The best replacement for aja is blackmagic.

The best alternatives to aja are rated as following:
★★★★★  blackmagic
★★★  matrox
★★  decklink
★  gaucho
★  black magic

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Speaking about aja, one probably shouldn't forget about davinci resolve.

Other search terms related to aja and their ratings are:
★★★★★ davinci resolve
★★★ avid
You have in mind a companion for aja and it is not yet here? Propose!



Google Trends

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aja
blackmagic
matrox