[mythtv] Audio-upmixing questions regarding #5900

foo bar foobum at gmail.com
Mon Nov 17 20:04:26 UTC 2008


2008/11/17 Yeechang Lee <ylee at pobox.com>:
> So there is an actual sound improvement? How is this done? Does the
> upmixer actually take the Dolby 2.0 mock-surround sound cues and
> convert them into the appropriate 5.1 tracks through the respective
> surround speakers?

Yes, the upmixer (detailed at
http://www.hydrogenaudio.org/forums/index.php?showtopic=52235) will
use Dolby Pro Logic, DTS Neo:6 and other cues to turn stereo into 6
channels. Well, 5 channels ( the LFE  channel is just a low-pass
filtered average of the original channels). It also does a very nice
job of upmixing plain old stereo (to my ears at least).

> I'd have thought all upmixing non-5.1 audio would
> do is to just turn it into a Dolby 5.1 carrier signal (forgive me for
> not using the right terminology) but still with only stereo sound.

That's a different step: after upmixing we encode to Dolby Digital
(aka AC-3). There's not much point encoding to AC-3 unless we've got
more than 2 channels to encode.

> In terms of performance, my Pentium 4 3.0GHz frontend has trouble with
> upmixing 5.1 audio off HD recordings; the normal 55-70% CPU usage
> turns into 90-99%.

Could you clarify? Do you mean upmixing (is the original audio track 2
channel or 5.1)?

"a few cycles" was probably an understatement. You could try a less
expensive upmix method (active simple, passive). Perhaps we should
also allow configuration of the bitrate we encode at. All of the HD I
get has 5.1 audio already..


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