cheap wideband spectrum analyzer from VGA to HDMI converter and USB HDMI capture - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 25, 2024, 03:16:05 am
content: VGA to HDMI converters are dirt cheap, impressive considering they contain 3x high speed ADCs capable of 148.5Msps for 1080p 60Hz. Sadly, they don't do continuous capture so they can't really be used for SDRs. But spectrum analyzers don't need continuous capture and one capable of viewing 60-70MHz of bandwidth in real time (that is, not sweeping) would be quite impressive by hobby standards. Double that if two channels can be used for I/Q mode, although that would require the HDMI capture device to support capturing RGB and not the downsampled YUV, even if only as snapshots.So my idea would be to have a microcontroller generating the sync signals to get the converter to operate. If a RF front end is used, that microcontroller could also coordinate sweeps to step to the next frequency as vertical blanking starts, giving it time to settle down before it starts capturing again.The issue I see is that the data would have a bunch of "holes" in it from the horizontal blanking. Apparently, that can be dealt with by using a "nonuniform FFT", although I'm not familiar enough with that to understand how to use it correctly.

author: moffy
author_rank: Super Contributor
timestamp: October 25, 2024, 03:58:01 am
content: Quote from: NiHaoMike on October 25, 2024, 03:16:05 amThe only non-uniform FFT, I can think, of is where the original signal is sampled at uneven times and a precision interpolator is used to recreate the signal and sampled at a consistent dt which is then used for the FFT. If that is the case it wouldn't help in this circumstance as the holes are way too large to interpolate, if not the case could you provide some reference for the "nonuniform FFT", thanks.

author: Someone
author_rank: Super Contributor
timestamp: October 25, 2024, 06:33:45 am
content: Quote from: moffy on October 25, 2024, 03:58:01 amProbably an incorrect addition of "Fast" to make FFT, when they really meant DFT. Which then points to the obvious:https://en.wikipedia.org/wiki/Non-uniform_discrete_Fourier_transformQuote from: Wikipedia: Non-uniform discrete Fourier transform

author: mawyatt
author_rank: Super Contributor
timestamp: October 25, 2024, 04:31:18 pm
content: The Non-Uniform FFT may be related to the Non-Uniform Sampling ADC, or NUSADC mentioned here, see #12:https://www.eevblog.com/forum/projects/measuring-full-20v-on-arduino-without-voltage-dividers/msg3658537/#msg3658537Best

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 26, 2024, 04:00:59 am
content: Found this for "raw" HDMI input with the MS2130 which involves setting some registers to bypass internal processing:https://github.com/steve-m/hsdaohNot sure if that will allow reading the individual channels with no downsampling. Although as it turns out, if a 1/2 downsample is as good as it gets, it would allow for a simplification of the input filters when operating in I/Q mode.It's also a bit ironic that between the $15 MS2130 HDMI capture stick and the $5 MS9288C VGA to HDMI adapter, all the parts to make a really fast digitizer are there but some extra bits are making things more difficult. On the off chance Macrosilicon is reading this, take your high speed ADC IP and your USB 3.0 IP and put them together into a new chip, you'll take the SDR and USB oscilloscope markets by storm.

author: metropolis
author_rank: Newbie
timestamp: November 30, 2024, 01:50:33 pm
content: What's funny is that this was the original idea I had for a couple of years before coming up with hsdaoh. After implementing osmo-fl2k I always wondered if the opposite direction would also be possible with an VGA adapter.Now that I have a transparent link by using a MS2130 (for at least for 2 out of the 3 TMDS channels), I think I should give this another go.Apart from the obvious HSYNC/VSYNC problem, there is another one however: if you look at the block diagram of the MS9288 for example, you see that it implements "Auto white balance" and "Video Process" blocks, which seem to be on by default.The chip has an I2C slave interface though, and I want to fiddle a bit with that. Hopefully there's a register somewhere that allows to turn the white balancing off.I also thought about using three synchronized, phase shifted adapters in parallel to get an uninterrupted stream. You need three, because when the second one is during its VSYNC period, it cannot fill in the HSYNC gaps of the first one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 01, 2024, 04:36:17 am
content: I'm now thinking that rather than running it at 1080p, instead run it at 1024x768 with a refresh frequency high enough to max out the dot clock limit of the chips. Then each line would have 1024 points which is a nice power of 2 to process with FFT. Or maybe 2048x1536 slowed down to stay within limits? (I'm not sure what screen resolutions the MS9288C would accept.)For arbitrary data from a FPGA, it looks like the CH569W would be a better bet with a pair of dev boards going for a bit more than $30.

author: metropolis
author_rank: Newbie
timestamp: December 01, 2024, 02:21:22 pm
content: I have those CH569 devboards here. The issue with it is that even if you use the parallel interface only in 8 bit mode, you still need 4 control lines. That is 4 more than HDMI. One of the cheapest FPGA boards, the Tang nano series, only has a limited number of pins, but already comes with an HDMI socket.For the low-cost end of things, I just ported it to the Raspberry Pi Pico2, where I can achieve 75 MByte/s. Not quite the speed you get with an FPGA, but still useful for some applications:https://github.com/steve-m/hsdaoh-rp2350If you want to spend a bit more money, you can just get the Tang Mega 138K or the upcoming Tang Mega Neo 60K, which already come with a CH569 onboard.I also read that the upcoming Tang Console 60K will have an FPC connector to connect to the PCIe host port of a Raspberry Pi 5, that'll be interesting as well, depending on the price.