Film-Tech Forum ARCHIVE: How to “calibrate” Perspect-A-Stereo card?

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Posted by Gary A. Hoselton (Member # 3447) on 02-18-2018, 06:59 PM:

Input is from Simplex 5-Star soundhead with reverse scan red LED into Component Engineering pickup, then thru CP-500, out Analog Accessory connector pins 35 and 37 Optical Right and Optical Left thru STA-1 stick-on amplifiers to Dolby SA-4 IN R and IN L, which go to Perspect-A-Stereo card inputs. The card’s outputs L, C, and R go to Analog Accessory connector ACCNRL, C, and R pins 19, 15, and 18, thence to a Perspecta User Format that Joel Miller programmed for me last summer.

The Perspect-A-Stereo card is upside-down in the SA-1, has one red LED and four green LED’s under the board. I’ve come by some used Perspecta calibration loops, with fairly worn hand-lettered labels on masking tape as follows:
A 400 Hz
B 35 Hz high
C 35 Hz low
D 25 Hz high
E 25 Hz low
F 400 Hz 50% w/ 25Hz high and 35Hz low.

I have no instructions for the Perspect-A-Stereo card, which has no adjustments on it. What do the LED’s indicate, and how do I use the loops with this card? My only adjustment now is gain in the STA-1 stick-on amplifiers.

Posted by Steve Guttag (Member # 268) on 02-18-2018, 07:46 PM:

The Perspect-A-Card requires NO calibration. The original tests were for the original "tuned" circuits.

The RED LED lights when there are NO tones to denote it is mono and the card will pass the audio out of center.

The next green LEDs indicate Left, Center and Right output. There is a 4th LED for surround, which was NOT part of the Perspecta process. However the card, if an extra jumper were soldered in, to allow a fake surround output if it senses all three signals are present. I've tried it; it wasn't a good effect.

The card was designed to go into a Dolby processor in place of a CAT150 so it has input pins for 300mV and 500mV nominal levels and output at 300mV nominal levels.

You want to hit the input levels as accurately as possible (I'd have to look up what the SA4 card cage is looking for...probably 500mV since it is a CP100 box) but I'd confirm, if I were you). Run Dolby-Tone to set the level, not your Perspecta loops. Once set, your loops should confirm that the card is decoding properly. Since Perspecta is tone level dependent, it is real important to get the input level right for the best results though it will still work to a degree if you are off (the tones control VCAs on the card to ramp the various channels up/down).

Posted by Gary A. Hoselton (Member # 3447) on 02-25-2018, 12:12 PM:

Thanks, Steve, I have it working. At my STA-1 stick-on amplifiers, I set each of L and R inputs to Perspect-A card at 500mVrms referenced to signal ground, threaded up a mid-50's MGM Droopy cartoon, and the red light went out and the L, C, and R green lights began going on and off. Most sound came from center speaker, I think because that is how the cartoon was staged, and beginning and end title music was splashed all across the screen.

Posted by Gordon McLeod (Member # 33) on 02-25-2018, 01:34 PM:

Which make of card do you have

Posted by Mark Gulbrandsen (Member # 72) on 02-25-2018, 02:18 PM:

In Germany they do Perspecta decoding with a DSP audio processor. It even includes the indicators for the control tones....

Mark

Posted by Gary A. Hoselton (Member # 3447) on 02-26-2018, 03:27 PM:

Gordon, I have the Perspect-A-Stereo card. I'm told it was made as a side project by some Dolby fellows in England.

Posted by Sam D. Chavez (Member # 1841) on 02-26-2018, 11:35 PM:

I don't know if the German ones are the same but a couple of Dolby London guys guys did the one you have. They've all retired by now.

Posted by Sam D. Chavez (Member # 1841) on 02-26-2018, 11:36 PM:

I don't know if the German ones are the same but a couple of Dolby Lodon guys guys did the one you have. They've all retired by now.

Posted by Marin Zorica (Member # 1716) on 03-05-2018, 04:14 PM:

For Germany, i think Mark is refering to guys from Karlsruhe cinema where they did programm BSS BLU dsp processor to do it.

Posted by Mark Gulbrandsen (Member # 72) on 03-06-2018, 09:00 PM:

Marin, You are correct.