48 kbps AAC Encoders Test - Q1 2006 Edition
Reply #53 – 2005-12-15 14:01:20
So you are pruposing a 32 kbit/s test, right? [a href="index.php?act=findpost&pid=350362"][{POST_SNAPBACK}][/a] At all. I just wonder about the choice of testing CBR or ABR -- or any controlled bitrate allocation encoding mode -- over VBR. The arguments I saw don't make sense to my mind. I could understand why CBR/ABR is privileged over VBR for low bitrate encoding: bitrate must be controlled for streaming. But I don’t know any form of internet connection which could handle CBR 48 kbps but not VBR 48 kbps. Either you have a 56K or less, or you have 128K or more. I don’t know anything between. With 128K, you can safely stream 48K with huge bitrate peak. With 56K, streaming 48 kbps file, even at constant bitrate, is unlikely. VBR could be discarded for the sake of “predictable size”. It’s indeed interesting for DVD backup (you proposed it, IgorC). But if the purpose of the test is DVD ripping, it wouldn’t make sense to use 44.1@PCM samples when all DVD are using 48KHz sampling rate and are rarely providing LPCM stereo tracks (but rather lossy encoded signal). A “DVD backup listening test at 48 KHz” would interest many people, but then, the chosen samples should obviously be in relation with the object of the test: several speech samples (with and without music), 48 KHz, high dynamic (both loud and very quiet samples) and last but not least AC3 and/or DTS samples as source. We can’t use the worst samples of previous listening tests as Ivan proposed it. Both arguments (streaming and DVD backup) are going against either the selected bitrate (48 Kbps vs streaming) or the possible application of the test settings (DVD Video ripping vs samples coming from CD Audio). We need some coherence. Are we looking for the best AAC setting in this pre-test? If the answer is positive, then it won’t make sense to discard VBR. Or does someone suddenly dispute the well-known fact that VBR is better than ABR/CBR? If not, if VBR encoders are available and if we’re starting a pre-test for 48 kbps encoders/profile, then we should at least give a try to existing VBR encoders. If we have to discard one encoder/setting (e.g. to limit listening fatigue), it would rather be a CBR one because they’re well-known to provide inferior quality to VBR encoding mode. ABR/CBR are maybe better than VBR at this bitrate, but if someone is tempted to claim this, he should prove it (the burden of the proof lies on people making unusual claims, and VBR<CBR is clearly unusual; so it must be proved – cf. TOS#8). Therefore, I can't see any valid reason to discard VBR from such test.