Weird issues with time left on disk readout...

Mike Harvey

Mike Harvey

Veteran
The other day I was shooting some interviews plus b-roll-- 1080p/24, LPCM audio, onto Sandisk Extreme 16GB cards. When I formatted the card right before the shoot, it told me there were 91 minutes on the card, which it normally says. However, after the shoot, it told me there was approximately 10 more minutes of space left than there should have been. Since the b-roll didn't require audio, we turned the wireless transmitter off, but I left the receiver on, so I just assumed it might have had something to do with getting no audio.

However, today after shooting approximately 45 minutes of interview (720p/24, LPCM) onto a different Sandisk Extreme, it told me that there was a little more than an hour left, even though before the shoot it told me there was 91 minutes on the card before the shoot. When I used the in camera card copy function to copy the footage onto a Transcend card (also formatted right before the shoot), it told me roughly the same thing.

The B-cam was my HMC150 recording to a Sandisk Extreme as well, and it did something similar. In the three years I owned that camera, I've never noticed that before, and I never noticed it until now in the five months I've had the AF100. I'm not saying it's hasn't happened before, only that if it has, I sure haven't noticed it until now.

So my question is, are you guys getting the same sort of thing, or are my cards (all of which are at least a year old) simply starting to wear out and I need to be buying some new ones? Is it even something I ought to be worrying about?
 
I think the AF100 has a variable bitrate. If it does the calculation is probably based on the most recent sample, or a maximum number. 24mbps and 21mbps are quite different.
 
I've seen variances of 2-3 minutes before and assumed that was the reason, just never 10-15 minutes before. Of course, at 24fps, and being that it's sit down interviews in doctors' exam rooms, there isn't a whole lot changing from frame to frame, so that makes sense. Usually when I'm shooting for that long, there is either more stuff changing in the background or it's at 60fps for sports. I just didn't think 3mbps would make that big a difference in time on card.
 
16 gb = 131072 mb
24 mbps = 5461s = 91m
21 mbps = 780s = 109m

s = seconds, m = minutes

So there's about a 20 minute difference between 20 and 24 megabits per second. That's a pretty extreme amount of bitrate change, but it should demonstrate how much of a variable there is. Add in the variable of the computer's estimates on time and you can get even more variation from the displayed numbers. Or I could be totally wrong, but it seems plausible to me.

My searches state that the AF100 records at a max 24mbps with a 21mbps average. Note that the initial reading you had corresponds to the maximum amount. This tells me that the AF100 displays the available time if it records at 24mbps. So as you record at a slower bitrate the time displayed will drop slower than real time. As I understand it PCM audio has a constant bit rate, so the audio input would not have any effect.
 
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