TiVo Community Forums Archives - VBR bitrates and lipsync on Series 1 Tivos in the US?

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VBR bitrates and lipsync on Series 1 Tivos in the US?
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Posted by: comdw

Over in the UK forum we've been experimenting with VBR (Save Disk Space option). For those who don't know, this feature doesn't work on a UK Tivo unless you manually modify the bitrates using Tivoweb resource editor (i.e. VBR&MAX bitrate are set to the same figure resulting in CBR).

Some of the people who have enabled VBR have audio/video sync issues when playing back their recordings, so we've got a couple of questions:

1) Do you guys get these lipsync issues when you enable Save Disk Space/VBR?

2) What are your standard bitrates for VBR as reported by tivoweb or the tvlog (VBR and MAX bitrate figures)? This might help us determine what bitrates to use to try and overcome the sync issues.

ccwf has already reported your CBR rates to be: 1630000, 2415000, 3202500, and 5215000bps, but we need to know both figures for VBR.

In the UK the standard CBR bitrates are 1700000, 2760000, 3660000 and 5960000 - all slightly higher than in the US probably because of our bigger vertical resolution (576 instead of 480).

The UK thread is here .

Posted by: pokegol

I record everything at medium which I modified to 2150000/2500000 using Tivoweb. I think the default was 2415000/2760000. I do have Save Disk Space turned on. I don't have any lipsyncing issues, none that I can detect anyway.

Hope that helps.

Posted by: comdw

Thanks pokegol, that seems correct - I found what I think are the default on another webpage:

Best: 5215000/5960000
High: 3202500/3660000
Medium: 2415000/2760000

Notice that VBR bitrate = MAX bitrate * 0.875. I'm wondering if this is a magic number that should be used to avoid syncing issues. In the UK we've been experimenting with numbers much further apart like 5960000/7836000. I think this might cause too much fluctuation in the bitrate and lead to the sync problems.

I did some bitrate analysis (graphs etc) here if anyones interested.

I'd still like to know if anyone else has ever seen audio/video sync issues on their Tivo when using Save Disk Space option?

Posted by: ccwf

quote:
Originally posted by comdw
ccwf has already reported your CBR rates to be: 1630000, 2415000, 3202500, and 5215000bps, but we need to know both figures for VBR.

Actually, those are the VBR target rates. The max/CBR rates are apparently the same as the UK defaults.

Any ideas how off the lipsync is? I've never noticed any lipsync issue here in the U.S. (and don't recall anyone complaining about it); but if it's just off a single frame, it might be subtle enough to have gone unnoticed.

If the problem is indeed significant, this could explain why TiVo didn't enable VBR on UK TiVos.

Posted by: HDTiVo

Anyone over there know if VBR can be used to record to DVDs? I think it can and is for commercial disks. If TiVo were to implement it for its DVDR combo units what might be the effects on recording quality and burn time?

What other issues might this bring up?

Thanks.

Posted by: comdw

ccwf - The lipsync problem is quite noticeable if you use a MAX bitrate that is alot higher than the VBR bitrate (e.g. we were trying figures like 5960000/7836000). Its enough to annoy you a bit but never gets really bad to the point where its unwatchable.

The good news is that with settings of 5215000/5960000 I haven't seen the lipsync issues (not yet anyway). I suspect that it probably is still there but not enough to notice unless you were really looking for it. The key seems to be to keep the VBR bitrate at 87.5% of MAX bitrate.

HDTivo - The DVD standard allows VBR as far as I know, but do the DVDR combo units support VBR? I heard that the Save to Disk Space option was only available on the Series 1 units.

Posted by: HDTiVo

According to the spec sheet from Pioneer, 810 supports VBR. However there is no indication that it is implemented and the specs only mention the 4 modes we all know and love.