Tom Morrow
August 13th, 2011, 04:18 PM
I just spent all morning measuring the noise on my Canon 60d DSLR, and I'd like to share what I learned for other Canon DSLR users.
This thread
http://www.dvinfo.net/forum/all-things-audio/498727-agc-disable-magic-lantern-vs-juicedlink.html
seems to indicate that the Canon DSLR firmware only adjusts gain with digital gain (ie multiplying the numbers after sampling), and that there is no way of adjusting the analog gain (before sampling) in the Canon firmware like Magic Lantern enables.
However my measurements appear to contradict this; if there was only digital gain then the noise would stay at the same level relative to the signal for all gain settings on the Canon. But I'm seeing the noise level (relative to signal) drop by about 10-12 db for high Canon gain versus for low Canon gain. I'd like to hear comments or suggestions for further experiments that could help determine once and for all whether Canon adjusts analog gain in the 60d firmware.
The excel graph of my results are attached.
To obtain this graph, I fed a -75dbu 1khZ sine wave from my NTI minirator MR2 into the left channel of the 60d, and left the right channel empty so I could observe noise level. I made one .mov file for each of the 63 gain settings, and manually played each one back in Audition while observing the onscreen level meter.
So I had tone level and noise level for all 63 of the level marks (except no noise data for marks below 11 because the noise didn't register on the -60db Audition level meter). I determined by trial and error that treating each level mark as 0.875db yielded a straight line for my tones. That number is different from the 0.375 that Jon predicted based on the chip data sheet... Hmm.
This graph below should theoretically just be two straight horizontal lines if gain was pure digital, but the slope of the noise line indicates to me that something other than simple digital gain is going on. If anyone wants the original spreadsheet data let me know and I'll send it. Please don't read too much into this data, as the manual measuring process means any value is likely to be off by 1-2db. And due to some mistakes keeping track of which click I was on, I ended up redoing the measurements between level marks 24 and 35; I'm guessing battery levels or room temperature changes are the reason for the shallow plateau in the tone levels in that region, rather than any real effects. However the overall trend seems clear... increasing gain lowers the noise level.
This thread
http://www.dvinfo.net/forum/all-things-audio/498727-agc-disable-magic-lantern-vs-juicedlink.html
seems to indicate that the Canon DSLR firmware only adjusts gain with digital gain (ie multiplying the numbers after sampling), and that there is no way of adjusting the analog gain (before sampling) in the Canon firmware like Magic Lantern enables.
However my measurements appear to contradict this; if there was only digital gain then the noise would stay at the same level relative to the signal for all gain settings on the Canon. But I'm seeing the noise level (relative to signal) drop by about 10-12 db for high Canon gain versus for low Canon gain. I'd like to hear comments or suggestions for further experiments that could help determine once and for all whether Canon adjusts analog gain in the 60d firmware.
The excel graph of my results are attached.
To obtain this graph, I fed a -75dbu 1khZ sine wave from my NTI minirator MR2 into the left channel of the 60d, and left the right channel empty so I could observe noise level. I made one .mov file for each of the 63 gain settings, and manually played each one back in Audition while observing the onscreen level meter.
So I had tone level and noise level for all 63 of the level marks (except no noise data for marks below 11 because the noise didn't register on the -60db Audition level meter). I determined by trial and error that treating each level mark as 0.875db yielded a straight line for my tones. That number is different from the 0.375 that Jon predicted based on the chip data sheet... Hmm.
This graph below should theoretically just be two straight horizontal lines if gain was pure digital, but the slope of the noise line indicates to me that something other than simple digital gain is going on. If anyone wants the original spreadsheet data let me know and I'll send it. Please don't read too much into this data, as the manual measuring process means any value is likely to be off by 1-2db. And due to some mistakes keeping track of which click I was on, I ended up redoing the measurements between level marks 24 and 35; I'm guessing battery levels or room temperature changes are the reason for the shallow plateau in the tone levels in that region, rather than any real effects. However the overall trend seems clear... increasing gain lowers the noise level.