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I don't know any really free AC3 encoder with any halfway complex psycho-acoustic model (similar to e.g. the one the LAME MP3 encoder uses). All the more complex AC3 encoders I know are commercial. So regarding freeware, Aften is still the best we have. And I did not say it is bad; it just could be better, and you may hear the limits when the bitrate gets sparse - like only 384 kbps for 5.1 audio, or even less.
640 kbps is not part of the DVD Video specs; there may be DVD authoring tools which reject such streams, there may be DVD players which contain AC3 decoders which are limited to at most 448 kbps.
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