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Why would a capture card look any worse? Isn't it capturing lossless video output? Just because of the re-encode?

yosito 4y ago
Capture is not lossless. Think about a photocopy machine, every copy loses a small bit of information. Recapturing video output is a similar situation.
- rowanG077 4y ago
  Why? Photocopy is obviously lossy since there is a very noisy digital-analog-digital conversion going on. But a capture card is capturing a digital signal. There should be no loss except for video decoding/encoding artifacts.
  - donkers 4y ago
    The capture itself could be lossless but would be ridiculously huge, re-encoding that to a usable file size will introduce some loss.
  - vel0city 4y ago
    You're not understanding how lossy compression encoders work. Try recompressing a JPEG a few dozen times. Or take an MP3 and export it from Audacity, open the export, export as MP3 again a dozen times and see what it sounds like. All those artifacts keep getting amplified every time you re-encode until it's practically just the artifacts. Every time you render and recompress you're losing information, it's lossy compression after all. https://youtu.be/icruGcSsPp0 https://youtu.be/icruGcSsPp0
    - steve1977 4y ago
      But the loss happens during re-encoding, not during capture.
      - Hackbraten 4y ago
        In practice, that difference doesn’t really matter because almost no one is going to store their captured, already-lossy material in a lossless format.
        porbelm 4y ago
        If you know you have to recompress and want to reduce unneccessary artifacts, you do. But beware that uncompressed video in 8 bpc (not HDR) 1080p @ 30 fps is 1,5 Gbps so you'll need 1,3 TB to store your 2-hour capture :)
      - vel0city 4y ago
        Lots of the time capture cards are returning a compressed video stream instead of raw frame data, at least for non-professional environments. I don't know too many amateur streamers handling SDI around their house.
    - icelancer 4y ago
      Thought for sure it was going to be the legendary Hank Hill JPEG meme: https://www.youtube.com/watch?v=QEzhxP-pdos https://www.youtube.com/watch?v=QEzhxP-pdos
      - vel0city 4y ago
        I just want a picture of a got-dang hot dog! Truly a classic.
    - botanical 4y ago
      But JPEG purposely discards information to save space. A digital stream is copied directly, only a codec would subject it losing information
      - jenadine 4y ago
        But if you want to share it or store it, it is not practical to keep the raw data. You will have to re-encode.
      - happymellon 4y ago
        Like the codec used to get the stream from Netflix to you, to be decompressed for the capture card (so lossless capture of a lossy source) and then back through x264/265 so lossy compression on a lossy compression. Just because there is a capture card in the middle doesn't stop it going through multiple lossy steps.
  - yosito 4y ago
    > except for video decoding/encoding artifacts Which is... lossy
Scoundreller 4y ago
The signal remains digital, but decompressed into a raw bitstream that would be many mbps (think how big lossless filesizes get). So it has to be re-encoded but you’re doubling the compression artefacts, and can only avoid them by really dialing up the bitrate. Maybe someone made a video encoder algorithm that’s tuned toward already compressed and decompressed video. Though I’m in the camp of watching for quality of the story etc. rather than the crispness of the video. If it’s not worth watching in 480p, it’s not worth watching in 4K either.
- rowanG077 4y ago
  A single re-encode will make a difference. But with the proper settings it will be almost unnoticeable.
  - deleted 4y ago
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Zardoz84 4y ago
I think it's both things. Netflix, and other platforms, don't send lossless streams to you. Even at 4k. Plus, you are re-encoding it. Its like doing a VHS copy from another VHS, or creating a new JPEG image from another. Always there is a loss of quality.
- porbelm 4y ago
  Well a 1080p stream at 30 fps would be 1,5 Gbit/s -- a little outside the spec of most people's internet tubes. And 4K UHD at 30 fps would be around 5 or 6 gigabit. It helps to capture the Netflix stream uncompressed to remove the extra compression step you'd otherwise get at capture time, and modern encoders are pretty good, I don't think most people would notice on a laptop screen. On a 40+ inch 4K TV though, it can be quite noticeable
- MikusR 4y ago
  It's closer to VHS to DVD.

Capture is not lossless. Think about a photocopy machine, every copy loses a small bit of information. Recapturing video output is a similar situation.

rowanG077 4y ago
Why? Photocopy is obviously lossy since there is a very noisy digital-analog-digital conversion going on. But a capture card is capturing a digital signal. There should be no loss except for video decoding/encoding artifacts.
- donkers 4y ago
  The capture itself could be lossless but would be ridiculously huge, re-encoding that to a usable file size will introduce some loss.
- vel0city 4y ago
  You're not understanding how lossy compression encoders work. Try recompressing a JPEG a few dozen times. Or take an MP3 and export it from Audacity, open the export, export as MP3 again a dozen times and see what it sounds like. All those artifacts keep getting amplified every time you re-encode until it's practically just the artifacts. Every time you render and recompress you're losing information, it's lossy compression after all. https://youtu.be/icruGcSsPp0 https://youtu.be/icruGcSsPp0
  - steve1977 4y ago
    But the loss happens during re-encoding, not during capture.
    - Hackbraten 4y ago
      In practice, that difference doesn’t really matter because almost no one is going to store their captured, already-lossy material in a lossless format.
      - porbelm 4y ago
        If you know you have to recompress and want to reduce unneccessary artifacts, you do. But beware that uncompressed video in 8 bpc (not HDR) 1080p @ 30 fps is 1,5 Gbps so you'll need 1,3 TB to store your 2-hour capture :)
    - vel0city 4y ago
      Lots of the time capture cards are returning a compressed video stream instead of raw frame data, at least for non-professional environments. I don't know too many amateur streamers handling SDI around their house.
  - icelancer 4y ago
    Thought for sure it was going to be the legendary Hank Hill JPEG meme: https://www.youtube.com/watch?v=QEzhxP-pdos https://www.youtube.com/watch?v=QEzhxP-pdos
    - vel0city 4y ago
      I just want a picture of a got-dang hot dog! Truly a classic.
  - botanical 4y ago
    But JPEG purposely discards information to save space. A digital stream is copied directly, only a codec would subject it losing information
    - jenadine 4y ago
      But if you want to share it or store it, it is not practical to keep the raw data. You will have to re-encode.
    - happymellon 4y ago
      Like the codec used to get the stream from Netflix to you, to be decompressed for the capture card (so lossless capture of a lossy source) and then back through x264/265 so lossy compression on a lossy compression. Just because there is a capture card in the middle doesn't stop it going through multiple lossy steps.
- yosito 4y ago
  > except for video decoding/encoding artifacts Which is... lossy

Scoundreller 4y ago

The signal remains digital, but decompressed into a raw bitstream that would be many mbps (think how big lossless filesizes get). So it has to be re-encoded but you’re doubling the compression artefacts, and can only avoid them by really dialing up the bitrate. Maybe someone made a video encoder algorithm that’s tuned toward already compressed and decompressed video. Though I’m in the camp of watching for quality of the story etc. rather than the crispness of the video. If it’s not worth watching in 480p, it’s not worth watching in 4K either.

rowanG077 4y ago
A single re-encode will make a difference. But with the proper settings it will be almost unnoticeable.
- deleted 4y ago
  [deleted]

deleted 4y ago

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Zardoz84 4y ago

I think it's both things. Netflix, and other platforms, don't send lossless streams to you. Even at 4k. Plus, you are re-encoding it. Its like doing a VHS copy from another VHS, or creating a new JPEG image from another. Always there is a loss of quality.

porbelm 4y ago
Well a 1080p stream at 30 fps would be 1,5 Gbit/s -- a little outside the spec of most people's internet tubes. And 4K UHD at 30 fps would be around 5 or 6 gigabit. It helps to capture the Netflix stream uncompressed to remove the extra compression step you'd otherwise get at capture time, and modern encoders are pretty good, I don't think most people would notice on a laptop screen. On a 40+ inch 4K TV though, it can be quite noticeable
MikusR 4y ago
It's closer to VHS to DVD.