On Sun, May 2, 2021 at 4:28 PM Clemens clemens@chair.audio wrote:
When there is no aliasing to worry about, i might set the cutoff to pi again... On low sample rates (22kHz), the lower cutoff is definitely noticeable.
I like this thesis you posted earlier https://www2.spsc.tugraz.at/www-archive/downloads/Mueller11_DopplerSRC_0.pdf
f(at) <-> 1/|a|*F(w/a) This (and two formulas that follow) is listed as Smith's algorithm. I actually got to speak with Julius Smith at the 2012 LAC about this formula. I asked, can't we do any better in terms than O(a*n) number of computations for a>1? He said nope!, but I still have some questions there. Playback with speeds less than 1 always use O(n), rather than O(a*n). I wrote an anti-aliasing external tabread4a~ that implements this formula per sample and works pretty well, except it becomes expensive when you transpose a few octaves up.
For a variable delay line (like vd~), the paper's contents are more relevant and maybe you should consider writing a vdsinc~ object next (once you've optimized this one). delread is more like a fixed, stationary delay line, and I think it's better to default as a literal interpolator (LP_SCALE = 1).
Btw. I just implemented sharing of the interpolation table of the delreadsinc~ object according to your suggestions. It counts the number of references and frees the pointer when no object uses it anymore.
I read through the changes. For those reading along see the rest
at line 348 of d_delay.c at https://github.com/chairaudio/pure-data/blob/feature/delreadsinc/src/d_delay... (in progress, current code re: the tables pasted below). 1. I wonder is this global table properly scoped for Pd. I seriously don't know. But it could be used for multiple objects, vdsinc~, tabreadsinc~, etc.... So you ought to think about how it could be re-used between classes and choose a scalable approach now. 2. I think the declared LP_SCALE variable is a bad approach as currently implemented. You can't change it at runtime. It can be used in the perform routine instead, with an additional argument to set the cutoff frequency per object. This is also good for your testing, as you'll be able to put objects with different LP_SCALE values side-by-side for comparison. 3. I'm convinced this is the right approach for making an optimal interpolator. Adding more length pays off over upsampling, because you evaluate the convolution at a fewer number of points. Your implementation needs a lot of tuning---you can squeeze out more performance. Focus on the inner most loops. Also, you'll have to compare some arm vs intel/amd platforms at some point. I think you're closer to the beginning than the end. This is maybe not what you wanted to research in the first place
... typedef struct _sigvdsinc { t_object x_obj; t_symbol *x_sym; t_float x_sr; /* samples per msec */ int x_zerodel; /* 0 or vecsize depending on read/write order */ t_float x_f; } t_sigvdsinc;
typedef struct _sigvdsinc_sharedtable { // sinc table is held in global variable t_sample sinc_array[SINC_LEN]; // derivative of sinc funtion for interpolation of sinc function table t_sample sinc_diff_array[SINC_LEN]; // reference count for shared pointer int ref_count; } t_sigvdsinc_sharedtable;
t_sigvdsinc_sharedtable *gSharedTable = NULL;
void sigvdsinc_initialize_sinc_table(t_sample* sinc_array, t_sample* sinc_diff_array) { sinc_array[0] = LP_SCALE; sinc_diff_array[SINC_LEN] = 0; float a0 = 0.35875; float a1 = 0.48829; float a2 = 0.14128; float a3 = 0.01168; for (int i=1; i<SINC_LEN; i++) { float idx = 0.5*(M_PI*i/SINC_LEN - M_PI); // four term blackmanharris after https://www.mathworks.com/help/signal/ref/blackmanharris.html float blackmanharris= a0 - a1*cos(2*idx) + a2*cos(4*idx) - a3*cos(6*idx); // blackmanharris windowed sinc function sinc_array[i] = sin(LP_SCALE*M_PI*(float)i/STEPS_ZC)/(LP_SCALE*M_PI*(float)i/STEPS_ZC)
- blackmanharris * LP_SCALE;
// calculate derivative for table interpolation sinc_diff_array[i-1] = sinc_array[i]-sinc_array[i-1]; } }
... void sigvdsinc_free(t_sigvdsinc *x) { // delete shared interpolation table if need be if(gSharedTable != NULL){ if(gSharedTable->ref_count==1){ free((void*)(gSharedTable)); gSharedTable = NULL;; } else{ gSharedTable->ref_count--; } } }