Just because your native media has a low bitrate does not mean that you should necessarily transfer to a low bitrate codec for editing (especially if the media you create is going to be used in your final output). You will still loose more data by recompressing to a low bitrate DNxHD codec than a higher one. Your only considerations should be how much drive space you can afford for the project and how good the footage needs to look at the end (ie where will it be delivered?). If you are short on space but want good quality footage because it will be broadcast or shown on good screens and not just on the web, then you should think about an online/offline workflow. Transcode all the footage to 36, make your edit and then relink the sequence back to the AVCHD footage, then export from that or transcode just the sequence to a higher bitrate.
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