Digital demodulator (SDR) for receiving legacy analog modulated wave I understand that in SDR, after the ADC the signal processing is fundamentally based on I/Q modulated waves where any discrete value that can be transmitted corresponds to a symbol which is represented as amplitude and phase through I and Q signals.
How can a modern SDR receive legacy AM waves?
The SDR uses I,Q components whereas legacy AM doesn't have any such component applicable.
I presume SDRs are able to receive and demodulate existing legacy AM,FM transmissions - please correct/add.
Do current SDRs receiving AM waves switch to a different technique wherein it doesn't use I/Q demodulation?
Sure, an SDR can transmit and receive conventional, analog AM. I've done it many times as an introductory exercise in my courses.
A quadrature signal $s_{\text{QAM}}$ is made up of two signals, $s_I(t)$ and $s_Q(t)$, that are transmitted over the same bandwidth and modulated in quadrature (i.e. with carriers phase-shifted by 90 degrees):
$$s_{\text{QAM}}(t) = s_I(t)\cos(2\pi f_c t) + s_Q(t)\sin(2\pi f_c t)$$
There is no restriction on the shape of these signals, beyond the physical limits of the hardware or the channel (in bandwidth or in power). In particular, they can be a DSB-LC AM signal!
$$s_{\text{QAM}}(t) = (A_m + m(t))\cos(2\pi f_c t) + 0\sin(2\pi f_c t)$$
The regular downcoversion and filtering process applied to $s_{\text{QAM}}$ will produce $A_m + m(t)$. You can also apply a traditional envelope detector. That's the beauty of SDR: it does not care about the meaning or structure of the in-phase and quadrature signals; you define that in software.
To conclude, note that not only it is indeed possible to transmit/receive AM with an SDR; you can actually transmit/receive two AM signals on the same bandwidth!
$$s_{\text{QAM}}(t) = (A_{m1} + m_1(t))\cos(2\pi f_c t) + (A_{m2} + m_2(t))\sin(2\pi f_c t)$$
You select one of the IQ branches and ignore the other, and then you can listen to one &quot;station&quot; or the other. Of course, this assumes that you are transmitting your own quadrature AM signals (for example, over one of the ISM bands).