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Date Submitted: 03/18/2013 12:23 AM
FFT
• Many ways to decompose an FFT • Simplest is radix‐2 • Computation made up of radix‐2 butterflies
A B
X = A + BW Y = A – BW
B. Baas, © 2012, EEC 281
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FFT Dataflow Diagram
• Dataflow diagram
– N = 64 – radix‐2 – 6 stages of computation
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 63
Memory Locations
Input
B. Baas, © 2012, EEC 281
Output
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Radix 2, 8-point FFT
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Radix 2, 8-point FFT
Radix 2, 16-point FFT
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Radix 2, 8-point FFT
Radix 2, 32-point FFT
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Radix 2, 8-point FFT
Radix 2, 64-point FFT
B. Baas, © 2012, EEC 281
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Radix 2, 256-point FFT
Radix 2, 256-point FFT
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Radix 4, 16-point FFT
B. Baas, © 2012, EEC 281
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Radix 4, 64-point FFT
B. Baas, © 2012, EEC 281
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Radix 4, 256-point FFT
B. Baas, © 2012, EEC 281
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Radix 2, Decimation-In-Time (DIT)
• Input order “decimated”—needs bit reversal • Output in order Critical path: • Butterfly:
B A B X = A + BW x Y = A – BW A car-sav add +
B. Baas, © 2012, EEC 281
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W
Radix 2, Decimation In Frequency (DIF)
• Input in order • Output “decimated”—needs bit reversal • Butterfly:
– Two CPAs – Wider multiplier A B X=A+B Y = (A – B) W Critical path: A B
+ (–)
W
x +
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B. Baas, © 2012, EEC 281
Radix 4, DIT Butterfly
• Decimation in Time (DIT) or Decimation in Frequency (DIF)
A B C D
V W X Y
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Bit-Reversed Addressing
• Normally:
– DIT: bit‐reverse inputs before processing
BIT REVERSAL
– DIF: bit‐reverse outputs after processing
• Reverse addressing bits for read/write of data
– – – – – – – – 000 (0) 001 (1) 010 (2) 011 (3) 100 (4) 101 (5) 110 (6) 111 (7) 000 (0) # Word 0 does not move location 100 (4) # Original word 1 goes to location 4 010 (2) # Word 2 does not move location...