UPDATE 6.18.10 Looks like my estimates below were pretty close. Leaked video comes out to 83.4 MB/ min and pictures about 2.6MB. (Read More)
One year ago today, I created a post to help those getting ready to buy the iPhone 3GS decide if going 32GB was worth the extra $100. Today I’m going to ask the same question of the upcoming iPhone 4 using the same template as last year.
The iPhone 3GS at 3MP has an average files size of about 1.3MB. The move to 5MP should make the average size about 2.7MB. So assuming you have 500 pictures on your iPhone the difference between the 3GS and 4 would be about 700 MB, not that big of a deal for that amount of photos. So just like the 3G to 3GS, the improved camera isn’t a reason to grab the 32GB model.
Video recording on the iPhone 3GS is VGA quality, 640 x 480 at 30 frames per second. This is the same as the new front facing camera on the iPhone 4. The calculations are assuming the same 24 bit depth as the previous models.
Using the same formulas as last year to calculate the size in kilobytes (KB) of one frame of uncompressed video:
Frame size K = ( [Pixel Width x Pixel Height x Bit Depth] / 8 ) / 1024
Where 8 represents an 8-bit byte, and 1024 equals the number of bytes per kilobytes.
To determine the file size of one second of uncompressed video, multiply the image size by the number of frames per second (fps).
To determine how compression affects file size, divide the file size by the compression ratio.
Me on the compression ratio last year:
Based on my research I found the compression ratio of standard MPEG-4 is about 30-40:1 and the newer H.264 (or MPEG-4 AVC) which the iPhone uses is about 1.5 – 2x better. For my cacluations below I will use a H.264 compression ratio of 60:1, just about in the middle of what I found.
This 60:1 ratio I calculated last year turns out to be a bit too high when looking at actual videos from the 3GS. Part of this might due to the fact that I’m not calculating the audio portion of the recording, but in practice a compression ratio of 40:1 is very close for the average recording.
NOTE This ratio might be off if the compression algorithm has been stepped up on the iPhone 4 with it’s speedier CPU, but this should be a pretty close estimate.
So let’s recalculate the 3GS size and compare that to the 4. This year let’s take a more moderate 250 mins of video usage vs the 500 I used last year. This would be about 50, 5 minute videos.
UPDATE A lot of comments I’m getting that 250 minutes is a crazy amount for a phone. This is just an example and assumes keeping videos on your camera roll and building them up over time. I know after 6 months or so my camera roll gets filled with hundreds of pictures and lots of videos. So yes, you can download them off the phone at any time, but I like to have videos and pics on my phone to show people. To make it easier to calculate how much you think you will use, I’ve added storage needed for 1 minute. So, for example, if you think you only need 30 mins of video on your iPhone 4 you are looking at: 89 MB * 30 mins…only 2.6 GBs.
iPhone 3GS video size for 250 mins:
( [640 x 480 x 24] / 8 ) / 1024 = 900 KB / frame
900KB/frame x 30 frames/sec = 27000 KB/sec
27000 KB/sec / 40 compression ratio = 675 KB/sec compressed
675 KB/sec * 250 min * 60 s / min * 1 MB / 1024 KB * 1GB / 1024 MB = 9.66 GB (39 MB / min)
This seems to be the wrong resolution, see updated version below
iPhone 4 video size for 250 mins:
( [960 x 720 x 24] / 8 ) / 1024 = 2025 KB / frame
2025 KB/frame x 30 frames/sec = 60750 KB/sec
60750 KB/sec / 40 compression ratio = 1518.75 KB/s compressed
1,518.75 KB/sec compressed * 250 min * 60 s / min * 1 MB / 1024 KB * 1GB / 1024 MB = 21.73 GB (89 MB / min)
UPDATE In the comments below and elsewhere people have been questioning my use 960 x 720 for the iPhone 4 HD resolution because the standard 720p is 1280×720. I was assuming Apple would stick with the same aspect ratio of the iPhone 3GS for video, 4:3. 720 in 4:3 ratio is 960. The new digital camera still shoots in this 4:3 aspect ratio as seen in the sample pictures on Apple’s website. So really I think this could go either way, but below I present the amount of storage that would be used if the iPhone 4 were using true 720p HD quality.
UPDATE 2 Watching the demo in the keynote, it looks like the video is in 16:9 aspect ratio. Guess all my naysayers were correct, but I’m happy to be wrong on this one…I’ll take letterboxed playback on the 1.5 aspect iPhone vs the weird 960 x 720 resolution I was thinking.
iPhone 4 video size for 250 mins if true 720p (1280 x 720):
( [1280 x 720 x 24] / 8 ) / 1024 = 2700 KB / frame
2700 KB/frame x 30 frames/sec = 81000 KB/sec
81000 KB/sec / 40 compression ratio = 2025 KB/s compressed
2025 KB/sec compressed * 250 min * 60 s / min * 1 MB / 1024 KB * 1GB / 1024 MB = 28.97 GB (118 MB / min)
If a higher 60:1 compression ratio is used by the more powerfull iPhone 4: 79 MB / min
So as expected more than doubling the resolution more than doubles the size of the video. So with the HD video recording even 64 GB could be a limiting factor. Hey Apple, were is our 64 GB version of the iPhone 4?
So like last year, the jump in the still camera mega pixels won’t affect your space enough for you to warrant an upgrade to 32GB if you are currently happy with your 16GB iPhone, but if you are planning on shooting a decent amount of video…that’s a different story. When you factor in all storage you’re already using for apps, music, and pictures you aren’t left with much. Even with a 32GB model most people won’t be able to hold anything near my calculated 250 mins which would take up about 60% (more like 90% with true 720p) of the total storage. So with this in mind I can’t recommend saving the $100, just go for the 32GB!
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