[mythtv-users] Max number of IVTV cards

[Michael T. Dean]

On 08/19/2004 01:49 AM, Kaushik Mallick wrote:

>I want to build a system to record 5 tv channels at
>the same time. I plan to use a ivtv card like
>Avermedia M179 for each channel. Is there any limit on
>maximum number of these cards I can use in a system
>(besides the limitation of pci slots on my
>motherboard)?
>
ivtv 0.1.9 supports up to 9 cards, but there was talk of reducing it to 
8 (and it may have happened for recent ck releases) because a) support 
for more cards complicates the programming and b) the DMA traffic 
generated by that many cards would probably kill any system in 
existence.  (Note, also, that the value 9 was chosen arbitrarily.  PCI 
buses can support more cards than that, and systems can have more than 
one PCI bus--so it was already constrained at 9.  However, due to the 
complexity of coding for an "unlimited" number of devices, drivers often 
impose arbitrary (but sensible) limits.)  See the thread starting at 
http://www.poptix.net/ivtv/Jun-2004/msg00888.html .

>Do I need more CPU power as the number
>of channels I need to record go up? I am hoping
>somebody have had run into a similar experience.
>Thanks beforehand.
>  
>
More than CPU power, you'll probably need luck.  Because the card is 
doing the encoding on-board, the CPU gets almost no load.  However, the 
system will have to deal with a significant amount of DMA traffic.  Some 
chipsets can handle it.; others cannot.

However, if you're doing things like automatic transcoding or commercial 
flagging after recording, you will need CPU power for that.  Basically, 
if you have 5 channels recording and are transcoding/commercial 
flagging, you'll either need to have a CPU (CPU's) that can 
transcode/flag faster than you can record or offload flagging to other 
systems (automatic transcoding currently cannot be offloaded).

Since the ivtv cards present so little CPU load, I would recommend 
getting an old system and making a second (and possibly a third) 
backend.  It will probably make your life much easier and your PVR much 
more reliable.

HTH.

Mike