Recording Digital TV broadcasts

[sjgore]

Hi,

I'm new to SageTV, and I was wondering if DVB broadcasts are recorded as-transmitted, ie. does SageTV just record the transmitted (MPEG) signal to disk without encoding, or does it encode it again?

In the sage.properties file, I found a setting called:
mmc/encode_digital_tv_as_program_stream=true

Does this mean what I think it means, that SageTV will ignore recording quality settings for digital TV, and just record it as-broadcast? I'm not referring to whole transport streams, just individual channels.

Thanks!
Steve.

[GKusnick]

Yes, that's correct. There's some processing involved in stripping out unwanted subchannels and such, but otherwise the digital content is recorded as broadcast.

[thborchert]

Hi,

while we're at this, two more questions, if I may:

1. I notice the file size per time recorded various wildly from channel to channel. Do channels broadcast in such different quality? How come it is not obviously noticable while watching, what with a factor of 2 or more in file size difference?

2. I have both a DVB tuner and an analog tuner in my Sage PC. What I find now is that the analog files are much bigger than the DVB files with my recording quality setting of "DVD Long Play". This of course begs the question: Do I waste HD space by recording at a quality that the analog broadcast doesn't really support? My very subjective impression was that picture quality improved when I switched the analog quality from Extra Long to just Long - and I also think I can see that digital recordings are even better in quality. However, the file size makes me wonder. Thoughts?

Thanks!

Thomas

[ke6guj]

Originally Posted by thborchert

Hi,

while we're at this, two more questions, if I may:

1. I notice the file size per time recorded various wildly from channel to channel. Do channels broadcast in such different quality? How come it is not obviously noticable while watching, what with a factor of 2 or more in file size difference?

Yes, different channles broadcast in different qualities. Some channels broadcast in 720P and others do 1080i, and that can show up as one twices as big as the other.

Plus, if a station reduces the bandwidth usage of a channel by reducing the quality, then they can use the unused bandwidth to send out a second or even third channel within the station's total bandwidth space.

[stanger89]

Originally Posted by thborchert

Hi,

while we're at this, two more questions, if I may:

1. I notice the file size per time recorded various wildly from channel to channel. Do channels broadcast in such different quality?

IMO it's a mistake to arbirarilly associate "quality" with "bitrate", but yes, different channels may well use very different bitrates at different times.

How come it is not obviously noticable while watching, what with a factor of 2 or more in file size difference?

My guess would be that it depends on the type of content. Some things are much easier to compress than others, thus they can have similar quality with drastically different bitrates.

2. I have both a DVB tuner and an analog tuner in my Sage PC. What I find now is that the analog files are much bigger than the DVB files with my recording quality setting of "DVD Long Play". This of course begs the question: Do I waste HD space by recording at a quality that the analog broadcast doesn't really support?

The only real answer is you have to experiment and see for yourself.

That said, I'd say no, you're probably not wasting space using a higher bitrate. The broadcast has two things going for them you don't, first they have pristine soure material for their encoders to work with, which makes encoding much more efficient.

Second, they have encoders that cost, probably on the order of 1000x more than ours, as such their encoders can squeeze a lot more out of an encoding with a lot less degridation than our encoders can.

My very subjective impression was that picture quality improved when I switched the analog quality from Extra Long to just Long -

I think that answers your question.

and I also think I can see that digital recordings are even better in quality.

Probably, MPEG encoding is a lossy opperation, each time you encode video you introduce more and more loss. Futher, as noted above, our encoders aren't near as good a what the broadcaster's use, and we have to deal with transmission losses (including playback and connection to the PVR card).

[thborchert]

Stanger,

thanks, that makes perfect sense. I'll experiment some more. I'd be interested to know what settings others use with analog SDTV and Hauppauge encoders.

Thomas