Coherent | Alias | Skip | Hiccap | PT-spurs | AQ-spurs
DDS background
DDS: Direct Digital Synthesis = point(memory location)-skipping at constant fc update(clock) - rate
[ vs. traditional PPC (point-per-clock) synthesis at variable clock-rate ]*
Finite memory requirement: coherent sampling
Discrete-time vs. continuous-time sinusoid
Nyquist-zones: images and aliasing
Point sampling, frequency conversion (folding)
Point(memory location)-skipping: temporal "scrambling"
Reversible permutation of samples, and ModT (re)plot
Jittered sampling: phase hiccap
Limited memory size, constant interpolation (virtual table)
Phase Truncation (PT) spurs: overlapped modulation structure
Where is the truncation "noise"?
// Tuning condition where there is
- no PT-distortion
- just one spur (worst case)
- spur pairs //
Amplitude Quantization (AQ) spurs: overlapped harmonic structure
Where is the quantization "noise"?
//Tuning condition where there is
- no AQ-distortion (only loss, scaling factor): f = fc/4
- coincident spurs (massive overlap): short numerical period
- close-in or "cross-over" spurs (fine offset): narrow band
- a "sea" of fine spaced spurs: long period //NRZ (non return to zero) mode DAC~~~~~
sin(x)/x spectral envelope (amplitude rolloff)
SINC interpolation
Exact interpolation with periodic samples (finite sum)
FFT artifacts: resolution (selectivity) or detection (sensitivity)?
Spectral leakage (bin splatter), picket-fence (scallop loss) and windowing
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e-mail: papay@hit.bme.hu