HD Software Encoders & CPU Usage

[Jesse]

Hello all,

I have not yet jumped on the v3 beta (beta stuff is not good at my house ). But in preparation for a final of v3 I am investigating HD.

Am I correct that all HD capture cards are software encoders? If so how much cpu will I be using for capture only? How much for capture and simultaneous playback of live HD? I ask because my sage machine is used to both capture and playback, it also has the client for my mvp installed, and is used to playback music in two or three zones at any given time. I dont usually have all this going on at once, but it is possible. I dont use the maching for anything other than the above mentioned media related stuff.

Anyone know of any HD hardware encoders in the works?

Machine specs: P4 3.0g HT, 1 gig ram, 9800pro, PVR500, MAudio D410 (x2), Intel D865perl mobo.

Thanks in advance,

Jesse

[Lucas]

Well HD TV is digitally encoded already so the HD cards just provide the digital stream which is in Mpeg2 form already.

Capturing in the sense of converting analog NTSC signals to Mpeg2 is not involved.

The only issue comes with playing the HD stream.

[stanger89]

An encoding HD in software would require on the order of a 10GHz CPU to do in realtime (based on current architectures).

As Lucas said, HD does not require encoding as it is sent as a compressed datastream.

[Mahoney]

An encoding HD in software would require on the order of a 10GHz CPU to do in realtime (based on current architectures).

Not doubting you, just eager to learn; why is that? By my reckoning a PAL SD encoder has to cope with 576 line frames at a rate of 25fps = 14400 lines to encode a second. A PAL 720p HD encoder would have to cope with 720 line frames at a rate of 50fps = 36000 lines a second. Now 14400 to 36000 is a big jump (2.5x), but not an exponential one.

If I do the maths taking theoretical horizontal resolution into account, its 10886400 pixels a second compared to 46080000; say 4.5x the amount of data to cope with.

[waynedunham]

Originally Posted by Mahoney

Not doubting you, just eager to learn; why is that? By my reckoning a PAL SD encoder has to cope with 576 line frames at a rate of 25fps = 14400 lines to encode a second. A PAL 720p HD encoder would have to cope with 720 line frames at a rate of 50fps = 36000 lines a second. Now 14400 to 36000 is a big jump (2.5x), but not an exponential one.

If I do the maths taking theoretical horizontal resolution into account, its 10886400 pixels a second compared to 46080000; say 4.5x the amount of data to cope with.

Ahhh, but don't forget the amount of data in each line. The lines may be longer (widescreen) and the resolution is higher so there's more data in each line.

Take a look at AudioVideo101.com

In the first paragraph of that page you'll see that a Std def digital frame (640x480) has 307,200 pixels and while the highest res HDTV frame (1920x1080) has 2,073,600 - that's over six times the number of pixels!

[stanger89]

HD has a data rate approximately 6x that of SD:

SD (NTSC) ~= 720x480x30x24 = 249Mbps
HD (720p) ~= 1280x720x60x24 = 1.3Gbps = 5.2x SD
HD (1080i) ~= 1920x1080x30x24 = 1.5Gbps = 6x SD

To further complicate the issue the PCI bus would be a limit as well:

33MHz x 32 bit = 1.056Gbps

Also consider that realtime software encoding of SD takes on order of 1.5-2GHz, so if you do the simple extrapelation, it's somewhere between 8 and 12GHz to realtime encode. Also consider that the newer codecs (H.264 and VC-1) are much more demanding than MPEG2.

[Jesse]

Hello and thanks for the info. I was quite puzzled as to why hardware encoding would be used for standard def and not HD. Now it all makes sense. I guess it is safe to assume that my P4 3.0g / 9800pro can handle playback with no problem?

Thanks again for all the great info.

Jesse

[Lucas]

Jesse,

I ve played 1280x720p wmv files on my 2.8 system without problems.

Microsoft recommends 2.4 or more for 720p and 3.0 and more for 1080i.
This is for wmv though.

I am not sure what it is for US Based HDTV but my guess is that it would be OK since mpeg2 decoding is not as intensive as other forms (higher compression) decoding.

In our side of the planet HDTV over the air is still a dream.

I base my comments on my experience with mpeg2 satellite based HDTV reception via a DVB-S card.

[stanger89]

Originally Posted by Jesse

Hello and thanks for the info. I was quite puzzled as to why hardware encoding would be used for standard def and not HD.

No encoding (on the receiving end) is used for HD. SD is transmitted as an uncompressed analog video stream, which must be captured, digitized, and compressed (or it would take 100GB/hour) to MPEG-2. HD is transmitted as a compressed MPEG-2 data stream, all that is done on the receiving end is to demodulate that MPEG-2 data stream and play it or write it to disk.

[salsbst]

Originally Posted by stanger89

No encoding (on the receiving end) is used for HD. SD is transmitted as an uncompressed analog video stream, which must be captured, digitized, and compressed (or it would take 100GB/hour) to MPEG-2. HD is transmitted as a compressed MPEG-2 data stream, all that is done on the receiving end is to demodulate that MPEG-2 data stream and play it or write it to disk.

Technically, I believe you mean to say that analog is transmitted as analog .

Digital cable and DirecTV (and OTA digital) transmit all channels as mpeg2, whether they are SD or HD.

[briands]

Originally Posted by stanger89

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To further complicate the issue the PCI bus would be a limit as well:

33MHz x 32 bit = 1.056Gbps

Also consider that realtime software encoding of SD takes on order of 1.5-2GHz, so if you do the simple extrapelation, it's somewhere between 8 and 12GHz to realtime encode. Also consider that the newer codecs (H.264 and VC-1) are much more demanding than MPEG2.

But a card with an onboard dedicated processor(s) might be an answer for both of these issues. What type of x86 processor do you think that Conexant on your Hauppauge card would compare to? Assuming scalability (maybe have a card that splits up the video to 6 or 8 of those chips) this may not be such a long putt after all...

[stanger89]

Originally Posted by briands

But a card with an onboard dedicated processor(s) might be an answer for both of these issues. What type of x86 processor do you think that Conexant on your Hauppauge card would compare to?

The problem is they aren't directly compareable. But basically the CX2341x does the work of about 1.5-2GHz of x86 CPU.

Assuming scalability (maybe have a card that splits up the video to 6 or 8 of those chips) this may not be such a long putt after all...

That is the ideal solution, but there seems to be no interest in producing a consumer-level hardware encoding HD capture card. Why?:

1) Everything we consumers have access to is already compressed when we get it.
2) The only reason for an end user to record uncompressed HD, is to bypass the copy/access protection mechanisms in place for it.

[stanger89]

FWIW, there is actually a card with hardware encoders capable of capturing full HD and compressing, realtime, into MPEG-2. It's the LSI Logic HDTVxpress.

[stanger89]

Of course, last time I found a price, it was five figures. ~$20,000 IIRC.