LHi analog distortion
I'm hoping someone can help me. If you have a LHi or LHe Plus and a waveform or oscilloscope, could you take a brief moment to recreate my test situation?
I recently switched from a (now obsolete) AJA LHe to a LHi card. I’m using the same computer, same OS and all the same analog connections. The LHi analog output has severe distortion when viewed on a waveform monitor. (see link below). The old LHe analog output has no distortion. The distortion is solidly locked to vertical sync (does not roll thru, like 60Hz hum) and the amount of distortion is directly correlated to the luminance values of the video image. A video image with no luminance change from the top to bottom (i.e. full field color bars) results in a nearly disortion free waveform. Leaving the analog output UN-terminated eliminates the distortion entirely (albeit at an un-terminated voltage).
AJA provided a second LHi card which displayed the same distortion. AJA support could not duplicate my situation at thier facility. They suggested my Tektronix WFM300A waveform (along with my Magni waveform and Tektronix oscilloscope) had DC restoration problems and changing the input coupling from DC to AC should correct the waveform (yes, I tried both AC and DC coupling on the oscilloscope).
So, I wrote to Tektronix for their opinion and asked what modification to my waveform monitor would fix my problem. They replied that all the scopes are showing me a fairly significant problem and suspected the distortion is being caused by a power supply problem. But, when I pointed out that the LHe had no issue in the same computer, the distortion from the LHi was locked to vertical sync and could literally be turned in to a sawtooth shape, they had no response. The waveform's internal jumper modification they pointed me to, had no effect on the displayed distortion.
Connect the analog video out to your scope/waveform and properly terminate the signal with a 75 ohm termination. Within your editing software create a 100% white horizontal bar, one-third the image vertical height, full width, center screen, over black background (similar to a narrow letterbox). Or, use the image in the link below. Display thru the editing software or the AJA control panel (as a test signal file). Switch to a vertical rate (2 Field) waveform and see if your card has the same distortion. Try with DC restoration ON and OFF. If you do see the distortion, remove the termination and see if the distortion disappears.
Any replies or assistance would be appreciated. I’m happy to provide more details or answer any questions.
Thanks so much,
A short follow-up;
I purchased a fourth waveform/scope to see if I could confirm or further analyze my problem.
With the "newer" waveform monitor I can change settings in software instead of using physical internal jumpers. Connected the same way as all previous tests.
Terminated vs. un-terminated: Same distortion when terminated, gone when un-terminated
DC Restoration On vs. Off: Same distortion, no difference
DC Coupling vs. AC Coupling: Same distortion, no difference
DC Restorer Slow vs. Fast: BINGO! Fast DC Restoration flattens waveform, no distortion
So, did I correct the actual problem? No. But now I too can hide and therefore ignore the underlying problem and get back to work.