Howdy All,
> -----Original Message-----
> From: Zia Mazhar [mailto:[EMAIL PROTECTED]]
> Sent: Monday, April 24, 2000 3:36 PM
> To: [EMAIL PROTECTED]
> Subject: Re: [MP3 ENCODER] mpglib bugfix
>
> Naoki Shibata wrote:
...
> > I made a patch to fix some problems of mpglib. And, I updated
> > in_mpg123 plugin.
> > Now, maximum differences of outputs of mpglib decoder and
> > l3dec 2.60 is 1 for each sample, as far as I tested.
...
> Really? That sounds great! I'll download it right away. By
> the way, how do you
> measure the difference? Isn't there any way to be sure that
> I'm getting the
> 100% accurate sound what has been encoded in an MP3 file?
If you have a C compiler, see my source for a utility called 'audiod' (audio
diff) below. (It may need some tweaking for I/O, etc. on your particular
platform.) It diffs two PCM audio files against one another and 1) tells
you their relative length, 2) tells you whether or not they differ, and 3)
outputs a histogram of the magnitudes of the differences (which may be quite
extensive if they are severely different). In the above case, it should
tell you that there were a number of differences of magnitude 1.
As to your second question, there is no such thing as the 100% accurate
sound that was encoded in an mp3. The precision of numerous internal
variables (not to mention the actual
audio I/O) is not specified in the MPEG spec, and rounding is not treated
consistently. Similarly, math libraries (especially floating point
division) on different platforms may not be 100% identical in their
operation. Thus 'conforming' decoders may disagree as to the precise output
PCM samples from a given input bitstream. However, this rarely leads to
absolute differences of more than an LSB; thus the above standard of
measurement is pretty prevalent.
Alex
-----
#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
void main(int argc, char *argv[])
{
int i,diff_index,diff_total=0;
int *diffs;
short sample1,sample2;
int samples=0;
int done1,done2,done=0;
FILE *in1,*in2;
if(argc!=3) {
fprintf(stderr,"USAGE: %s infile1 infile2\n",argv[0]);
exit(1);
}
if((in1=fopen(argv[1],"rb"))==NULL) {
fprintf(stderr,"ERROR: Could not open file %s\n",argv[1]);
exit(1);
}
if((in2=fopen(argv[2],"rb"))==NULL) {
fprintf(stderr,"ERROR: Could not open file %s\n",argv[2]);
exit(1);
}
diffs = (int *)calloc(SHRT_MAX-SHRT_MIN+1,sizeof(int));
// Prime the pipeline
fread(&sample1,sizeof(short),1,in1);
fread(&sample2,sizeof(short),1,in2);
done1 = feof(in1);
done2 = feof(in2);
while(!done1 && !done2) {
diff_index = abs(sample1-sample2);
if(diff_index) diff_total++;
diffs[diff_index]++;
samples++;
fread(&sample1,sizeof(short),1,in1);
fread(&sample2,sizeof(short),1,in2);
done1 = feof(in1);
done2 = feof(in2);
}
if(!done1) {
fseek(in1,0,SEEK_END);
fprintf(stdout,"File %s is %d samples longer than file
%s...\n",
argv[1],(ftell(in1)>>1) - samples,argv[2]);
}
else if(!done2) {
fseek(in2,0,SEEK_END);
fprintf(stdout,"File %s is %d samples longer than file
%s...\n",
argv[2],(ftell(in2)>>1) - samples,argv[1]);
}
else
done++;
if(diff_total) {
fprintf(stdout,"The files differ...\n");
fprintf(stdout,"The differences were distributed as
follows:\n");
for(i=1;i<SHRT_MAX-SHRT_MIN+1;i++) {
if(diffs[i]) {
fprintf(stdout,"\t%d differences of
magnitude %d\n",diffs[i],i);
}
}
if(done)
fprintf(stdout,"For a total of %d differences over
%d samples.\n",
diff_total,samples);
else
fprintf(stdout,"For a total of %d differences over
the %d overlapping samples.\n",
diff_total,samples);
}
else {
if(done)
fprintf(stdout,"The files are identical.\n");
else
fprintf(stdout,"The overlapping portions of the
files are identical.\n");
}
free(diffs);
fclose(in1);
fclose(in2);
}
--
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