Video capture bandwidth on firewire

[LongStar]

Hi,

does somebody know what is the bandwidth use when capturing DV video on a firewire port ?

Firewire-a is suppose to be 400Mb/s max speed. But I doubt that all of this 400Mb is used.

Also, If I'm not mistaken, the PCI interface is 133Mb/s. So if i add a PCI Firewire card... I have a bottleneck at 133 Mb/s ?

So what could be the advantage of using a Firewire 800 then ?

Thanks !

--------------------------------
LongStar
"If everything seems to be going
well, you obviously don't know what
the hell is going on !" (Murphy)

 

[wahnula]

Everybody's lying when it comes to speed. The trick is to catch when they use mbps or MB/s. The first is mega BITS per second, or "millions of bits". The second (MB with a capital "B" is megabytes per second or "millions of bytes", with each byte = ~8 bits (8.39, I think).

So, 480 mbps = 57.2 MB/s, well under the PCI bus's rating, which is also a lie. They love selling us stuff in theoretical numbers but I never believe any of it, just know hat USB 2.0 is faster than USB, Firewire 800 is faster than Firewire 400.

I also never believe people when they say how much they paid for something or how much they make. It's a rough world.

Tony

"If it can't take it, I don't want it

 

[stduc]

You can't accuse them of lying LOL - Being economical with the truth - almost certainly. You do have to pay attention to capitalization though - as already pointed out MB is Mega Bytes and Mb is Mega bits. But what is often not clear is whether one Mega is 1000,000 or 1048576 ! (decimal or binary)

The problem - & I am right behind you here - is that they always quote the best number they can justify. However obtusely. So you get burst speed, or fastest speed, never average expected speed or anything more realistic. They also always include control bits and overhead and never quote you real data throughput. They also tend to quote decimal speeds not binary speeds.

So, take a 100Mbps Ethernet - with overhead you may get 8MBS as a sustainable transfer rate if you are lucky. (100/12.5 to convert bits to bytes and take control bits into consideration)

So to answer the original question. 133 PCI is 133 MBps burst - around 70-90 MBps sustainable.

Firewire is 400 Mbps and FW2 is 800 Mbps burst - so FW2 is around abouts 60MBps sustainable. Therefore the PCI bus should be able to cope providing it is not involved in transferring much else (video or disk transfers for instance)

Hope that helps.

[navy]When I married "Miss Right" I didn't realise her first name was 'always'. LOL[/navy]

 

[wahnula]

stduc

Thanks for adding your educated analysis and clarification. It's like the old days of amplifier power ratings, before they standardized to rms (root-mean-square, the most reliable and accurate way of listing a power amp's power). There was IPP (Instantaneous peak power) which is right before it bursts into flames) and EIA (Electronics Institute of America) and other ratings that were many times the actual rms rating. A 25-Watt rms amp could be rated at 100 Watts EIA and 200 Watts IPP. Now, with everyone using rms, you need to look at the THD, wattage tested, and spectrum tested(20-20KHz) to find the liars.

This same game is happening right now with PSUs, there is no standard for rating so some test at 25C and others at 40C, some have 60% efficiency and some 85%, it's deliberately deceptive and confusing.

One thing I did learn in my research to answer the original question is find agreement that Firewire is better than USB at video transfer, with some F/W 400 units outperforming supposedly-faster (480 Mbps) USB 2.0 at this task.

Tony

"If it can't take it, I don't want it