211,15552,19005,22025,24140,25018,24521,22729,19919,16534,13106,10173,8196,7484,8148,10086,12993,16413,19808,22646,24479,25023,24192,22116,19120,15673,12319,9582,7895,7521,8517,10728,13807,17272,20577,23203,24739,24944,23787,21448,18292,14818,11569,9057,7675,7640,8959,11423,14646,18123,21305,23695,24920,24784,23310,20730,17447,13976,10864,8600,7537,7844,9473,12163,15499,18953,21984,24116,25014,24539,22764,19968,16589,13157,10211,8217,7484,8129,10049,12943,16359,19759,22608,24460,25025,24215,22156,19171,15726,12367,9618,7911,7515,8492,10686,13756,17220,20530,23171,24727,24954,23814,21492,18346,14871,11614,9088,7686,7629,8929,11377,14592,18069,21261,23666,24910,24796,23342,20776,17501,14028,10907,8627,7544,7829,9438,12116,15445,18902,21943,24091,25010,24556,22800,20016,16642,13207,10252,8239,7486,8110,10010,12893,16305,19709,22571,24441,25028,24236,22195,19221,15781,12415,9653,7928,7511,8467,10644,13704,17165,20484,23138,24712,24960,23842,21534,18397,14924,11660,9118,7698,7621,8899,11333,14539,18017,21217,23638,24901,24807,23372,20821,17554,14080,10950,8653,7548,7813,9403,12068,15390,18852,21902,24067,25008,24573,22836,20065,16695,13257,10291,8260,7486,8089,9973,12844,16251,19660,22534,24420,25029,24259,22234,19272,15835,12463,9689,7944,7507,8442,10604,13652,17112,20437,23104,24698,24967,23869