GH4 Bits and Bytes - which is it on the GH4 specs

T

True North

Well-known member
The GH4 spec's show that the camera writes video at anywhere from 50mbps to 100mbps to 200mbps.

My understanding is that 'mbps' stands for Megabits per Second.

It is also my understanding that if the specs were meant to represent Megabytes then it would or should be displayed as MBps.

There are 8 bits to 1 byte

So my question is as follows;

Are the motion picture data rates shown on the GH4 Spec sheets in Megabytes per second or Megabits per second?
 
Barry_Green said:
bits.
Click to expand...


I have always thought it was 'bytes' but manufacturers just weren't paying attention to the details (using a small b instead of a capital B) on how they displayed their data rates.


So let's take another Panasonic camera that you are very familiar with, the AG-AC90. The data rates when recording AVCHD 60p are listed as 28mbps or 28 megabits per second by your interpretation of what Panasonic means when they use mbps to list data rates .


However, when I record about 50 minutes at 60p on my AG-AC90 the file size is about 8.5GB. So while Panasonic lists the data rate as 28Mbps it should be listed 28MBps as the camera is clearly recording 28 megabytes (not megabits) per second.


Now, when you look at the specs for the GH4, when you shoot in Mov or MP4 you will be shooting with data rates anywhere from 100mbps to 200mbps. However on the same spec sheet it lists AVCHD as having record data rates of 17-28mbps. So unless 'mbps' means different things when used in conjunction with different codecs I have to believe the GH4 shoots at data rates of 100-200 megabytes per second.


Where am I going with this? Some people have noted that they intend to use the new Kingston memory card for the GH4. Problem is the new Kingston card has a listed write speed of 80MB/s -Megabytes per second which depending on what interpretation is actually true, then the write speed of the Kingston Card is not high enough.


I am still uncertain about whether the top end data rates for the GH4 are 100 to 200 Megabits or Megabytes. I think it's Megabytes.
 
True North said:
So let's take another Panasonic camera that you are very familiar with, the AG-AC90. The data rates when recording AVCHD 60p are listed as 28mbps or 28 megabits per second by your interpretation of what Panasonic means when they use mbps to list data rates .


However, when I record about 50 minutes at 60p on my AG-AC90 the file size is about 8.5GB. So while Panasonic lists the data rate as 28Mbps it should be listed 28MBps as the camera is clearly recording 28 megabytes (not megabits) per second.
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I reckon your figures are consistent with 28Mbs - Mbs=megabits/sec (which is what I'd expect it to be - b=bits, B=bytes)

50 minutes =3000 seconds, multiply by 28 gets 84,000 Mb.

Divide by 8 gives 10,500 MB, then finally divide by 1024 to get just over 10.25 GB.

But the quoted 28Mbs of AVC-HD is a MAX datarate - not average - your figure of 8.5 GB total implies an average datarate of just over 23 Mbs, which is what I'd expect.

(If it was 28megabytes/second, that would be 28x3000 MB total = 84,000 MB. Divide by 1024, and you'd expect to get just over 82GB total.)
 
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Yep, 200 Mega BYTES per second would be quite insane. I'm not sure if true uncompressed 4K RAW would even be that high (basically a BMP for every frame). 200MB/s would mean you could record 5 minutes and 20 seconds on a 64GB card. As it is, with the 200 mb/s (mega bits) that the GH4 will record 1080p at will give you approximately 52 minutes for 64GB which is quite a bit of storage.
 
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It's megabits. Every Panasonic camera, from the first digital tape cameras on up to the top-of-the-line varicam, quote their data rates in megabits. And that includes the GH1, GH2, and GH3, and certainly includes the GH4 as well.

There is zero possibility that the GH4 will be recording 200 megaBytes per second. Zero.

The AC90 records a max of 28 megabits per second. I just recorded one minute of footage on an AC90, and it turns out to be 185 megabytes. According to the windows "properties" window, that works out to an average data rate of 25,250 kilobits per second, or 25.25 megabits per second.

If it was megaBytes per second, then the file size would be 25.25 x 60, or 1.52 gigabytes. But it isn't. The file size is 185 megabytes.

Absolutely, unquestionably, without a doubt and guaranteed, the data rates being reported and the way the cameras are rated is in megabits (mbps), and absolutely not in megaBytes (mBps).
 
Barry, I love your posts.

Barry_Green said:
It's megabits. Every Panasonic camera, from the first digital tape cameras on up to the top-of-the-line varicam, quote their data rates in megabits. And that includes the GH1, GH2, and GH3, and certainly includes the GH4 as well.

There is zero possibility that the GH4 will be recording 200 megaBytes per second. Zero.

The AC90 records a max of 28 megabits per second. I just recorded one minute of footage on an AC90, and it turns out to be 185 megabytes. According to the windows "properties" window, that works out to an average data rate of 25,250 kilobits per second, or 25.25 megabits per second.

If it was megaBytes per second, then the file size would be 25.25 x 60, or 1.52 gigabytes. But it isn't. The file size is 185 megabytes.

Absolutely, unquestionably, without a doubt and guaranteed, the data rates being reported and the way the cameras are rated is in megabits (mbps), and absolutely not in megaBytes (mBps).
Click to expand...
 
One point of interest, the M is always capitalized in the abbreviation. The b can be lowercase (megabits) or upper case (megabytes).

Mb/s - Megabits (video recording)
MB/s - Megabytes (hard drive transfer rates)
 
Yup, looks like a case of user error in calculating the order of time. I suspect a step was missed off seconds to minutes to hours.

We almost alway measure by Megabits in regards to video recording so Panasonic aren't wrong.

There are very few cameras that shoot actual 200MB/s.

In perspective:

5DIII raw video at 1080p roughly is about 800Mb/s or 100MB/s (about 5 mins on a 32GB card). BM4KC is about the same but 4K and using Prores instead of raw.

Also big productions like The Amazing Spider-Man, Prometheous, The Social Network etc shot on Red 5K with a bitrate of 75MB/s with the compressed raw redcode.
 
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