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I'm just curious if this could be a way of transmitting stereo?

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Yes I and Q are in quadrature and therefore orthogonal so they are naturally isolated. For example you could consider QPSK as two independent BPSK channels transmitted on the same carrier frequency (or you could consider it having twice the spectral efficiency as BPSK since you can send twice as much data at once within the same bandwidth, and at the same SNR level).

To maintain isolation in the SDR you need to have good IQ balance or correct for IQ quadrature and amplitude error. Correcting for analog errors with software as depicted in the diagram below is easy to do: if the data content is equally distributed in the long term then the data can be used directly, and this would be a reason to "whiten" the actual data with scrambling prior to transmission. Otherwise training sequences could be used for such calibration of the receiver hardware- to the extent that is needed to be sufficient for audio purposes. Amplitude is adjusted by comparing the rms magnitudes of I and Q, and phase is adjusted by correlating I and Q (average of the product of I and Q over all samples) and scaling by the amplitude (to get phase).

IQ Correction

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In SDR could you isolate I and Q

Refer to Dan's answer please – it's the more concise than what I could write.

I'm just curious if this could be a way of transmitting stereo?

It would, but you'll probably have a hard time finding such a system in reality, though. It's not a technically sensible way:

  • as Dan said, you need an analog-wise good or digitally well-corrected receiver
  • atop of that, you need phase synchronization (otherwise, how would the receiver know what is I and what is Q?)
  • and that phase synchronization needs to finely tracking the phase of the receiver – for a phase offset of 10°, you get cos(10°) of your I receive energy to bleed into your Q, and vice versa. And, no two oscillators in this universe are the same, so, a phase variation at the receiver is inevitable

The facts that you

  • either need to build an expensive analog mixer/filter/amplifier chain that is very well balanced, or do the correction in digital domain after digitizing the signal,
  • you need either a continuous, relatively strong carrier for a receiver PLL to lock on, or some kind of time-discrete synchronization symbols, and
  • that in all likelihood, you'd want to digitize your stereo audio and transmit it that way (since that is spectrally more efficient than just sending analog audio, especially if you have a highly correlated stereo signal in there),

means that you'd really just go and use the one RF channel that you have to transmit a stream of digital data, containing compressed audio as complex symbols (using both I and Q at once). That's just what all the audio broadcasting standards do!

  • DAB: FM-radio replacement multicarrier system doing differential phase shift keying per subcarrier, supports stereo through MPEG-1 Audio Layer 2 (MP2)
  • Sirius/Sirius XM: US-American Satellite radio: phase-shift keying system, proprietary digital codecs yielding satellites doing stereo
  • DRM: Short/Medium/Longwave-radio replacement radio, multicarrier QAM-carrying system working on veeery long channels, supports stereo through MPEG-4 HE-AAC
  • DAB+: basically DAB with longer frames and better codecs, MPEG-4 audio does stereo
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