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Externals in C – Sample rate retrieval

Dec 06 2010 | 10:02 am


I did a search for this beforehand but couldn’t find anything so I apologise if I’m repeating something that’s been posted here before.

I’m building an external object for Max in C for a school assignment. We were given a basic template for an object that lets audio pass through, and we’re modifying it into a 4-tap delay.

I’ve got the signal vectors (s_vec) coming through the object, and also the vector size of the signal (s_n), both of these are picked up fine in my perform routine. For some reason I CAN NOT get the perform routine to pick up the sample rate of the incoming signal.

I’ve added sp[0]->s_sr to my dspAdd section, and then added it to the perform routine in the same way I have s_n being read in, but when I use post to check the value of both values, the s_n prints as whatever the buffer is set to on the dac~ object unsurprisingly and the s_sr prints as 0. If I use post just after the dspAdd section however, the sample rate reads as 44100.

I can elaborate or post screenshots if this isn’t altogether clear.


Dec 06 2010 | 10:36 am

"I’ve added sp[0]->s_sr to my dspAdd section, and then added it to the perform routine in the same way I have s_n being read in…"

maybe you’ve got this covered, but t_signal struct ref defines s_n as int and s_sr as float. did you make sure to assign it to a float variable(and you can also make it explicit(‘typecast’ as below)) in your perform routine?

something like this:

void yourobject_dsp(t_mhnmna *x, t_signal **sp, short *count) {

dsp_add(yourobject_perform, 5, x, sp[0]->s_n, sp[0]->s_sr, sp[0]->s_vec, sp[1]->s_vec;


t_int *yourobject_perform(t_int *w)
t_yourobject *x = (t_yourobject *)w[1];
int n = (int)w[2];
float f = (float)w[3];
t_float *in = (t_float *)w[4];
t_float *out = (t_float *)w[5];


i could be wrong(too tired to run a full code test right now, need sleep), but try it out if you haven’t already… hope it flies.

*Never fear, Noob4Life was never here!*

Dec 06 2010 | 12:49 pm

Yea, that’s exactly what I’ve got. Here’s an exact replica of those two parts of code in my object:

dsp_add(mspExternalTPerform, 8, x, sp[0]->s_vec, sp[1]->s_vec, sp[2]->s_vec, sp[3]->s_vec, sp[4]->s_vec, sp[0]->s_n, sp[0]->s_sr);

The outlets are because I’m creating a delay with a quadrophonic output. All those outputs work, as does the slot with s_n in it.

My perform routine (spaces are for ease of reading here):

t_int* mspExternalTPerform(t_int* w)

t_mspExternalT* x = (t_mspExternalT*)(w[1]);
float* signal_in = (float*)(w[2]);

float* signal_out_one = (float*)(w[3]);
float* signal_out_two = (float*)(w[4]);
float* signal_out_three = (float*)(w[5]);
float* signal_out_four = (float*)(w[6]);

long num_samples = (long)(w[7]);
/* this was given to us as a long rather than an int */

float sample_rate = (float)(w[8])

then a bunch of code for my delay etc.

That last line should assign the sample rate to a flat variable named "sample_rate" right? But when I use this:

post("sample rate is: %f", sample_rate)

right after it, it returns a zero. However, the same line placed after my dsp_add line prints 44100.



Dec 06 2010 | 1:32 pm

The quick and simple answer is that we don’t support passing floating point values as arguments to dsp_add. Only pointers and integers.

Typically one accomplishes what you are looking to do by caching the sr value in your object struct. Or you could pass as a pointer to a float for that signal’s sr field (you can assume this signal memory will persist).

Sorry for the inconvenience.

Dec 06 2010 | 2:03 pm

That explains why I can’t get it to work!

Ok, so I’m a little confused about how to do this now, could you elaborate on the mentioned methods? How do I catch the value in my object struct? This is in the

typedef struct _mspExternalT

section at the top, right?


Dec 06 2010 | 2:38 pm

Yes. Add a floating point variable in your object struct. Assign it in your dsp method.

typedef struct _myobject
    t_pxobject  ob;
    float        sr;

and in your dsp method:
x->sr = sp[0]->s_sr;
Dec 06 2010 | 2:44 pm

Thank you so much, got it working now. Now if I can actually get the delay to work properly…


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