Re: Half length output with libsox for stereo -> mono

[Ulrich Klauer <ulrich@chirlu.de>]

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Stephen Paterson wrote:

> I can't get it to work with libsox. I've made a tiny modification to  
> example3.c that should do the same but no matter what I try I always  
> end up with a file that is exactly half the length of the one I get  
> from sox (or some crazy audio).

example3 is buggy. It doesn't take into account that sox_add_effect()  
modifies the "in" signalinfo, which is why it isn't a good idea to  
pass it a signalinfo that is still in use elsewhere, or to call  
sox_add_effect() with identical in and out signalinfos.

What is happening here is that in->signal.length, after the  
sox_open_read(), contains some value like 882000, for a 10-second  
stereo 44100 Hz input file. After adding "rate", the value will have  
changed to 160000, and after "channels", to 80000. Because it is the  
same memory location all the time, the input format handler (from  
sox_open_read()) will see this value as well and think that its file  
is only 80000 samples long. It will therefore stop reading after 80000  
samples (/2 channels, /44100 Hz = 0.907 seconds), and that is all that  
is processed by the effects chain.

See the attached diff for how to use a copy of the signalinfo instead.

Ulrich

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--- main-pre.c	2013-04-10 17:12:23.000000000 +0200
+++ main.c	2013-04-10 17:20:10.000000000 +0200
@@ -4,6 +4,7 @@
 int main(int argc, char * argv[])
 {
 	static sox_format_t * in, * out;
+	sox_signalinfo_t c;
 	sox_effects_chain_t * chain;
 	sox_effect_t * e;
 	char * args[10];
@@ -13,13 +14,14 @@
 
 	assert(sox_init() == SOX_SUCCESS);
 	assert(in = sox_open_read(argv[1], NULL, NULL, NULL));
+	c = in->signal; /* NB: deep copy */
 	assert(out= sox_open_write("converted.wav", &in->signal, NULL, "wav", NULL, NULL));
 
 	chain = sox_create_effects_chain(&in->encoding, &out->encoding);
 
 	e = sox_create_effect(sox_find_effect("input"));
 	args[0] = (char *)in, assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
-	assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);
+	assert(sox_add_effect(chain, e, &c, &in->signal) == SOX_SUCCESS);
 	free(e);
 
 	out->signal.precision = 16;
@@ -29,20 +31,20 @@
 	if (in->signal.rate != out->signal.rate) {
 		e = sox_create_effect(sox_find_effect("rate"));
 		assert(sox_effect_options(e, 0, NULL) == SOX_SUCCESS);
-		assert(sox_add_effect(chain, e, &in->signal, &out->signal) == SOX_SUCCESS);
+		assert(sox_add_effect(chain, e, &c, &out->signal) == SOX_SUCCESS);
 		free(e);
 	}
 
 	if (in->signal.channels != out->signal.channels) {
 		e = sox_create_effect(sox_find_effect("channels"));
 		assert(sox_effect_options(e, 0, NULL) == SOX_SUCCESS);
-		assert(sox_add_effect(chain, e, &in->signal, &out->signal) == SOX_SUCCESS);
+		assert(sox_add_effect(chain, e, &c, &out->signal) == SOX_SUCCESS);
 		free(e);
 	}
 
 	e = sox_create_effect(sox_find_effect("output"));
 	args[0] = (char *)out, assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
-	assert(sox_add_effect(chain, e, &in->signal, &out->signal) == SOX_SUCCESS);
+	assert(sox_add_effect(chain, e, &c, &out->signal) == SOX_SUCCESS);
 	free(e);
 
 	sox_flow_effects(chain, NULL, NULL);

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