Changing bitrate & resolution (Mode 0)

sanderton · 08-21-2003, 10:16 AM

I know you can change the bitrate and resolution used by the MPEG chip on US stanadalone Series 1s.

Has anyone tried this with a UK machine?

The resolution settings for US machines are well understood, but do we know what the numbers are for ours?

Sneals2000 · 08-21-2003, 11:09 AM

Quote:

Originally posted by sanderton
I know you can change the bitrate and resolution used by the MPEG chip on US stanadalone Series 1s.

Has anyone tried this with a UK machine?

The resolution settings for US machines are well understood, but do we know what the numbers are for ours?

I was interested in this as well - as I find the horizontal subsampling the most annoying thing about Tivo (especially the chroma) However ISTR that whilst people have had a good level of success modifying US Tivos to run at 720 samples horizontally, when this was tried on UK models there were problems with positioning and green bands appearing to one side of the picture?

I think that people HAVE managed to improve the picture quality at the standard resolutions by increasing the bit rate though.

mrtickle · 08-21-2003, 01:10 PM

I did a lot of experimenting with this last week as it happens! Knowing that I was going to replace the drive I had nothing to lose!

Assuming you have tivoweb of course and know how to use the resource editor (press return to submit each and every change you make; select the "update resources" link after doing them all). I measured these resolutions (*):

0 = 720x576
4 = 544x576 "best"
2 = 480x576 "high"
1 = 352x576 "basic"/"medium"

Selecting resolution "0" (I modified my "high" setting as I don't use it much) did have the problems you mentioned. Basically the picture is offset to the left quite a bit and up a little bit. But the increase in resolution was certainly noticeable.

The other main thing I noticed was that we don't have VBR. Even with "save disc space" turned on, tivo still doesn't use VBR mode. I'm suspicious that this is because both the "DBSBestVBRBitrate" and "DBSBestMAXBitrate" are set to the exact same value (for example Best quality with Sky digital lineup). When you start a recording the tvlog says "using CBR, bitRate=5960000, maxBitrate=5960000" nomatter whether you have Save Disc Space on or off. If you then set the VBRBitrate to a different (lower) value, tivo reports that it is using VBR mode in the logs.

I also changed the live buffer to Medium

There is also this thread:
http://www.tivocommunity.com/tivo-vb...hlight=bitrate

I set my "Basic" to 800000 and 850000 values (NB: this made it use VBR mode, even in Basic!). It looked fantasically awful - like old AVI files where the colour disappears and you get blotches of grey. It also made the tivo say I had 314 hours of Basic quality on my 138GB drive.

HTH

sanderton · 08-21-2003, 01:16 PM

Interesting; I will have a play.

If VBR is not set up right (It's the AltBitrate settings for VBR mode isn't it?) then are those who report lower quality suffering from a placebo affect?

(Personally I've never been able to tell the difference so I feel vindicated!)

I only use Basic for Radio, so will se how low you can go!

mrtickle · 08-21-2003, 01:31 PM

Quote:

Originally posted by sanderton
Interesting; I will have a play.

If VBR is not set up right (It's the AltBitrate settings for VBR mode isn't it?) then are those who report lower quality suffering from a placebo affect?

To answer the question - dunno

But I don't think that the AltBitrate page controls VBR. You'll see two values for each quality setting - I think the one with "VBR" in it controls VBR.

My suspicion is that the Alt page is for the 2nd tuner on DirecTivos. Annoyingly, all guides seem to say "change them all just in case" without any more discussion

On my machine I didn't touch the Alt page - and I only edited the DBS values on the main Bitrates page. This was after trial and error of setting all 6 possibilites (DBS, CAT, Roof in Bit and Alt) to slightly different values and seeing which values made it into the logfile!)

Quote:

(Personally I've never been able to tell the difference so I feel vindicated!)

Indeed. I set Best from 5960000, 5960000 to 5660000, 5960000. This made it start to use VBR. An hour of Best went from 2580MB down to 1800MB (reported sizes from tivoweb) and I couldn't really tell unless I looked very hard that it was different.

Quote:

I only use Basic for Radio, so will se how low you can go!

Yes that was the idea of the chap on the other thread

. I noticed that even on the Sky digibox radio screens, where the only thing that changes is the time - once a minute - it takes a very long time to settle down to a clear image!

sanderton · 08-21-2003, 06:25 PM

OK, I've done a bit of fiddling!

Here are some stills of the results .

They are quite large, I'm afraid; the page is ~2Mb.

Here are the Sky red dots in the two resolutions:

To do this I reset Medium resolution to be resolution = 0 and upped its bitrarte to 7000000.

Although the screengrabs show a green area on the right, it does not appear on my TV. I see a perfectly normal looking picture. It seems from comparing the captions that there is a slight left shift - but it's not noticeable unless you are looking for it.

You can also see from the artifacting on the caption that the bitrate increase has had a noticeable effect.

The "real" resolutions of the two (ie, the picture area, not black or green bars) are 533 x 568 in Best and 677 x 568 in my edited Medium, so the resolution increase is real!

I may have to lash out on another 120Gb drive to allow me to up the bitrate and res!

mrtickle · 08-21-2003, 06:53 PM

Cool, glad you got it working! I can only just see the green bar on my TV too - although I do have the overscan reduced to minimum.

Brownedger · 08-22-2003, 03:53 AM

Are you saying that VBR doesn't actually work on the normal Tivo's or just the upgraded Turbonet Tivo's?

sanderton · 08-22-2003, 04:23 AM

All TiVos.

Brownedger · 08-22-2003, 06:32 AM

Ok is it working in the latest American release ver.3.0?

sanderton · 08-22-2003, 07:26 AM

I assume so. To be honest it looks like a mistake by the people rolling out the UK version - the configuartion file variables just look like they are set wrong, with the max and min bitrate set to be the same!

I'm going to try boosting Best by leaving the minimum at the current CBR level but giving it the option of adding maybe 1Mbps at peaks to see if that helps it with fast moving sports.

NickDvl · 08-23-2003, 06:44 AM

For us lesser mortals, how many hours of recording time can you get out of those higher bitrate/resolution settings on a 120GB drive? Are there any other side-effects besides the green strip on the right? Do the settings survive a reboot?

I've always found the horizontal resolution somewhat lacking on widescreen broadcasts, would be great if it could be improved...

LJ · 08-23-2003, 07:56 AM

Is there a list of the resolution modes somewhere? Is there a mode 3?

I tried a search of the forums and read the Philips datasheet but couldn't find anything.

Mode 0 does look much better, but on my TV the 'wide screen' mode chops a bit too much off the left hand side for it to be useable.

NickDvl: Yes, the settings survive reboots - you have to reboot after you've changed the settings to make them take effect.

What's the lowest bitrate anyone's managed to get working? Down at around 600000 I get a "kb per second too small" message in tverr (and other errors in the kernel log) and the rate appears to default to some higher value.

mrtickle · 08-23-2003, 10:15 AM

Note that changing the resolution doesn't alter the recording capacity, only changing the bitrate does. When I changed my high from res 2 to 0 the reported capacity was the same, because the bitrate was the same. The result is that the same amount of movement will produce more artefacts than before unless you increase the bitrate as well.

According to the other thread I found tivo defaults to Best quality if the bitrate is too small. When it happened to me it basically lied in the tvlog (said it was using the bitrate I had set, but didn't). I didn't think of checking the other logs!
650000 was too low when I tried. 675000 worked.

Dapper Dan · 09-09-2003, 08:06 AM

Quote:

Originally posted by sanderton
I assume so. To be honest it looks like a mistake by the people rolling out the UK version - the configuartion file variables just look like they are set wrong, with the max and min bitrate set to be the same!

I'm going to try boosting Best by leaving the minimum at the current CBR level but giving it the option of adding maybe 1Mbps at peaks to see if that helps it with fast moving sports.

Have you tried this yet ? I'd be interested to know how you got on.

As an aside, I was reluctant to up my bitrate in case I caused the mpeg chip to overheat, though for all I know it may be operating well within it's design limits. Can anyone comment ?

rharnwel · 09-09-2003, 08:17 AM

Quote:

Originally posted by Dapper Dan
Have you tried this yet ? I'd be interested to know how you got on.

As an aside, I was reluctant to up my bitrate in case I caused the mpeg chip to overheat, though for all I know it may be operating well within it's design limits. Can anyone comment ?

Surely the higher the bitrate, the less demand on the MPEG chip....?

sanderton · 09-09-2003, 09:13 AM

I'd be surprised if you could damage the chip from a software setting.

Dapper Dan · 09-09-2003, 10:34 AM

I was thinking of it as a form of overclocking. I'm sure that someone will correct me if I'm wrong, but if you make a chip work harder it runs hotter, this in turn increases the rate of electron migration which eventually causes the chip to fail.

iankb · 09-09-2003, 10:55 AM

But, as rharnwel says, the higher the bitrate, the lower the amount of work required to compress it.

Ian.

sanderton · 09-09-2003, 11:02 AM

The chip is good for up to 15Mbs so, I doubt I'll stress it at 7. The HD is another matter!

http://products.sel.sony.com/semi/PDF/CXD1922Q.pdf

Dapper Dan · 09-09-2003, 11:04 AM

Quote:

Originally posted by iankb
But, as rharnwel says, the higher the bitrate, the lower the amount of work required to compress it.

Ian.

I'm sure that you're both right, it justs seems so counter-intuitive that you should be getting higher quality for less effort, surely you have to pay somewhere along the line ?

Dibblah · 09-09-2003, 11:41 AM

Quote:

Originally posted by Dapper Dan
I'm sure that you're both right, it justs seems so counter-intuitive that you should be getting higher quality for less effort, surely you have to pay somewhere along the line ?

Disk space.

Cheers,

Allan.

sp99 · 09-09-2003, 11:55 AM

Quote:

Originally posted by mrtickle
I did a lot of experimenting with this last week as it happens! Knowing that I was going to replace the drive I had nothing to lose!

Assuming you have tivoweb of course and know how to use the resource editor (press return to submit each and every change you make; select the "update resources" link after doing them all). I measured these resolutions (*):

0 = 720x576
4 = 544x576 "best"
2 = 480x576 "high"
1 = 352x576 "basic"/"medium"

Selecting resolution "0" (I modified my "high" setting as I don't use it much) did have the problems you mentioned. Basically the picture is offset to the left quite a bit and up a little bit. But the increase in resolution was certainly noticeable.
HTH

Broadcast spec for a composite signal at 720 x 576 only has 702 visible (active ? ) lines with the rest being used for VBI, windscreen switching etc..
- I can't remember for component signals but maybe Tivo have used this as their guide.

It could be worth a try .

p.s. have lurked for a year, so thanks to everyone for your informative posts.

sanderton · 09-09-2003, 11:59 AM

The resolutions are all either DVD or DVB standard resolutions or bastardised versions of them (horizontal form one and vertical from another), so presumably are influences by the encoder/decoder chips options.

In the USA TiVos tend to be 480 x 480 I think.

mrtickle · 09-09-2003, 06:59 PM

The USA series 1 tivos have exactly the same horizontal res as the figures above, but with "480" everywhere I put "576".

As far as DVD goes, I believe both 720x576 and 702x576 are valid. I hear what you say about VBI but in the case of the UK tivo the shift up/left is way off. Presumably all the other settings to define the screen were left at the US default timing values, which is fair enough as this resolution isn't available to the user normally so would never be used.

HTH

LJ · 09-10-2003, 01:33 AM

Originally posted by mrtickle
Presumably all the other settings to define the screen were left at the US default timing values, which is fair enough as this resolution isn't available to the user normally so would never be used.

Wonder if we can change them... Resolution 0 looks better than res 4 (unsurprisingly!).

xxxx · 09-10-2003, 03:35 AM

An interesting topic.
Could those who have experimented give a full list of settings that they think look best for RGB widescreen on a UK Tivo? Disk space no object. With and without VBR would be nice.

On the odd occasions when I have experimented with this (in order to get recordings that are directly compatible with DVD resolution) I have become bogged down with settings that only work on US Tivos. In the end I gave up.

Banned uses aside, I would really like to improve on the UK Tivo "best" setting for day to day recodings as I can clearly see the quality drop from live and have plenty of disk space. I suspect that I am not alone.

Dibblah · 09-10-2003, 04:19 AM

Unfortunately, one thing that MPEG doesn't handle too well are sudden transitions. With the green bar running down the side of the screen, you have a constant "drain" on available bitrate.

Unless we can tune the parameters of the SA7118E, the 0 resolution is mostly useless.

Oh. And yes, it's easy to tune it - However, on every channel change it gets reset. I was looking a while back for a 'portable' way to effectively copy PALMod (Tridge's), but got lost too quickly.

Cheers,

Allan.

sanderton · 09-10-2003, 04:55 AM

I don't think the green bar has any effect on anything. It seems to just appears on screen grabs and in PC video editing software which is aware what the pixel size of the image is! Certainly it does not appear on my TV. The sharpness of the transition from black to green in the still suggests to me that it's not a part of the MPEG encoded signal, just an artifact of the display software.

The main problem with Resolution 0 is that a part of the far left of the screen is not encoded at all. This appears to be a pretty small part, if you look at the pics I posted above, but you will always have that trade off.

The MPEG chip is capable of far higher bitrates than TiVo uses, but with a recording stream and a playback stream happening simultaneously you get very close to the limits of the IDE/HD infrastructure.

LJ · 09-10-2003, 05:41 AM

Originally posted by Dibblah
And yes, it's easy to tune it

Yeay!

However, on every channel change it gets reset.

Ah

I was looking a while back for a 'portable' way to effectively copy PALMod (Tridge's), but got lost too quickly.

Have you got a link to Tridge's site handy? How do you mean portable?