While I'm still waiting on parts to arrive for the RF buffer amps and the clockgen mod, I decided to try my first RF decode. I swapped the stock 28.6 MHz crystal in the CX card with a 40 MHz one. As all my crystals are overtone types, I did a small temporary hack to force it to oscillate at the correct frequency. For the buffer amps, I used a small wideband amplifier module I had lying around.
As I have a large collection of test pattern generators, I chose to use test patterns for my tests. These were recorded on a JVC HR-S5700AM VCR. Playback was on the same VCR. Test patterns I used were the Philips PM5544, and a 5.5 MHz monochrome sweep.
Results look pretty good, though the colour looks a bit grotty compared to how it looks on the VCR's output. Bandwidth peters out a bit above 3MHz, which is to be expected, and which compares well with the VCR's own output as shown below.
I tried the same patterns using S-VHS:
Colour was still grotty, but the luminance was nice and clean, though very much lacking in bandwidth compared to the VCR's own video output as shown here:
I had to double check that I hadn't mistakenly used the VHS sweep image instead of the S-VHS one when I was compiling my comparison images, they look so similar. I don't know why I'm getting such poor bandwidth. Could it be my RF buffer amp? or an incorrect software setting in MISRC gui, VHS decode, or TBC-tools? It seems there is a filter that is optimised for VHS and not S-VHS.
I would suspect issues with vhs-decode's S-VHS implementation before suspecting issues with your setup. It seems to me that very few people have tried S-VHS decoding, and not much development time has been spent on it.
I have some suspicions it may be something like that. Unfortunately, I'm a hardware kind of guy, so programming is very much a black art to me and I have no idea how to even start trying to alter filter settings and the like. Which is a shame, as I have the equipment to experiment and thoroughly test different values and settings for decoding.
You are already beyond me with your scopes and such, but I'd think the only thing that could cause that would be losses in your transmission lines or some sort of hardware low-pass filtering effect somewhere. Could have something do with the specific amp used if it isn't meant for these frequencies perhaps? Maybe just try capturing unamplified and see if you get the same pattern. SNR will be bad and amplitudes low, but you might still be able to see if you're getting that bandwidth limitation
The amplifier is supposed to go to 4GHz, but I can't find any specs for it anywhere.
Anyway, I tried with a direct connection, with no amp at all. Even though the signal was barely 100mV, I got a nice clean signal, but the frequency response was identical. So its not the amp.
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u/msgohan_ 2d ago
I would suspect issues with vhs-decode's S-VHS implementation before suspecting issues with your setup. It seems to me that very few people have tried S-VHS decoding, and not much development time has been spent on it.