Author Topic: Sub Carriers  (Read 2428 times)

tstuart

  • Newbie
  • *
  • Posts: 2
Sub Carriers
« on: May 27, 2014, 10:27:55 pm »
Using a Pira175 with latest firmware and FM Scope software.
When watching the MPX screen I notice that the RDS level goes up, even when I don't have RDS in the signal. I do have an SCA, 67, and I'm assuming that's what is being detected.
Looking at the spikes I can see the SCA in the correct place, and even indicating the proper level.
Is there a way to have the software/hardware detected sub carriers and break them out, not just RDS?

Jan

  • Hero Member
  • *****
  • Posts: 586
Re: Sub Carriers
« Reply #1 on: May 28, 2014, 08:13:26 am »
There's no special support built-in for the US services based on SCA. The issue is that use of the SCA channel is not fully covered by global broadcast standards. There are several methods of use, some of them are proprietary and sometimes the parameters of the implementation are not well defined. For reliable and accurate adjusting of the SCA channel level the best way is an oscilloscope tied in parallel to the SCA modulation input while knowing the FM modulator's kHz-to-Volts sensitivity.

The situation is different in the case of RDS. Peak deviation caused by RDS signal can be calculated from the level of the FFT spectrum components because the relation between RDS peak deviation and average energy of the RDS signal is almost constant and well known in advance. The P175 does it that way. This unfortunately does not apply to SCA.

Demodulated signal is well covered (flat) in the P175 from DC to 80 kHz. So a contribution of the 67 kHz SCA channel to overall peak deviation is correctly indicated for usual SCA signal levels. Turning off the stereo pilot and audio you can adjust the deviation caused by the SCA channel within the typical tolerance of +/- 2 kHz, by reading the overall peak deviation.