Author Topic: Stereo channel seperation  (Read 4231 times)

BROADCAST

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Stereo channel seperation
« on: October 12, 2011, 01:18:16 am »
Hi,
Is it possible for the analyzer to show channel seperation value say in db ?

This would be great to adjust the encoders pilot phase and subcarrier level to get the best null for max separation, for instance 1khz tone in on left only and then be able to use the analyzer to trim the encoder.

If the analyzer show db levels of both channels,nulling of the opposite channel would be easily achieved,and give a db value of seperation.

Paul.

Jan

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Re: Stereo channel seperation
« Reply #1 on: October 12, 2011, 09:51:55 am »
I think this question has been already answered in old forum section. So in short: the device expects professional and ready to use broadcast equipment where are no trimmers for separation adjustment. Nowadays professional stereo encoders integrated in one box with FM exciter can offer a separation of more than 60 dB. This separation cannot be measured "from air". There is no affordable FM receiver on the world that can provide this separation. Serious separation measurement requires sophisticated measuring method, of course excluding entire RF path.

BROADCAST

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Re: Stereo channel seperation
« Reply #2 on: October 12, 2011, 07:50:26 pm »
Hi Jan,
I do use the baseband scope method to set the phase and subcarrier level on the 8100a optimod.

But as the transmitter can alter the baseband amplitude and phase, so the encoder needs to be adjusted to compensate for this.

IIRC the 8200 optimod has no pilot phase, but it does have subcarrier amplitude adjustment, to give compensation.


Jan

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Re: Stereo channel seperation
« Reply #3 on: October 13, 2011, 04:48:54 pm »
I understand what you mean, unfortunately a real solution is not known to me for this situation.

The transmitter should be good enough to affect the phase and amplitude of MPX compoments minimally. Compensation on the stereo encoder does not give real solution as you can effectively compensate the separation loss only at one frequency, say 1 kHz, but not in whole audio band. Better solution seems to be ensuring amplitude flatness of the FM modulator in whole MPX range 30 Hz to 60 kHz, especially in critical range 10 kHz to 60 kHz as at lower frequencies the flatness is usually good in all cases. This can be measured at selected modulation frequencies using carrier null effect with a precision of about 0.05 dB, i.e. comparing modulation sine signal level and FM deviation caused by this signal.

However the MPX flatness of any analogue FM transmitter will never be 100%. This is one of the reasons why real high separation (not only paper-like) is achievable more likely using DDS modulator and fully digital stereo encoder.