Compression question

[tawd1992]

I've been using a Scientific Atlanta 8300 dvr for a couple of months now & I'm really surprised how well the picture looks on this compared to my Sage recordings (using a Hauppauge 150 tuner card.) There is a little motion blur with the dvr, but not nearly as much as my Sage recordings. Also the picture quality is pretty similar with both, maybe a tad better with the dvr.

I was really surprised when I found out that the dvr is recording @ 1GB/hr. My Sage recordings are @ 2GB/hr. If I try & record @ 1GB/hr with Sage (actually .9 GB/hr) the recordings look horrible. It seems that the dvr is using mpeg2 compression, so how is it that the recordings look as good if not better at half the bitrate?

[tawd1992]

Anyone?

[AngelofDeth]

One reason is your cable DVR doesn't encode anything. It just plays the bitstream coming off the wire. The cable company takes the highest quality source, then squeezes it down to that 1.1GB/hour. If you then re-encode that signal down to 1.1 again, its not going to look good. Also, if you are outputing to an SD set via your PC, its not likely to look as good as something with a dedicated hardware decoder such as your cable dvr, or media mvp.

[stanger89]

Originally Posted by AngelofDeth

=The cable company takes the highest quality source, then squeezes it down to that 1.1GB/hour.

Using a very good, very expensive encoder.

[tawd1992]

Originally Posted by AngelofDeth

One reason is your cable DVR doesn't encode anything.

Originally Posted by Stanger89

Using a very good, very expensive encoder.

Ok, now I'm even more confused. Does the dvr encode (compress) the video or not? My main concern is how do I get dvr like quality out of my htpc. It sounds like it would be too expensive to do this, if it's even possible?

[blade]

Originally Posted by tawd1992

Ok, now I'm even more confused. Does the dvr encode (compress) the video or not? My main concern is how do I get dvr like quality out of my htpc. It sounds like it would be too expensive to do this, if it's even possible?

What they are saying is that the signal is already encoded before it is broadcast using very high quality equipment and a clean source. When the DVR receives the already encoded signal all it does is write it to the disk.

Your HTPC on the other hand is receiving an analog signal and must encode it to mpeg2. Your source is obviously not as clean, you're limited to single pass encoding, and your hardware isn't as good.

[blade]

Originally Posted by tawd1992

There is a little motion blur with the dvr, but not nearly as much as my Sage recordings.

Have you adjusted the spatial and temporal settings using the Hauppauge tweak tool? The default settings cause way too much motion blur. I have temporal at 4 and spatial at 0.

[dvd_maniac]

I just downloaded the tweak tool and I do not see anything for the PVR-USB2 devices. Is this because there is no way to set these tweaks for that device?

[stanger89]

Originally Posted by tawd1992

Ok, now I'm even more confused. Does the dvr encode (compress) the video or not? My main concern is how do I get dvr like quality out of my htpc. It sounds like it would be too expensive to do this, if it's even possible?

Well, let's make this as confusing as possible.

There are two cases here (assuming cable) the first is the analog channels, these chanels are analog on the cable line, the DVR will have an encoder to create MPEG files for these, much like a PC PVR does.

The other case is digital channels. These start as uncompressed, clean video at the cable company head end. The cable provider then uses very high quality, very expensive compression equipment to create an MPEG-2 transport stream out of the uncompressed video, and sends this out the cable line. Then the DVR just grabs this MPEG-2 data and sticks it on the disk.

What I was getting at above, is that PC PVR cards (as well as DVRs for analog channels) use "commodity" encoders, these, while not bad, are not nearly as good as the ones the cable company uses to encode the digital channels. A good comparison of the "quality" difference of the encoders is that something like the PVR 250 requres about 6-8Mbps for to achieve similar quality to what the cable company equipment can do at 1-3Mbps.

There are a couple issues facing PC PVRs that aren't facing a provider's DVR:

First PCs generally don't have dedicated NTSC outputs, this is why the PVR 350 and Media MVP are praised so much, they have the same sort of decoder/output as a DVR. You'll notice that for analog channels there's really no quality handicap for the PC other than this.

Second, is for digital channels, the DVR doesn't encode these, it captures them raw, so there's no loss in the recording operation. PCs on the other hand, suffer from genrational losses, because the video is compressed once on the way to you, decompressed by the set top box, and compressed again by the PC PVR card. So there's some unavoidable loss there.

[tawd1992]

Originally Posted by blade

Have you adjusted the spatial and temporal settings using the Hauppauge tweak tool? The default settings cause way too much motion blur. I have temporal at 4 and spatial at 0.

Yes I did the registry tweaks (both set to 0) & it made no difference whatsoever. I've heard that some cable providers compress their signal so much that you will get artifacts & motion blur no matter what you do. I have Time Warner cable, do they compress their signal a lot? Like I said, even their dvr has a little motion blur which really surprised me.

[tawd1992]

Big thanks to everyone for explaining the differences between a cable company dvr & an htpc.