Why -3db gain?

Ken Tanaka · December 30th, 2002, 02:46 PM

Andre,
Confucious said: "Never argue with man who have technical reference manual for camera and knows what he read." ...which Don has and does. <g>

Actually, isn't this getting awfully thick and irrelevent to Charles' original question?

Robert Knecht Schmidt · December 30th, 2002, 04:48 PM

Well I'm interested in the inner workings of the XL1S.

_____ thick and irrelevant
|
V

Robert Knecht Schmidt · December 30th, 2002, 04:51 PM

If the XL1S gain control is analog, why is the knob designed to be discrete adjust (versus being a continuously variable pot)? That is, why can't I turn the knob between settings to get +4 dB etc.?

Andre De Clercq · December 31st, 2002, 04:32 AM

I didn't know that confuscious (+/- 500 B.C.) already knew about the existance of technical ref manuals for camera's...Because I was for over 30 years, toroughly involved in circuit and chip design for simular signal processing, and I know about the strip down exercises to be done for so called ref manuals to make them useable for repair people, I am not impressed by the "interconnected blocks" style wiring diagrams which are published through such channels...Sorry Don. Happy Newyear anyway!

Andre De Clercq · December 31st, 2002, 05:51 AM

Because the unusual 9-bit processing mentioned by Don I searched and found some info from Chris Hurd http://www.dvinfo.net/canon/skinny.php. Now I understand why Jeff Donald allways sees banding in the sky on his DV camera(XL1s I suppose) where it is seldon mentioned (although possible)in other DV articles. The XL1s apparently has only a 6-bit equavalent grayscale resolution... of course resampled on a 8-bit structure for the DV format. This makes me also conclude that gamma and WB are done in the (9-bit) digital domain. These are the two processes by which you "loose bits": 2 for gamma (4xgain at black) and 6 db for WB (=1-bit). Other processes ( pixel shift, noise reduction...) don't reduce grayscale resolution.

Jeff Donald · January 2nd, 2003, 06:11 AM

That makes a lot of sense. Thanks for doing the research on it. It's very useful information.

Jeff

Don Palomaki · January 2nd, 2003, 07:02 PM

> ... If the XL1S gain control is analog, why is the knob designed to be discrete adjust ...

The gain control is a switch that selects a specific voltage step on a voltage divider. That voltage is sent to the control/logic circuit. Depending on the voltage level you get the differnt gain settings, or automatic gain. The analog gain is selected in descrete steps, which works for video. If the control directly controlled the gain via a potentiometer (like many stereo volume controls) it could introduce noise, especially as it ages and would be hard preseed to provide logic selections as well.

As for the 6-bit grayscale. Not sure how that computes. Would be easy to check. Someone shoot a gray wedge and read the luma levels in the output. 6-bit implies only 64 possible values for the luma signal. The XL1 DSP shows 9-bit input, 8-bit output.

FYIW, the old L2 and Ai-Digital camcorders Hi-8 system used 6-bit for the color portion of their internals DSP.

December 30th, 2002, 02:46 PM	#16
Ken Tanaka Retired DV Info Net Almunus Join Date: Dec 2001 Location: Chicago, IL Posts: 6,943	Andre, Confucious said: "Never argue with man who have technical reference manual for camera and knows what he read." ...which Don has and does. <g> Actually, isn't this getting awfully thick and irrelevent to Charles' original question? __________________ Lady X Films: A lady with a boring wardrobe...and a global mission. Hey, you don't have enough stuff! Buy with confidence from our sponsors. Hand-picked as the best in the business...Really! See some of my work one frame at a time: www.KenTanaka.com

December 30th, 2002, 04:48 PM	#17
Robert Knecht Schmidt Wrangler Join Date: Feb 2002 Location: Cleveland, Ohio Posts: 1,933	Well I'm interested in the inner workings of the XL1S. _____ thick and irrelevant \| V __________________ All the best, Robert K S Search DVinfo.net for quick answers \| The best in the business: DVinfo.net sponsors

December 30th, 2002, 04:51 PM	#18
Robert Knecht Schmidt Wrangler Join Date: Feb 2002 Location: Cleveland, Ohio Posts: 1,933	If the XL1S gain control is analog, why is the knob designed to be discrete adjust (versus being a continuously variable pot)? That is, why can't I turn the knob between settings to get +4 dB etc.? __________________ All the best, Robert K S Search DVinfo.net for quick answers \| The best in the business: DVinfo.net sponsors

December 31st, 2002, 04:32 AM	#19
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	I didn't know that confuscious (+/- 500 B.C.) already knew about the existance of technical ref manuals for camera's...Because I was for over 30 years, toroughly involved in circuit and chip design for simular signal processing, and I know about the strip down exercises to be done for so called ref manuals to make them useable for repair people, I am not impressed by the "interconnected blocks" style wiring diagrams which are published through such channels...Sorry Don. Happy Newyear anyway!

December 31st, 2002, 05:51 AM	#20
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	Because the unusual 9-bit processing mentioned by Don I searched and found some info from Chris Hurd http://www.dvinfo.net/canon/skinny.php. Now I understand why Jeff Donald allways sees banding in the sky on his DV camera(XL1s I suppose) where it is seldon mentioned (although possible)in other DV articles. The XL1s apparently has only a 6-bit equavalent grayscale resolution... of course resampled on a 8-bit structure for the DV format. This makes me also conclude that gamma and WB are done in the (9-bit) digital domain. These are the two processes by which you "loose bits": 2 for gamma (4xgain at black) and 6 db for WB (=1-bit). Other processes ( pixel shift, noise reduction...) don't reduce grayscale resolution.

January 2nd, 2003, 06:11 AM	#21
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	That makes a lot of sense. Thanks for doing the research on it. It's very useful information. Jeff

January 2nd, 2003, 07:02 PM	#22
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,489	> ... If the XL1S gain control is analog, why is the knob designed to be discrete adjust ... The gain control is a switch that selects a specific voltage step on a voltage divider. That voltage is sent to the control/logic circuit. Depending on the voltage level you get the differnt gain settings, or automatic gain. The analog gain is selected in descrete steps, which works for video. If the control directly controlled the gain via a potentiometer (like many stereo volume controls) it could introduce noise, especially as it ages and would be hard preseed to provide logic selections as well. As for the 6-bit grayscale. Not sure how that computes. Would be easy to check. Someone shoot a gray wedge and read the luma levels in the output. 6-bit implies only 64 possible values for the luma signal. The XL1 DSP shows 9-bit input, 8-bit output. FYIW, the old L2 and Ai-Digital camcorders Hi-8 system used 6-bit for the color portion of their internals DSP.