Early ISA bus sound cards were half-duplex, meaning they couldn't record and play digitized sound simultaneously, mostly due to inferior card hardware (e.g., DSPs). Later, ISA cards like the SoundBlaster AWE series and Plug-and-play Soundblaster clones eventually became full-duplex and supported simultaneous recording and playback, but at the expense of using up two IRQ and DMA channels instead of one, making them no different from having two half-duplex sound cards in terms of configuration. Towards the end of the ISA bus' life, ISA sound cards started taking advantage of IRQ sharing, thus reducing the IRQs needed to one, but still needed two DMA channels. Many Conventional PCI bus cards do not have these limitations and are mostly full-duplex. It should also be noted that many modern PCI bus cards also do not require free DMA channels to operate.