[mythtv] IVTV VBI reading

[Bruce Markey]

Hans Verkuil wrote:
> On Friday 26 January 2007 21:07, Bruce Markey wrote:
...
>> which is set from mythtv-setup General, second page. This turns
>> off VBI for all cards on all hosts regardless of type and disables
>> Closed Caption for all frontends even if the information is already
>> stored in the recordings.
> 
> That's the one. In the end it turned out that this setting is off on a 
> new installation so there's no problem. I was told that it was on 
> initially, but that was obviously wrong information.

Okay. My concern that I discovered is that this affects both
the frontend and the backend. This should be decoupled and
possibly the backend settings could be per card type, per card
or per input.

>>> It can record it perfectly,
>> I disagree.
> 
> I think that bttv will get a better picture because as you correctly say

My terse point wasn't that bttv, ivtv, myth, mpeg4, mpeg2,
ffmpeg or anything else is perfect but that working around
issues is not the same as perfection.

>>>  but scaling does introduce slight amount of
>>> ghosting. Whether this is a driver, firmware or hardware bug is
>>> something that I need to look into one of these days.
>> This seems to be a severe issue if only the extreme maximum
>> resolution is considered to be acceptable. A 720x480 recording
>> file with the same relative bitrate as 480x480 would be 50% larger
>> for no (or very little) visible difference.
> 
> To put this into perspective: the driver has been around for several 
> years and I have had only a handful of complaints about this. I'm 
> pretty sure it can be fixed somehow but it's simply not high prio at 
> the moment.

1) I didn't have your email address 2) I'm complaining now and
3) I could assemble a lynch mod if that's what is needed ;-).
There was another thread on these lists about ghosting so there
have been at least ten messages on this issue just this week.
If the response is always that you must record at 720x480 then
people will do what they have to do but that doesn't mean that
there isn't a problem.

What if I were to tell you that you could create files two-thirds
the size with no visible loss in quality? That your current disks
could hold 50% more hours of recordings and that there would be
less network traffic for remote playback, lower decode CPU time,
faster file copy, faster deletes, less I/O contention, fewer
prebuffer pauses, faster commercial flagging, faster transcoding
or no need for transcoding, etc.? I think everyone would like that
even if they are not complaining now. I think a slick marketing
department could put out an impressive press release about the
vast improvements of the new version if this were fixed =).

>>> The default bitrate
>>> set by ivtv is sufficient for good quality encoding at that
>>> resolution.
>> Now you've peaked my curiosity. What is this default bitrate
>> and why is it a good choice?
> 
> Let me see: VBR 6Mbps, peak 8Mbps. Audio 224 kbps. I believe these 
> values were originally derived from the Windows driver, and I've never 
> had any complaints :-) According to Video Demystified MPEG-2 was 
> targeted for broadcast-quality video at bit rates of 4-9 Mbps, so ivtv 
> is within that range. Nothing terribly scientific, I'm afraid. 

Good answer. I recall that early versions used 16000 as the
peak because this was the max and 8000 as the average I guess
because that was half the max. The myth default resolution was
480x480 so this was overkill by any measure. At the time, most
people assumed that the default must be right and would complain
about how big hardware encoded files were (this is before any
HDTV). I changes the myth defaults to 4500/6000 at 480x480 and
this wasn't terribly scientific either. I did some test recording
to find a bit rate that would create hardware encoded files of
approximately the same size as my software encoded files at the
time. 4500/6000 at 480x480 is similar to 6000/8000 at 720x480
but a slightly higher relative bitrate. However, I suspect that
many people use 4500/6000 at 720x480. This is perfectly acceptable
but may have some artifacts with higher motion like sporting
events.

In my myth profiles, I set LiveTV to a slightly lower 432x480 but
a higher bitrate. This keeps the file size (processing, throughput)
down but does an even better job with high motion and I use this
profile for sports. I set the Low Quality profile for 'talking
heads' news, old b&w movies and animation. Animation is easy to
compress and using 352x480 with a bitrate on the order of 1800,
I get files of about 3/4GB/hr that look perfectly fine. This is
another area where being locked to 720x480 only would be a little
disappointing.


While googling for info about the sampling clocking for bt8x8
I found this interesting article and thought I'd pass it along.
While this isn't the article I was looking for and doesn't have
the info I was actually seeking, it has gads of info about
sampling rates, resolutions and capture sizes.

http://www.doom9.org/index.html?/capture/sizes_advanced.html

--  bjm