I've been messing around with MythTV for a few months now and something has always bothered me. Why does it take a 2.0Ghz+ CPU with decent RAM and a nice tuner card in order to get acceptable performance?
A Series 1 Tivo has a 54Mhz PowerPC CPU and I'm not sure how much RAM. But I know it must not have even 512MB. I have a Linksys WRT54G Wireless router with a 200Mhz CPU and 16MB RAM. I also have this neat new device called Neuros OSD (Open Source Device) (link here http://www.zipzoomfly.com/jsp/ProductDe ... ode=280894 ). It has a 200Mhz ARM CPU and can record video to storage memory cards. None of these products have any performance issues.
So what is it about a non proprietary set up such as MythTV that makes it such a performance hog? Here are my suspicions:
1. The Linux kernel is compiled specifically for the proprietary device.
2. Since each proprietary device only needs specific hardware, a lot of overhead and CPU cycles are saved.
3. The makers of these devices have just tested them like crazy to fine tune performance.
4. They don't use X Windows which is a performance hog? (I know the Linksys does not but unsure of the other two)
5. They were all programmed in a low level language. (But Myth is mainly in C I thought)

Any ideas? This problem really perplexes me.

The reason that a TiVo can get by with such a wimpy CPU is that all of the video is handled by dedicated encoder and decoder chips. Handling video is very hard work, but the TiVo sidesteps the issue by not having the CPU handle video at all. The CPU in a series 1 TiVo isn't even capable of playing an mp3 file, much less handling video.

On a general purpose PC, however, you don't have the specialized hardware, to the CPU has to handle more of the grunt work with the video and audio streams.

You don't need a 2 Ghz machine, though. With a tuner card that encodes to MPEG2 and an nvidia video card that is capable of assisting with decoding, I run my myth box on a 1Ghz Celeron, and it's every bit as snappy and responsive as you'd want it to be. I don't foresee needing more horsepower until I eventually upgrade to and HD television.

Mythbox CPU power requirements have been a matter of interest to me, and has resulted in my building three Mythboxes; IBMs with Pentium 4s of 1 ghz, 1.8 ghz, and 3.2 ghz (for HD)

I found the 1.0 machine works fine as a one card Mythbox on analog TV (and have read others have had satisfactory results using as slow as a 500 mhz processor)

What with swapping drives, I had digitalHD content on it, and while Myth would stutter playing them, Xine could play HD files of 6.4 g/hr, (probably 720p) and stuttered on 7.4 g/hr. files (probably 1080i). The machine has a MX400 video card, and currently runs R5D1 with the Myth .20 upgrade.

The "family's production machine" is a 1.8 ghz. It runs R5B7 and requires no help from me to keep it running for them. ("If it ain't broke, don't fix it")
It has two analog cards (250 & 350), will record two streams & play one with no problem, record HD (temp. experiment), but stutters playing it back. Xine will play 720 &1080 digital TV files OK, but is more hassle than the family will put up with to find the Myth files with Xine....
(Has a FX5200 video card)

My 'play-with' box is the 3.2 ghz... It has two ATSC cards (Air-to-PC 0.2 DVB)
This is fast enough to record two HD streams & play one... but the commercial flagging still has to run overnight. That process is a CPU hog.
I'm currently running Mpeg2lib & kernal settings for playback, getting 60% cpu usage total. I'm looking forward to the 'latest Nvidia driver' setup getting ironed out, because it may have the potential to drop video processing demand 30% when using the Xvmc and Bob playback settings. This one also has a FX5200 video card.

Hope my signposts on the road help !

Interesting that you can record two streams and play one with the 1.8Ghz machine. Standard Tivo's can't do that. But also, if the baseline for the lowest acceptable possible Myth setup is a 500Mhz CPU then it is still a far cry from the Tivo's 54Mhz PowerPC CPU.

Tivos CPU is only used to display the interface and to coordinate the encoder and decoder chips, it doesn't really need to do much.

_________________
Paul Turpie
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My Mac Quadra 840av could record and play pack video, usually in real time (sometimes it skipped a frame or two). It was a 40MHz 68040 with a 66MHz DSP coprocessor. It had about 8MB of RAM.

If you wanted to do the same as a Tivo today, you would get a FireWire DV converter for the output, a FireWire converter or tuner for the input, a big FireWire hard disk, and a very small computer to initiate the data transfers between them all. A few megahertz should be all it requires.

_________________
| Nigel Pearson, nigel.pearson.au@gmail.com
| "Things you own end up owning you" - Tyler, Fight Club

I'd guess some proprietary tweaking as well, but I'm curious how the Series 3 can handle two HD recordings without blowing up

I can hardly do one with an Athlon 64 3200+ and not get an occasional skip/pause at 50% cpu utilization

It was my understanding that the Series 3 just recorded the raw files and didn't actually encode anything, but I could be wrong (although does it have an HD MPEG encoder???)

It seems that with SD content, a Myth box with a PVR-xxx card is okay.

Don't get me wrong. I love my Tivo, but I have a hard time justifying $650 for a Series 3, plus $20/month and not have the ability to record to DVD (Tivo To Go does not exist in Series 3 and probably never will if Cable Labs has their way), and no MRV (doesn't exist in Series 3 and *may* for SD content if Cable Labs has their way).

With my "near dragon 1.1", I can record THREE HD streams and watch another, with no trouble. I'm using the newest nVidia driver, with mrfarenheight's tweak to xorg.conf. If you're running R5E50 with the stock nVidia drivers, look into this fix - it really works.

Even using the latest Nvidia drivers did not do a thing (for CPU utilization). Even with Xvmc either.

The CPU utilization is NOT with "X" either.

It's 40-55% CPU utilization doing "watch TV" or playback of HDTV and it shows "mythfrontend" as sucking up the CPU.

x.org is like 3% MAXIMUM during this time.