What factor increases storage requirements for digital recordings?

The factor that increases storage requirements for digital recordings is higher density audio channels. This is because higher density audio channels typically refer to a greater number of audio tracks or a higher bit depth and sample rate in recordings. When audio is captured at greater fidelity, such as using more channels to record different instruments or voices separately, or at higher sampling rates for improved sound quality, the amount of data generated increases significantly. This results in larger file sizes, which directly translates to greater storage needs.

In contrast, using external microphones does not inherently increase storage requirements; rather, it can enhance audio quality without affecting the data size. Shorter recording sessions would generally lead to smaller file sizes as less data is recorded. Operating system updates relate more to software performance and capability rather than the actual data structure or size of recordings. Therefore, higher density audio channels are clearly the factor that drives up storage needs the most.