Jitter Test Signal (J-test Signal) with MATLAB

[merlijnv]

I’d very much like to have an audio file based on Julian Dunn’s exact description for jitter testing (http://www.nanophon.com/audio/diagnose.pdf). As far as I know this file isn’t available for downloading. I’ve come to understand that such a signal would be possible to create with a program like MATLAB. However after some intense browsing a came to the conclusion that the scripting is beyond me. I’ve copied the original explanation from Dunn’s paper below. Maybe somebody is willing to figure it out.

Kind regards,

Merlijn

A test signal has been developed in order to stimulate worst-case levels of data-jitter. This signal has
two components. The first is an un-dithered square wave with a period of 4 samples. A cycle of this
is shown here in hexadecimal notation (hex):

C00000 C00000 400000 400000

On conversion to analogue at a sample rate of 48kHz this signal would produce a sine wave with an
amplitude of -3.01dBFS at 12kHz.

This is added to an undithered 24 bit square wave of amplitude 1 least significant bit (1 LSB) and dc
offset of -1/2 an LSB. This square wave is repeated at low frequency. In the case of the signal used
in these tests a rate of 250Hz was selected. This adds an undithered square wave of amplitude -
144dBFS and a negative dc offset of -150dBFS. The two values used for this square wave are 0 and
-1 (FFFFFF).

The combination of these signals results in the following 192 sample cycle of 24-bit data values:

C00000 C00000 400000 400000 (x 24) BFFFFF BFFFFF 3FFFFF 3FFFFF (x 24)

The low frequency coherent alternation in the values of the 22 LSBs produces strong jitter spectral
components at the repetition rate and its odd harmonics. A low frequency was used to stimulate jitter
at a frequency that will not be attenuated significantly by a receiver clock recovery circuit. The high
frequency component was selected for convenience when making jitter measurements of DACs; this
is discussed in a later section.

[wakibaki]

What do you want it as? A .wav file? Of what duration?

I haven't written a file from code probably since I wrote C under DOS, so it'll probably not arrive until tomorrow, unless somebody can write it off the top of their head.

w

[wakibaki]

OK, here's a zip file (down at the bottom), it contains a file called jitter.exe. If you run it it will ask you to press a key to start, it takes a while to run, then it'll ask you to press a key again to indicate that it's done.

It'll create a file in the same directory, jitter.wav, of 17,280,044 bytes, the .wav header and one minute of stereo sound @ 48ksamples, 24 bit resolution.

I've looked at the file with a hex editor, it looks OK to me, there's a mixture of Big- and Little-endian data in there, God knows what Microsoft were up to.

Here's the code, it compiles under Microsoft QC25, if there are any problems you can look and see if you can see any errors, I haven't attempted to debug the output in any way apart from look at it in the hex editor, I haven't tried to run it.


/*******************************************\
* jitter.c *
* 14/02/2011 *
* wakibaki *
\*******************************************/

#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <ctype.h>

void main();

void main()
{
FILE *fp;
long length, fileptr;
unsigned char d;
int i;

printf("\jitter\n\n");
printf("Press a key to continue...\n");
getch();
length=15000;
if((fp=fopen("jitter.wav","wb"))!=NULL){
d='R'; /* Chunk ID*/
fputc(d,fp);
d='I';
fputc(d,fp);
d='F';
fputc(d,fp);
d='F';
fputc(d,fp);
d=0x24; /* Chunk Size, 36+No of samples */
fputc(d,fp);
d=0xAC;
fputc(d,fp);
d=0x07;
fputc(d,fp);
d=0x01;
fputc(d,fp);
d='W'; /* Format */
fputc(d,fp);
d='A';
fputc(d,fp);
d='V';
fputc(d,fp);
d='E';
fputc(d,fp);
d='f'; /* SubChunk 1 ID*/
fputc(d,fp);
d='m';
fputc(d,fp);
d='t';
fputc(d,fp);
d=' ';
fputc(d,fp);
d=0x10; /* SubChunk 1 size */
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x01; /* Audio Format*/
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x02; /* MumChannels */
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x80; /* Sample rate */
fputc(d,fp);
d=0xBB;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00; /* Byte rate */
fputc(d,fp);
d=0x65;
fputc(d,fp);
d=0x04;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x06; /* Block Align */
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x18; /* Bits Per Sample */
fputc(d,fp);
d=0x00;
fputc(d,fp);
d='d'; /* SubChunk2 ID */
fputc(d,fp);
d='a';
fputc(d,fp);
d='t';
fputc(d,fp);
d='a';
fputc(d,fp);
d=0x00; /* SubChunk2 Size */
fputc(d,fp);
d=0xAC;
fputc(d,fp);
d=0x07;
fputc(d,fp);
d=0x01;
fputc(d,fp);
for(fileptr=0;fileptr<length;fileptr++){
for(i=0;i<24;i++){
d=0x00; /* Data - Little Endian */
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0xC0;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0xC0;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0xC0;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0xC0;
fputc(d,fp);

d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x40;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x40;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x40;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x00;
fputc(d,fp);
d=0x40;
fputc(d,fp);
}
for(i=0;i<24;i++){
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xBF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xBF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xBF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xBF;
fputc(d,fp);

d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0x3F;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0x3F;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0x3F;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0xFF;
fputc(d,fp);
d=0x3F;
fputc(d,fp);
}
}
fclose(fp);
}
else{
printf("\nError opening files...\n");
exit(1);
}
printf("\jitter\n\n");
printf("Press a key to continue...\n");
getch();
exit(0);
}

w

[Pano]

It seems to work. Here is an FFT. I had to convert it to 16 bits, but you get the idea.

[merlijnv]

Great!

Thank you very much for the effort. It seems to work at my pc as well. I think I understand what you’ve done. I will be using the file hopefully to test the DAC’s of a Dolby Lake Processor. I’m curious to the differences between it’s PLL and ASRC.

Regards,

Merlijn

[merlijnv]

On second thought, would it be possible to also create a 16 bit 44.1 kHz version using the same method. I’ve come to understand that the following parameters (https://ccrma.stanford.edu/courses/4...cts/WaveFormat) need to be changed:

• ChunkSize (10584036 or A17FE4 HEX)

• SampleRate (44.1 kHz or AC44 HEX)

• ByteRate (176400 or 2B110 HEX)

• BlockAlign (4 or 4 HEX)

• BitsPerSample (16 or 10 HEX)

• Subchunk2Size (10584000 or A17FC0 HEX)

• Data (C000 C000 4000 4000 (x 24) BFFF BFFF 3FFF 3FFF (x24) at 192 sample cycle*)

*This cycle rate or number of samples should stay unaltered to maintain the 4/1000 and 1/4 sample rate frequency ratio.

Kind regards,

Merlijn

[SoNic_real_one]

Originally Posted by Pano

It seems to work. Here is an FFT. I had to convert it to 16 bits, but you get the idea.

The added fake "jitter" was at the LSB in a 24 bit signal. I don't know how did you convert THAT to 16 bit...

[merlijnv]

And yet this way of working produces bit accurate results that comply with Dunn’s original paper. I’ve downloaded at least one version of the 16 bit test signal claimed to be the “one”. But closer inspection using a hex editor shows small imperfections. I for one hope that this method can be used to create a 16 bit version.

Regards,

Merlijn

[SoNic_real_one]

I don't think it can be done. The whole ideea is based on using a "smaller" 24bit step to simulate the jitter of the "bigger" 16bit signal. You need to feed that 24 bit as a 16 bit signal to the receiver to be perceived as jitter.
If you have the same 16 bit for both, you will end up with just regular signal, and that will NOT be seen as jitter by the receiver.

[Pano]

Originally Posted by SoNic_real_one

The added fake "jitter" was at the LSB in a 24 bit signal. I don't know how did you convert THAT to 16 bit...

I had to convert to 16 bit to read it in my FFT sotware, as it did not like the 24 bit version. Does not mean you have to use it that way.