P2 Recording Times; The Numbers Do NOT Add Up

BobDiaz

BobDiaz

The Professor
I was going through the numbers for recording time at 100Mb/second and found something that does not add up right.

If we are recording at 1080@60i and take the number of bytes per second required, times 8 to convert to bits, and divided by the compression ratio plus the uncompressed audio, the number 100Mb/second is very close to what I get. No surprise there.

Working the other way, 8GB = 8 minutes of video at the maximum data rate (100Mb/second), the answer comes out weird. A GB is equal to 1,024 MB, but for this calculation I will use the lower number of 1,000.

1,000MB (1 minute of video & audio) x 8 (8 bits = 1 Byte) / 60 (seconds in a minute) = 133.333...Mb/minute!

Now I know that there must be some overhead, BUT according to my calculations: An 8GB P2 Card has 6GB of video/audio and 2Gb of overhead. That seems WAY TOO MUCH! :shocked: That's 1 byte of overhead for 3 bytes of data.


(1) The camera records at a higher data rate than 100Mb/second OR (2) The P2 cards hold more than the 4GB=4 minutes, 8GB=8 minutes times.


This is a request for anyone with an HVX-200: Start with an empty card and record until the camera says out of memory.

How long is the clip you recorded?

What files were created and how big is each file?

Even with a short clip, it would be interesting to know the running time of the clip and how big it is. My PC can't play the native files, so I'll need someone else to look it up.


Bob Diaz
 
Bob

Everytime I read one of your posts I flashback to College and my roommate who was a physics/engineering fanatic. Every Sunday night I'd have a gaggle of scientists arguing, drinking, and smoking in my livingroom and while I could never quite grasp everything or even most of what they talked about I still felt just a little bit smarter being in their company. That's how your posts make me feel.

Somebody get this man his Data!
 
Maybe they use some kind of error correction. Remember, a 3,5" floppy disk has a capacity of 2 MB, but you can only use 1,44 MB of them.
 
Freshly formatted a 4GB card, and set it to recording 1080/60i.

It stopped at 4 minutes, 11 seconds.

Don't know what's wrong with your math, but the card does what the chart says...
 
Barry_Green said:
Freshly formatted a 4GB card, and set it to recording 1080/60i.

It stopped at 4 minutes, 11 seconds.

Don't know what's wrong with your math, but the card does what the chart says...
Click to expand...

How big is the file?

How much of the P2 card's 4GB is taken up by formatting it I wonder?
 
Barry_Green said:
Freshly formatted a 4GB card, and set it to recording 1080/60i.

It stopped at 4 minutes, 11 seconds.
Click to expand...

4:11 at 100 MBit/s should give you a 3.06 GB file.

If the file is bigger than that, the camera records more than 100 MBit/s.

If 4 GB doesn't mean "4 Gigabytes" but "4,000,000,000 Bytes", a 4 GB has actually 3.73 Gigabytes. Still some MBytes missing.
 
Yeah but you're forgetting that it's not just the video file. For each clip there's a video file, four audio files, an XML file, a BMP thumbnail file, then there's the directory structure and its attendant text file, and ... well, I think that's about it.

A chock-full 4GB card reports in Windows Explorer that it is 3,720,918,946 bytes.

The audio folder contains four audio MXF files, each is 24,217,152 bytes, taking up a total size on disk of 96,993,280 bytes.

The CLIP folder contains one XML file of 2,892 bytes, taking up a total size on disk of 32,768 bytes

The ICON folder contains one BMP file of 14,454 bytes, taking up a total size on disk of 32,768 bytes

The VIDEO folder contains one MXF file of 3,624,032,992 bytes, taking up a total size on disk of 3,624,042,496 bytes

The exact runtime of the file is 00:04:11.28
 
Barry,

Thanks for the numbers, I printed them out and will do a little "crunching" on them.


Thanks,


Bob Diaz
 
Hey, while we're playing "let's make poor Barry do some recording of his office wall", how long does it record for if you set the camera to 720p/24p? (The equivalent setting might be different, as I don't have the camera).

I was hoping there'd be some space savings there, although there might not.

Barry, when you have 4-8 minutes. ;)
 
No proxies on the small camera. They're possible on the big camera, but the small camera doesn't get 'em. There's a folder for proxy, but it's always empty.

720/24pN gives you 10:15 of recording time.
 
Update

Update

Thanks to Barry, I now know how the numbers add up. Before I go over what I found, let me first talk about "Under Specing" and "Over Specing (Puffing)" the product.

"Over Specing" or Puffing the Product's specifications is where a manufacturer or manufacturers provide a specification that is really better than what the product will do. A good example of this is around 10 years ago when USA monitors were sold as 17", but the maximum diagonal image was around 15" to 15.5". The 17" was the outer dimensions of the tube and not the viewable area. Sometimes you can spot Over Specing when you see wording like, "Up to ..."; this means under ideal conditions in a lab after testing 10,000 units we found 1 unit that would do this, as for all the other units, dream on sucker!

With "Under Specing" the product is when the product is a bit better than what the manufacture says it is. This may sound funny to most people, but there is a reason. When I worked at EIZO (Nanao) USA, the computer monitors were specified for a given video bandwidth, like 100MHz. (Higher bandwidth was only one of several factors that made for a sharper image.) Yet, if that was the specification, none of the monitors would max out at that point. While it varied, they might max out at around 110MHz. Why publish the lower specification? Simple, it allowed some wiggle room for manufacturing to deal with variations. Some might be 115MHz and some might be 105MHz, but the key thing is that all were AT LEAST 100MHz bandwidth.


As far as the numbers I've seen so far, Panasonic is Under Specing the product's data requirements at 1080 and 720. This means that the camera is a bit better (as far as data) than what Panasonic says it is.

(1) Recording time is a bit ABOVE 4 minutes for 4 GB. OK, 11.28 seconds isn't exactly a huge amount, it's only 4.7% more time, BUT it's better than getting 3 minutes 48.72 seconds and later be told by Jan, Well it's CLOSE to 4 minutes. If a card holds more clips the total time will be a bit less, but even if every clip is only 5 seconds long, the extra overhead wouldn't cause you to drop below a total for 4 minutes. The one factor I can't take into account is variations in M-JPEG file size. It may be possible for a high bandwidth image to eat up more time and give you less than 4 minutes; however I have no test data either way to be sure.

As a side note, Berry said he got 10 minutes 15 seconds when recording 720@24p. This is 2.5% more than the Panasonic specification, not much, but even a little extra time helps.

(2) Recordings MAY exceed the 100Mb/second data rate. This may sound like a bad thing, BUT is really a good thing. The audio records at around 3Mb/second and this never changes, because it is always uncompressed. The video on Barry's clip was recorded at 115Mb/second (using base 10 million) and 110Mb/second (using computer million). The clip and icon information is too small to mater. This data rate translates to a compression ratio of 5.75:1 (Panasonic specification 6.7:1). However, there is some extra overhead in the video's data that increases the compression number. While I don't know the exact overhead, in order to make the compression ratio match 6.7:1; for every 6 bytes of data, there would need to be 1 byte of overhead. This is WAY too high for any overhead. The one thing I can say for sure is the compression ratio is between these two numbers and remember, a lower compression ratio is always better because you'll have less compression noise hidden in the image.

(3) The data takes a little less space than we first understood. In the DV Expo, Jan has said the maximum data rate is 1GB/minute. This is an easy number to remember and if we round the correct number, you will get 1GB/minute. Figure that at maximum data rate, the camera uses 0.93GB/minute. Not a huge difference, but if you're trying to squeeze as many files onto that 300GB hard drive, that is really LESS THAN 300GB, every little bit helps.

I'm estimating the data rate is around 0.36GB/minute using 720@24pN. Just remember that a 4GB card uses about 7% of it's space for file overhead. These are things like the FAT (File Allocation Table), Directory, Error Detection, and Correction information.



Bob Diaz
 
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