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[linrad] RE: Question about time delay compensation

Hello Alberto and all,

> In another forum we were discussing about phase and time compensation,
> and someone came up saying that Linrad is capable of compensating a time
> delay that may exist in e.g. the left channel of a sound card, but not 
> in the
> other one, or viceversa.  This because some testings have measured that
> cheap sound cards exhibit about 14 microseconds of delay on one channel
> with respect to the other, probably due to the fact that there is just a 
> single
> ADC, time-multiplexed between the left and right channels. This time delay
> plays havoc with the ortogonality of the I and Q signals.
> A fixed time delay means a phase difference varying with the frequency, so
> the normal methods of phase compensation do not work in this circumstance.
> What is needed is an adjustable fractional time delay filter.
> Does Linrad have such a feature ? If yes, could you Leif please 
> tell me the
> routine where it is implemented ? I am more than willing to copy it  :-)
Linrad makes the I/Q balancing in the frequency domain. The reason is that
it is designed to work with analog anti-alias filters of very high Q and 
therefore Linrad must be able to handle phase and amplitude unbalances that
change rapidly with frequency.

The actual I/Q balancing is in fft1.c, but for this routine to work you have
to set up the proper correction function with a calibration routine.

There is a price to pay however. Balancing in the frequency domain will not 
allow better rejection than about 70 dB. I do not quite understand why. The
balancing depends on whether the frequency is exactly on a fft bin or between
two of them. I have been satisfied with 70 dB so I did not spend time trying to
make it better. Probably temperature drift and other hardware inadequacies
makes it useless to do the balancing much better.


Leif / SM5BSZ


> Thanks for any answers
> 73  Alberto  I2PHD