суббота, 18 декабря 2010 г.

Using SA to measure and correct sound card digitizer errors.

Using SA to measure and correct sound card digitizer errors.

This article had been written completely on the basis of letters sent by SA users.

All sound cards and A/D converters have some clock error. This is especially true if converters are commercial and cheap ones, like PC sound cards and similar. Professional and expensive converters exhibit a much better clock stability and jitter.

To correct this error, one must know nominal parameters of the signal under test. If these parameters are known, it is quite easy to correct digitizing clock error using SA.

The SA method of “Correction of BR” is quite good for this job, but perhaps using resampler as data input is a better procedure.

The correction factor will have to be measured for any digitizing speed and mode.
Bear in mind that for cheap cards, the speed can vary due to various factors, so if high precission measurements are required, a new calculus should be carried out.

For a good measurement, a quite big signal is needed.

This method is useful for PSK,FSK,MFSK and other modulations. To use it with OFDM, some more operations should be carried out using SA.

Ideally, an external signal like GPS 1000 Hz or a signal from a high end signal generator will provide the best results. Also, a radio timing signal should be good enough.

The best way to understand the subject is an example.
A well known signal as Stanag-4285 (sampled at 8000 sps) will be used to show the method.

We know 4285 has a nominal speed of 2400 sps. Since a frame is 256 symbols, the frame time must be 106,666666 mS. This is the value that should be obtained if using the VMW feature of SA when signal structure is perfectly vertical.

As we can see, the mesured value is 106,69241.

Correction factor =measured value/nominal value=106,69241/106,66666= 1,000241
Error PPM=(correction factor-1)* 1000000= (1,000241-1)*1000000= 241 PPM.

Real Digitizing speed= 8000*1,000241 = 8001,931
Real modulation speed (Br) = 2400/1,000241 = 2399,421739

Measured SA Br= 2399,41

Now, lets go to correct the signal using calculated BR in SA.

The signal parameters are almost perfect, so we can save it and after this procedure, we can be quite sure the new signal will be demodulated using any comercial demodulator.

Also, we know the correction factor for the used card in that speed.

As soon as parameters of a sound card in the record channel are received, then it is possible to realize precise measurements of replay( reproduction) channel for sound cards, which have separate record/reproduction channeles.

You can perform these measuremens simply by having connected an output of such card with an input, playingback a known/synthesized signal/file. Measurements should be realized by the technique described above. As the error of the channel of record is already known, it can be easily considered, and there will be only an error of the channel of reproduction/replay/playback.