What is meant by “direct sampling” in software defined radios?

In traditional superheterodyne receivers, the incoming radio frequency (RF) signal is mixed with a local oscillator to convert it to a lower intermediate frequency (IF) before further processing. Direct sampling bypasses this mixing stage entirely. In a direct sampling Software Defined Radio (SDR), the antenna's incoming RF signal is fed directly (often after some basic bandpass filtering) into a high-speed Analog-to-Digital Converter (ADC). The ADC digitizes the entire chunk of the RF spectrum at once, allowing all subsequent demodulation and filtering to be done mathematically by software or digital signal processors. The other options describe different SDR architectures or digital processes. For instance, generating I and Q signals using a switching mixer is a characteristic of a direct-conversion or zero-IF receiver, not a direct sampling receiver where the raw RF itself is digitized first.