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Subject: RE: Quick questions
From: BlueChip (cs_bluechip_at_webtribe.net)
Date: 2003-10-20


>Hi BlueChip,
>
>I don't want to start a flame on the best stereo mode.

No me neither, audio appreciation is entirely subjective.
I hope I did not sound aggresive in my writing. If I did, I'm sorry, it
was not intended that way :)

>I was just interested if I had something wrong in mind (I didn't look it
>up before I wrote my
>message).

Precisely what I did - posted what I "knew" and then read your
contradiction - as I have no idea where my original "knowledge" came from,
I just did a search and posted all the interesting stuff I found.

> I will comment on your links in the following if you don't mind.

Please do, I would certainly like to be corrected if I am wrong :)

>BlueChip wrote:
> > http://www.geocities.com/altbinariessoundsmusicclassical/vbr-e
> > ncoding.htm#ch11 "Joint Stereo looks for signals that are identical
> > in the left and right channel and if it finds any they are encoded as
> > mono"
>
>I found that one, too. Sounds like a non-techie explanation of encoding sum
>and difference to me.

hmmm, maybe - not sure on that...

> Signals that are really completely identical in the
>left and right channel, even for the short time of a frame, don't exist
>(except you copy one channel digitally)
>
> > with regard to the post on "you do not lose quality"
> > === THIS IS NOT TRUE AT ALL ===
> > Whatever you may read from the net - I can personally assure you
> > otherwise. And Yes, I HAVE been put through the "blind test"
> > procedure by two of my friends now.

ooohhhhh, that DID sound a little OTT didn't it - sorry, I was just
covering my ar53.

>What encoder did you use? I read that some commercial xing encoder had a
>very poor joint stereo implementation in early times.

Good question - these tests were carried out a couple of years back, when
we (my peers and I) were all using Fraunhoffer.

This is JUST "my opinion", there may be others that agree ...I don't know?
I now use "LAME v3.93 -m s -b 160 -q 0" ...this gives me an acceptable
compromise between quality and compression.
Fraunhoffer is poor - the radium optimisations are 'allegedly' detrimental
- I think I originally read that at mp3proclub.
Xing is blindingly fast, and gives (imho) abysmal results

> > http://www.mediatwins.com/FAQ/mp3_stereo_modes.htm
> > "However, for some audio, Joint Stereo MS may create a 'flanging' or
> > 'swishing' effect...
>
>Yes, I read this one, too. It says "for SOME audio...". Of course, if you
>have extremely wide stereo signals or two mono signals, you gain nothing by
>using joint stereo. LAME automatically switches to normal stereo in theese
>cases, but they are very rare. In normal audio, you gain bandwith or quality
>by using joint stereo.

"Blue Oyster Cult - Don't Fear The Reaper" can give especially bad results,
as ironically, did one of my own recordings.

I suppose it was a few especially bad experiences, and all the flanging you
get on downloaded filez, that just said to me "don't take the risk" so I
just never use it any more.

> > http://www.mp3machine.com/discussion/messages/780.shtml
> > quotes William Schelter, Nils Faerber, Alexander Leidinger, Oct 13,
> > 2001: "Using mid/side stereo [MS] inappropriately can result in audible
> > compression artifacts. Too much switching between mid/side and regular
> > stereo can also sound bad."
> > ...which was given as a definition of Joint Stereo
>
>Again, I also found this site. It's not a definition of JS, it's just
>describes the switching between JS and normal stereo with difficult signals.
>The sentence which would come next in your citation is the real important
>statement:
>"To determine when to switch to mid/side stereo, LAME uses a much more
>sophisticated algorithm than that described in the ISO documentation, and
>thus is safe to use in joint stereo mode."

:) Another mighty fine reason to use LAME
...but I did not want to make the presumption that everybody uses LAME, and
also did not want to get into the "my compressor is better than yours"
argument.

> > http://www.iis.fraunhofer.de/amm/techinf/layer3/
> > "Joint stereo coding takes advantage of the fact that both channels of a
> > stereo channel pair contain far the same information. These stereophonic
> > irrelevancies and redundancies are exploited to reduce the total
> > bitrate." ...whatever that means
>
>That's what I wrote.
>If you have a normal stereo mp3 with 192kbps, you get 96kbps for each
>channel. If you use joint stereo instead, you can use (I'm just guessing
>numbers here) 160kbps for the mixed signal (sum of left and right). The
>difference between left and right are very small, therefore 32kbps is enough
>to encode it. The result is better quality for the main information.

hmmmmmm, you've got me wondering now - me thinks it's time for some new
experiments :)

> > I cannot find defitions that agree with ANYTHING I have read so far,
> > including my own post!!!
>
>The technique you were describing could work, but I think it's not what is
>known as joint stereo. All surround systems use the fact that deep
>frequencies can't be located easily and have only one subwoofer. However,
>the threshold frequency for this effect is in the range under 200Hz,
>therefore you'll practically gain nothing.

Yes, it all made sense when I first posted - but I must say that everything
you have written makes perfectly good sense.

>Andreas Stemmer

Hang Loose Andreas,

BC



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