Streamer aspect ratio

Discussion about this major DATV Project. See https://wiki.batc.org.uk/The_Portsdown_Transmitter
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G3GJA
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Streamer aspect ratio

Post by G3GJA » Thu May 06, 2021 11:13 pm

I've noticed on GB3EY that the streamer feed as seen at batc.org,uk/live/gb3ey has an aspect ratio closer to 5:4 by the time signal has gone through the repeater's Ryde receive and its Portsdown streamer. Monitoring the input signal locally confirms that the source has a 4:3 aspect ratio.

From years back I seem to remember an issue in some computer generated video caused by the assumption that the PAL pixels are 1:1 ratio whereas they are in fact oblong; I don't recall the exact details. Is this related to the horizontally squashed display?

Clive G3GJA

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Re: Streamer aspect ratio

Post by g0mjw » Fri May 07, 2021 8:06 am

More likely the many compromises with systems being built and still using PAL even in 2021. It's maybe time to go completely digital within the repeater and arrange it such that what comes out is effectively what goes in. Sent it 4:3 stream 4:3, send 16:9, stream 16:9, send 64:27...

Mike

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Re: Streamer aspect ratio

Post by G8GKQ » Fri May 07, 2021 9:48 am

Hi Clive

The hardware in the EasyCap captures at 720x576 25 Hz, and by default the Portsdown streamer software retains this resolution. I believe that the pixel aspect ratio is 1:1 but haven't looked at it in detail. So that makes the image aspect ratio 5:4.

You could probably set a correcting pixel aspect ratio in the ffmpeg call here: https://github.com/davecrump/portsdown- ... a.sh#L1519 or set the Video Width to 768. Not sure how changing the video width would affect performance though.

Dave

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Re: Streamer aspect ratio

Post by G3GJA » Sat May 08, 2021 10:48 pm

Thanks for the info Dave. I'll take a look at the code but I don't pretend to know what to tweak in FFMPEG.

The problem appears to get progressively worse as the content moves through the system. See the graphic below. System for this test comprised of a Portsdown 2020 using the internal testcard transmitted to GB3EY, received on its Ryde Receiver and retransmitted by a DTX1. The Porstdown Streamer captures the video feed to the DTX1.

Input and output RF captures were monitored by Minitioune. Screen shots were taken of both and the Streamer output and then the three images stitched together and their height normalised to 576 pixels without altering the aspect ratio.

Aspect ratio comparison.jpg
Aspect ratio comparison.jpg (192.8 KiB) Viewed 1195 times
Left hand pictured as transmitted @ 1.33:1 Middle picture as seen relayed by GB3EY @ 1.21:1 using Ryde Rx and the right hand shows the Streamer ouput. The aspect ratio is now 1.13:1 or pretty much square.

It seems to me that there are two stages of aspect ratio distortion in play here; firstly in the Ryde Receiver and secondly in the streamer chain. I'm assuming that the DTX1 doesn't add to the mix here. I've not seen any evidence that it does.

Has anyone else noticed this?

Clive G3GJA

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