Necessity of video compression

baghera8 · #1 10-07-2003, 02:58 PM

Being new to the HTPC hobby, I have a few questions I would like to pose to the members of this forum. I apologize for the length of the posting, but I was attempting to be as succint as possible, while still conveying all the necessary nuances.
My questions mainly relate to the necessity of constant video compression for PC-based PVR functionality.

1) From reading postings on this and other HTPC forums, I get the impression that the SageTV (I am not trying to discredit this excellent piece of software in any way, merely using it for the sake of an example) Cable/SAT video stream is always compressed, even when viewing live TV, without using any of the pause/rewind functionality. What is the reasoning behind this, as I would think that there would be a substantial quality advantage to putting the video out uncompressed, unless, of course, compression is necessitated by pausing, forwarding, etc.?
I realize that the look of the video would be changed noticeably when switching from live TV to a recorded stream, but given a reasonable amount of calibration, this would be a minor drawback to obtaining improved video quality.

2) The first question leads me to ask the following: for a card employing an onboard MPEG encoder (such as the Hauppauge boards), is the encoding mandatory or can it be called by the PVR software on demand? Basically, I would like to know whether the stream obtained from the tuner/video-in inputs of the Hauppauge card can be viewed uncompressed, utilizing the onboard encoder module only when encoding is called upon (allowing for encoding of arbitrary content, not necessarily the current video input, essentially offloading some encoding-related processing off the CPU) , or is the video passed through the card automatically MPEG2 encoded? If the latter is the case, wouldn't having a powerful CPU and a standard tuner card be of a huge advantage to empoying onboard MPEG encoding, as it (at least theoretically) allows for uncompressed video?

3) If, indeed, it is possible to utilize the onboard encoder on demand, does the same possibility exist for the external USB tuners, such as the Hauppauge WinTV-PVR-usb2?
If, on the other hand, the external tuners are locked into putting out an MPEG encoded stream, wouldn't that constitute a major drawback for such units as compared to their PCI counterparts? Even if no current software is unable to display uncompressed live video, the possibility of such a feature in future versions of the software would obviously be of little use to anyone utillzing an external tuner with mandatory compression.

4) Some people on this forum have suggested utilizing the S-Video output of the external tuner units in order to avoid the processing performed on the stream by the PC hardware/software. If used in this fashion, isn't ALL the PVR functionality lost, as the external tuner is simply putting out the video received via its coax/composite interface? Or is the tuner actuall receiving the video stream to display from the PC via USB?

A big thank you in advance goes out to anyone who might be able (and willing

) to shed some light on the above points.

jmeeks · #2 10-07-2003, 04:51 PM

1) The content is always recorded to facilitate the PVR functionality. That is the default mode of operation and there is no other. I think the quality is great with SageTV. It looks better on the computer than the regular TV with a straight broadcast signal.

2) I think you can not obtain the raw stream on the Hauppauge cards. However with the provideo card and SageRecorder, I believe you are actually watching the raw stream in the LivePreview window of SageRecorder.

3) I'm not sure what you mean by "PCI counterparts". Like I said in 2 above, I believe with the right combination of card and software in some instances the raw stream can be accessed, I just don't think that capability exists with the Hauppauge products.

4) No. the S-Video out is outputing the Mpeg2 encoded content and with SageTV2 the OSD from SageTV with also be viewable via the S-Video out.

stanger89 · #3 10-07-2003, 05:59 PM

1) I agree with jmeeks. The quality through my WinTV 250 is better than direct to my TV. I've played with Dscaler and a BT878 card in the past and even though the 250's output is compressed it is noticeably better than the uncompressed stream from the BT878 card (that was a WinTV-PVR PCI - the original). Going uncompressed does not automatically guarantee better quality.

2) It is actually possible to view the raw stream from a Hauppauge 250/350. I've done it with graphedit, you just render the "Preview Video" pin of the PVR PCI II Capture filter. But jmeeks is right, the card is always encoding. However, as I said above there is little/no advantage to doing that. These cards are designed to encode constantly, and do a good job of it.

The problem with software encoding is that it take so much power that your computer is next to useless while recording. If you use your computer you risk dropped frames or lower quality recordings.

3) USB doesn't have the bandwidth for uncompressed video. USB2 might be close, but probably couldn't reliably.

4) dito

If you really want live uncompressed video you're best bet is probably Dscaler with something like a Flyvideo2000/3000. I just don't see any benefit, if the source were say DVD quality it might make a difference, but from the cable I've had experience with (both good and bad), most of the PQ problems I've encountered are in the source, not the MPEG encoding (unless you choose an anemic bitrate).

Crazedz · #4 10-08-2003, 03:24 AM

Ditto the quality on hardware tuners is every bit as good.

baghera8 · #5 10-08-2003, 05:52 PM

Thank you all for your responses to my questions.
Many of my concerns have been answered, but I am afraid that I didn't make some of my points as clear as I had hoped.

1) "It looks better on the computer than the regular TV with a straight broadcast signal." "Going uncompressed does not automatically guarantee better quality." I am perfectly aware of the above facts. However, by definition, the output of any lossy compression method is inferior to the input. Now, additional filters (such as the Dscaler, etc.) may improve the quality of the signal well above that of the original [B]unprocessed[B] video, but properly applied filters on the uncompressed stream should always produce output that is superior to a filtered compressed stream. Clearly, the question arises as to the magnitude of improvement, and whether or not it can be detected by the viewer. I can safely say that, to my eye, very few real-time MPEG-encoded streams are completely free of compression artifacts. If the possibility exists for viewing a stream uncompressed, and there's no need to store said stream, I will go for the uncompressed source without hesitation. One of the reasons I have posed the original question was the fact that I am considering the Hauppauge cards, and even though the current software might not support uncompressed live viewing, I would like to at least consider some cards that do allow for it, as a step toward future-proofing my system to some degree. A system that can do both compressed and uncompressed video definitely allows for more choices down the road. Another reason for my preference for an uncompressed stream is the greater availability of real-time filters that readily work with such streams.

2) "The problem with software encoding is that it take so much power that your computer is next to useless while recording. If you use your computer you risk dropped frames or lower quality recordings." A 2.5Ghz+ P4 can easily handle real time software encoding without dropping a single frame. Obviously, the previous statement does not hold true if you are in the middle of an Unreal Tournament deathmatch.

As related to point 1 above, my preference for software encoding stems from the fact that it can be applied on demand. I would be very interested to see a hardware encoder allowing for on demand encoding of an arbitrary stream, but, my searches for such a beast have been fruitless.

3) "USB doesn't have the bandwidth for uncompressed video. USB2 might be close, but probably couldn't reliably". I apologize for not being more specific in my use of the term "USB". My original question was in reference to USB2 (hence the use of WinTV-PVR-usb2 as an example), which I believe has more than enough bandwidth (480Mbps) to handle uncompressed video at resolutions well above 720x480.

4) "No. the S-Video out is outputing the Mpeg2 encoded content and with SageTV2 the OSD from SageTV with also be viewable via the S-Video out." I find this to be a very interesting point. Does this mean that an external tuner such as the WinTV-PVR-usb2 can be configured to display arbitrary video from the PC via its S-Video output? For example, would it be possible to display a myHTPC menu on my TV via the device? If only certain data (such as the OSD) can be displayed via the unit, wouldn't I have to switch my TV from one input to another if I were to, say, browse the web after watching some live TV?

The FlyVideo cards definitely merit consideration, as I have heard very good things about their tuner quality. However, without a hardware MPEG encoder, they are not a viable choice for my Epia M10000 based system.

I hope the above comments have shed some light onto the reasoning behind the original questions.

stanger89 · #6 10-08-2003, 06:25 PM

1) Yup, you're absolutely correct, you lose something when you use lossy compression. I currently store all my music as lossless ape since I can't stand mp3 compressed music, so I understand where you're coming from. The best way to watch raw-live video is with Dscaler, period. I doubt any app will catch up with Dscaler in the near future. With an Epia you could get a FlyVideo for Dscaler viewing of live TV, and get a WinTV-PVR-USB2 for your SageTV duties.
I was just trying to make the point that while raw video is theoretically better,

2) The reason I made the comment about software encoding is this: I've got a P4 2.4b with 512Gb ram and a Radeon 9500, plenty to record TV with software. The problem is, while standard TV requires very little power, HDTV maxes out my system. I don't want to have to wait for a recording to finish before I can watch something. With a hardware encoder I don't have to worry about the recording if I want to do something CPU intensive, like watching HDTV.

3) I realize USB2 is rated at 480Mbit/s, and uncompressed video is under 400Mbit/s but I'm not sure how efficient USB2 is or if there would be latency problems.

4) PVR-USB2 doesn't have an S-Video out (in only). The PVR-350 however has a hardware MPEG decoder that outputs S-Video. Currently only video is output by the 350, but Jeff/Dan have told us that with V2 the entire SageTV UI will be able to be output by the 350. I know of no other app that can output a UI through the 350, so yes you would have to switch inputs on the TV to browse the web (unless someone makes a web browser plugin for V2, then that might be displayable through the 350).

If you've got any more questions just ask, I'll be glad to answer any I can.

p.s. Has anyone else noticed Jeff/Dan have been a little scarce lately? Maybe they're getting V2 ready for us to try yippie.

Narflex · #7 10-08-2003, 07:47 PM

You guys are doing a great job of answering a lot of the questions now...so we haven't need to post as much as we used to.

jmeeks · #8 10-08-2003, 09:24 PM

baghera8,

Based on your comments in 1 and 2, are you sure you want a PVR? That's the whole point of not subscribing to your position of "If the possibility exists for viewing a stream uncompressed, and there's no need to store said stream, I will go for the uncompressed source without hesitation." If you don't have a need to store the stream why wouldn't you just watch TV? Why bother with a PVR?

To get PVR functionality, you have to store the stream, and that's what the basic function of SageTV is a PVR.

Crazedz · #9 10-09-2003, 05:42 AM

Actually i kinda see what he's getting at i think.

In the past i have done some recordings that i wanted to recompress before i had gotten a dvd burner i wanted to get some recordings out to cd or vcd and wanted them to look as good as possible. So i did some checking on the web (after some failed atempts to get a good enough looking end video that would fit on a cd) and from other sources that all said if you want your end recording to look as good as possible then you should make sure it was recorded in as high a quality as possible or uncompressed that way when you recompress it you will come out with a better end video than if you had started from a lower quality compressed recording.

I think thats part of where his concern lies over being able to record some shows looking as close to the orriginal as possible so they look better when recompressed down later, though i could be wrong on that.

His concerns about getting an uncompressed stream remind me of mine early on before i got a hardware tuner and a dvd burner. So i thought that thats probably at least part of why he's concerned about getting an uncompressed stream.

At anyrate baghera8 if im wrong on that then just disreguard my ramblings but if you could more clearly outline the reasons why you are concerned about getting an uncompressed stream then maybe we could more clearly address them. Because i myself am kinda at a loss otherwise as to why you would want to record an uncompressed stream otherwise given the amount of space that can consume on a drive.

baghera8 · #10 10-09-2003, 02:43 PM

Stranger89, you've made some excellent points on the potential problems arising from pushing your system to its theoretical limits. USB2 streaming and software encoding might (then again, they might not

) be good enough for your TV stream today, but HDTV comes along, and all of a sudden, your system is choking on every other frame. Giving myself at least some headroom for the relentless march of technology is just another reason I am being so anal about building an HTPC system.
Thanks for all the great detailed info, BTW!
Would it be correct to assume that in order for SageTV to output its OSD via the Hauppauge S-Video output, it would need to insert the OSD into the MPEG encoded stream? If this is the case, even with the appropriate plug-ins, would functions such as web browsing, weather reports, etc. be feasable despite potential text legibility issues?
In response to jmeeks's question of "If you don't have a need to store the stream why wouldn't you just watch TV? Why bother with a PVR?", I would pose another: What percentage of video streaming through your PVR actually gets stored (either permanently for recording purposes, or temporarily for pausing/rewinding live TV)? I would guess the answer for a typical PVR user would be no higher than 5-10%. Hence, while I am certainly interested in PVR functionality, I would like to ensure the best possible video quality for the 90% of the time I am just watching TV without utilizing any of the functions requiring storage of the stream.
Crazedz, to answer your question as to my reasoning for wanting an uncompressed stream, I'll refer you to my 2nd posting of this thread. To clarify a few points, I am NOT looking to store the uncompressed video, just to utilize it for live TV viewing, as well as to keep my options open as to any filters, compression codecs, etc. I might be interested in employing down the line.

dkardatzke · #11 10-09-2003, 03:02 PM

Actually the typical PVR user never watches "Live TV", they are most always watching programs in timeshift mode. I would say I only watch about 1% of TV "Live" and that's usually the news in the morning.

Quality really shouldn't be of a concern because depending on the quality setting you can get as good, if not better quality from recorded TV as from Live TV. All depends on your video decoder and the quality setting you are using to record.

I would say based on your needs above, you don't need a PVR application, you just need a DVR application so you can set recordings once in a while. The true nature of a PVR is that it's always recording so you can Pause and Instant replay Live TV at any time. You'll find this is common with TiVo, ReplayTV, us and a few of the other PC-based PVR apps.

Just my $.02.

Dan

stanger89 · #12 10-09-2003, 04:17 PM

Just like Dan said, 99% of my TV watching is things that have already been recorded. The only times I watch "Live TV" are for a few minutes of the Weather Channel in the morning, and teh occational news broadcast, and in those cases I generally just use my TV's tuner.

If you go for SageTV, or any other PVR for that matter, most likely you'll almost completely stop watching "Live TV". With a PVR such as Sage, all the programs you want to watch are there when you want to watch them, you don't have to make time to watch things anymore. It's really nice not to have to worry about being there to watch your favorite show.

Since PQ is a top concern for you I'd get one or more 250Gb HDDs and use the Max 5.6Gb/hr setting and you should be good to go. That setting is 15Mbit/s I think almost the same bitrate as HDTV BTW.

Crazedz · #13 10-10-2003, 05:02 AM

Same here i guess that's why we couldn't understand right off what you were meaning (at least i couldn't) about getting an uncompressed stream.

baghera8 · #14 10-11-2003, 03:18 PM

Thanks to some of the recent posts, I have come to realize that one's viewing habits are changed significantly thanks to the timeshifting capabilities of a PVR. I suppose I haven't yet been exposed to the wonderful world of not having to worry about watching a show during its actual air time. I have seen the light!

However, I do watch quite a bit of sports, and I doubt that either myself or my buddies would be willing to wait even 5 minutes to watch the latest in our respective sports of choice.

I still stand by my statement of preferring the flexibility of an uncompressed stream (different compression codecs, filters, etc.), as I do not see the problem with implementing such functionality in software, as long as the hardware can handle the real-time compression without the aid of dedicated encoders. I am aware of the fact that a PVR is always recording in order to enable time-shifting (a functionality I am very much interested in, hence my preference of a PVR over a DVR), but I still fail to see any reason why the live uncompressed stream cannot be displayed when the time-shifting functionality is not being used at a particular moment. Once the user has paused or rewound the live stream, the recorded compressed stream can take over. If the user has caught up to the live feed, the uncompressed stream can be shown again. I can certainly see potential issues with a shift in quality, but, so far, I haven't seen any posts as to other disadvantages of this approach, or why this functionality cannot be implemented. Quite a few people are insistant on posting about the quality of the compressed video, but please refer to my 2nd post for my view on this subject.
Thanks all for your help!

stanger89 · #15 10-11-2003, 05:37 PM

Just to correct a misconception you seem to have, the "Live" not quite live recorded stream is delayed about 1sec not 5min.

Now, there are a number of programs that do software encoded PVR, like Snapstream and Showshifter, so yes, there is no technical reason why that can't be done. SageTV however has been designed from the ground up to use HW encoding TV cards.

Now, about switching on-the-fly between encoded and raw streams.
While theoretically there wouldn't be a disadvantage, I'd imagine that technically this would be much more difficult to impliment than the current setup, not only due to keeping track of which stream to display (raw, compressed), but also the processing of the raw video, that's a massive project in and of itself.

I'm curious what kid of display device you're using, HDTV, standard TV. Just curious because the main reason the encoded stream can look better than the raw one is because it's deinterlaced. Now from my experience interlacing artifacts, combing, are the most anoying ones. If you're going to be using an HDTV you're definitely going to want to deinterlace the video. With a compressed stream the decoders take care of that, but for raw video, Dscaler is in a class by itself. That's why I recommended a Flyvideo+Dscaler to watch live TV and Sage for PVR duties.

The short answer is that while it would be possible to impliment what you describe, it's probably not likely to happen due to lack of demand. That's just a guess as I have no affiliation with Sage, I'm just a satisfied customer.

Beelzebub · #16 10-12-2003, 02:15 PM

If you build the right system, you be will extremely impressed with the Quality. I can't evan really tell the difference between a direct feed, and the feed going through Sage. The color actually is more vivid and has more depth then My digital cable box. The DVD quality is as good if not better then my friends 720p DVD player. I own a Tivo, I let the account run out, and its now sitting in the closet my HTPC blows it away.

Here's what I recommend for Hardware/Software
- AMD Based Processor 1000 MHZ or higher.
- Geforce FX 5200 or Higher, using Nvidia NVDVD codecs
- (2) PVR-250
I'm using both the Gateway OEM Version ($89 on ebay) and the Retail. The only differnce between the 2 is the OEM runs a little Hotter, and It doesn't have support the remote. The Hauppauge remote sucks anyway, I ended up puchasing a Phillips Windows MCE remote for $20 on ebay.

- M-Audio Revolution
I use a SB Audigy 2, but thats only because I didn't know about this card. BTW this card supports 7.1 and 24biit audio at 192kHz. It doesn't get much better then that.

- SageTV or MythTV (Linux)
There is no other PVR software worth mentioning, everything else is crap.

- 2 Hard Drives. One for Windows, One for recording video

A setup like this will run you around $800 - $1000, but you will Have a complete HTPC, that rivals any Tivo/ReplayTV, High End DVD Player, Gaming System, MP3 Jupebox, Internet Box.

robn · #17 10-12-2003, 07:53 PM

I actually have a question about the video cards. I notice that a lot of people here have mentioned the Geforce FX 5200 (and higher) cards as being of choice. Currently I have a radeon 9700. The technical reviews I have read have given the ATI cards better ratings .. but most all of them focus on gaming. So, is the quality difference with the GeForce cards greater when it comes to video output like TV?

Thanks!

Beelzebub · #18 10-13-2003, 12:47 AM

Quote:

Originally posted by robn
I actually have a question about the video cards. I notice that a lot of people here have mentioned the Geforce FX 5200 (and higher) cards as being of choice. Currently I have a radeon 9700. The technical reviews I have read have given the ATI cards better ratings .. but most all of them focus on gaming. So, is the quality difference with the GeForce cards greater when it comes to video output like TV?

Thanks!

I prefer NVidia because more games are optimized for Geforce cards. Where as more 3D Bench mark programs are optimized for ATI cards. As for TV/out quality both are very close. I have a friend who has a ATI Radeon 9200 256MB DDR. I really couldn't tell a difference when plugged into his 60 " Mitsubishi HDTV using the DVI port. As for gaming, I get a better FPS then his Radeon. but In all reality I couldn't tell the difference. He has a faster CPU then mine though. He's using a Athlon 2600, were as this is a 1800.

I do know this, with my FX5600 and the NVDVD codecs, I get almost perfect picture quality, no artifects and the motion is smooth. It is actually as good as my XCard, which is currently out of the system.

baghera8 · #19 10-15-2003, 11:36 AM

stanger89, I am currently using a standard TV, with plans to switch to an HDTV around the same time as I finish building my HTPC. Thanks for the tip on Dscaler, as I am definitely interested in deinterlacing the signal before it gets to my HDTV. You make some good points on the quality advantages of a processed stream, and I agree wholeheartedly. However, I must stress my previous argument on the fact that any processing done on a compressed stream, can be done at least as well or, likely, better on an uncompressed one.
You've mentioned using a FlyVideo card for live TV viewing (losing any pause/rewind/etc. functionality), and a Hauppauge with SageTV for PVR. At the present time, that setup certainly seems to be the best way to go. What I fail to understand is the lack of interest in producing PVR software that excels at both and does NOT depend on HW decoding, allowing for the best possible live TV viewing, while losing none of the PVR functionality. Why not have the software support not only the excellent tuner cards, such as the FlyVideo boards, but also tuner-less cards such as the amazing Winnov capture cards, for people who are interested in the best possible quality, and do not need tuner functionality (HDTV boxes, most channels being scrambled in certain markets, etc.)? As it is, quite a few people seem to mostly utilize external tuning solutions (IRBlaster, etc.).
Moreover, it seems that every response to my original post has focused on my misgivings on the quality of the compressed stream. As of yet, no one has responded to my points on the obvious flexibility of the uncompressed stream, where the compression can be controlled via the PVR software.
I am not trying to make any digs at SageTV, as it is clearly a quality piece of software. I am merely trying to understand some of the design rationale behind the product.
Beelzebub, thanks for the info on your HTPC setup. I was actually going to start a new thread on advantages/disadvantages of various HTPC form factors (Micro-ATX vs. Mini-ITX vs. cube, etc.). Hope to see you there!

stanger89 · #20 10-15-2003, 06:14 PM

Quote:

Originally posted by baghera8
...However, I must stress my previous argument on the fact that any processing done on a compressed stream, can be done at least as well or, likely, better on an uncompressed one.

No argument here, didn't mean to make it sound that way if I did.

...What I fail to understand is the lack of interest in producing PVR software that excels at both and does NOT depend on HW decoding, allowing for the best possible live TV viewing, while losing none of the PVR functionality.

Lack of demand. Now why isn't there demand? For me it's simply this: TV isn't of sufficient quality to start with. Now if we were talking a DVD or HDTV quality source there is no way I would stand for compression artifacts.

Why not have the software support not only the excellent tuner cards, such as the FlyVideo boards, but also tuner-less cards such as the amazing Winnov capture cards, for people who are interested in the best possible quality, and do not need tuner functionality (HDTV boxes, most channels being scrambled in certain markets, etc.)? As it is, quite a few people seem to mostly utilize external tuning solutions (IRBlaster, etc.).

The reason for Sage is that there aren't any cards with HW encoders that don't have tuners. In general if you don't need a tuner the prolink Xcapture is also a good card.

Moreover, it seems that every response to my original post has focused on my misgivings on the quality of the compressed stream. As of yet, no one has responded to my points on the obvious flexibility of the uncompressed stream, where the compression can be controlled via the PVR software.
I am not trying to make any digs at SageTV, as it is clearly a quality piece of software. I am merely trying to understand some of the design rationale behind the product...

Not sure what kind of answer you're expecting. Software PVRs were the first ones, look at Snapstream, Showshifter, MythTV(Linux). All started as software encode PVRs like you describe. The problem with those is that is that you don't get the best of either world. The software recording quality isn't very good, and the live TV quality is nothing like the benchmark Dscaler. At least that was the case last time I tried them, before Sage. So before SageTV/HW encoders, we could have great PQ with no PVR ala Dscaler, or go with a software PVR and have OK live TV, poor recordings, and High CPU usage.

Now Sage was designed specifically for HW encode only for that reason. With HW encoding you get good recordings, good live tv, and essentially no CPU usage.

Two last things I'll mention are:
Outside of Dscaler there isn't much for filtering/processing of a live video signal, definitely nothing as good, at least not on a consumer level.

Second, your source is probably worse than you think. A standard TV tends to obscure not only much of the detail in the signal but also many of the artifacts. Unfortuantely TV tends to look worse on an HDTV than on a regular TV since HDTV's are capable of showing every tiny detail, be it actual detail in the picture or noise and artifacts.

You make some good points. In theory, if sources were of DVD quality or greater it would not be desireable to unnecessarily compress the incoming stream as the compression artifacts would be the primary artifact. However with any TV signal I've experienced, deficiencies in the source are much worse than anything compression introduces (with my cable at school my 250 actually cleaned up much of the noise in the signal).

I hope that helps a little.

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