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Understanding precision sync

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Offline ottokar

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Re: Understanding precision sync
« Reply #20 on: 23 / March / 2018, 07:36:57 »
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Hello,
 I have a quick question about this topic:

 My understanding is that there was a big change in how usb triggering happened for older cameras (pre 2010 or so) compared to newer ones (like the sx150).
 I believe that older cameras will take a shot within 1 or 2 millisec of the usb trigger, and that this whole discussion applies only to newer cameras - is that correct?
thanks,
andrew
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Offline waterwingz

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Re: Understanding precision sync
« Reply #21 on: 23 / March / 2018, 08:10:18 »
Quote from: ottokar on 23 / March / 2018, 07:36:57
My understanding is that there was a big change in how usb triggering happened for older cameras (pre 2010 or so) compared to newer ones (like the sx150). I believe that older cameras will take a shot within 1 or 2 millisec of the usb trigger, and that this whole discussion applies only to newer cameras - is that correct?
I do not believe that to be true. Can you post a link or was that from memory?

The CHDK USB remote code was completely rewritten in 2012 but it's the same for every camera.

That being said, it's possible that some cameras are better than others with the precision of their "sync" but I don't believe anyone has compiled a complete list.

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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline andrew.stephens.754365

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Re: Understanding precision sync
« Reply #22 on: 23 / March / 2018, 09:30:19 »
Edit (30 Nov 2018):
all comments below the double horizontal rule here were made before realising that vnd's precision sync code was added to chdk trunk in 2014.

The entire comment is probably misleading so probably best to ignore but will leave in place for the record only. 

Quote from: waterwingz on 23 / March / 2018, 08:10:18
Can you post a link or was that from memory?

Perhaps this, although I can't easily see the original base for that:

Quote from: waterwingz on 23 / October / 2012, 18:55:39
Possibley using one of the newer cameras with "lousy" sync.  I can probably patch the assembler files for you if they don't follow the task hook convention that vnd is using.

Do you happen to know the value of GPIO_TICKS_TO_INTERRUPT 0xC0F0713C for A1200 to help prove/otherwise my theory (i.e. Edit2) here:
https://chdk.setepontos.com/index.php?topic=13363.msg136480#msg136480
« Last Edit: 30 / November / 2018, 08:05:33 by andrew.stephens.754365 »
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Offline waterwingz

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Understanding precision sync
« Reply #23 on: 23 / March / 2018, 09:49:08 »
Quote from: andrew.stephens.754365 on 23 / March / 2018, 09:30:19
Perhaps this, although I can't easily see the original base for that:
Quote from: waterwingz on 23 / October / 2012, 18:55:39
Possibley using one of the newer cameras with "lousy" sync.  I can probably patch the assembler files for you if they don't follow the task hook convention that vnd is using.
To clarify, while the variation in the length of the interval between remote release and shutter release seemed to get much worse on some newer cameras, there was never a time when older cameras would always sync reliably within a millisecond or so.  There have always been outliers.
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline andrew.stephens.754365

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Re: Understanding precision sync
« Reply #24 on: 23 / March / 2018, 11:59:02 »
Edit (30 Nov 2018):
all comments below the double horizontal rule here were made before realising that vnd's precision sync code was added to chdk trunk in 2014.

The entire comment is probably misleading so probably best to ignore but will leave in place for the record only. 

Quote from: waterwingz on 23 / March / 2018, 09:49:08
To clarify

Ok.

Quote from: waterwingz on 23 / March / 2018, 09:49:08
There have always been outliers.

My current understanding/theory is that the vast majority of these outliers have been associated with current counts greater than the "critical value" at switch release and that the smaller the fraction of a standard count this critical value is then the longer the potential time between the larger than critical value count and the end of the subsequent standard period (i.e. worse sync results). 

For S95, reading the testing sync thread, the std period is 272 & critical value is 224. On switch release, immediate sync within 1 or 2ms should, nearly always occur in 82% of cases. Assuming a std period of 32ms the 18% of outliers should, mostly, vary between around

(((272-225) / 272) + 1) x 32ms = 37.53ms  & 32ms

in absolute terms from switch release.

If the A1200 critical value was a much smaller fraction of the std period it could help explain (or not) why no shot occurred immediately on switch release and the sync variances were all between 40 & 63ms - the critical count would need to be very low to account for the 63ms timing though which dampens the theory.

At critical count 541 and std period 620, SX150IS sync (i.e. nearly always within 2ms of immediate switch release) is 87% and inherently better than S95.

I am not aware of any absolute sync measurement times other than your own diagram.
« Last Edit: 30 / November / 2018, 08:07:08 by andrew.stephens.754365 »
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Offline ottokar

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Re: Understanding precision sync
« Reply #25 on: 24 / March / 2018, 20:52:18 »
Hi,
 about this:

Quote
I do not believe that to be true. Can you post a link or was that from memory?

if I go to this link here:
https://chdk.setepontos.com/index.php?topic=7758.0

some quotes from the first page are:

(about SDM but i understand that the sync code is the same )
Quote
Tests with SDM (an early breakaway from the original CHDK code) typically synch to within a fraction of a msec :-

also:
Quote
On the G12 and SX30 this time is very consistent, on the newer IXUS310 and SX40 it can vary by up to 50 msec.

So far we haven't found out why this is the case or any way to overcome this.

and:
Quote
Unless using a camera newer than SX30/G12, you should achieve synch of a fraction of a msec.

thanks,
Andrew
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Offline andrew.stephens.754365

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Re: Understanding precision sync
« Reply #26 on: 25 / March / 2018, 11:29:49 »
Edit (30 Nov 2018):
all comments below the double horizontal rule here were made before realising that vnd's precision sync code was added to chdk trunk in 2014.

The entire comment is probably misleading so probably best to ignore but will leave in place for the record only. 

Quote from: philmoz on 01 / March / 2012, 20:55:19
On the G12 and SX30 this time is very consistent, on the newer IXUS310 and SX40 it can vary by up to 50 msec.

So far we haven't found out why this is the case or any way to overcome this.

Phil, you don't happen to know the std period & "critical" count for any of those models?
« Last Edit: 30 / November / 2018, 08:07:41 by andrew.stephens.754365 »
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