0 Replies Latest reply on Dec 15, 2017 12:47 PM by CalebCZ

    HDMI to VGA conversion delay

    CalebCZ

      Hi all, I'm posting this here just in case anyone else wants to know and possibly to compare results with other users.

       

      Recently, I needed to convert HDMI signal from AMD HD7970 graphics card to analog VGA signal. I was aiming to get as little conversion delay as possible, because too big delay would be bad for gaming. First I tried a noname chinese converter, but that one had poor output signal quality - the image looked washed out. So I moved to JDA213 converter by j5create, which was supposed to conform to VESA Video Signal Standard (VSIS) Version 1, Rev. 2. It probably does, because the image looked as good and crisp as from HD7970's native analog output. So I set out to measure its conversion delay. I prepared two male 15-pin VGA connectors to which I soldered 75 ohm load resistors between green signal and its ground (pins 2 and 7) and between vertical sync and its ground (pins 14 and 10). I plugged first connector to HD7970's native VGA output and the second one to JDA213's VGA output. Then I plugged the JDA213 converter into HD7970's HDMI output. I hooked 4-channel digital oscilloscope with four 10:1 probes to the resistors and displayed the signal waveforms at 1024x768@60 Hz resolution. With this setup, I measured conversion delay of 4.3 ms. To make sure I was measuring the correct delay, I run a test pattern that changed every second or so and then captured the change with the oscilloscope.

       

      Afterwards, I opened the JDA213 converter to see what's inside. It's based on Chrontel CH7101 circuit, so other converters that use this IC should have similar conversion delay. I was actually quite surprised (and pleased) it was only 4.3 miliseconds, or about 1/4 of the frame rate. If anyone knows or measured delays of other converters, please post them here!

       

      Edit: I forgot to mention one important thing: the JDA213 converter does NOT work in BIOS, the connected VGA device goes to sleep during boot sequence. I've tested a few similar converters and I found that Hama 99054569 doesn't suffer from this shotcoming. Unfortunately, it has slighly worse output VGA signal quality.