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Thread: HD-PVR has poor image quality

  1. #11
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    Re: HD-PVR has poor image quality

    Okay. It's hard to know exactly what's different without seeing what you are seeing. Are you running a custom kernel (I thought you were on Ubuntu 12.04)? It might be helpful to make a short recording of the same program in Linux and in Windows and then examine the characteristics of the file so we can see what exactly is recording differently (bit rate, resolution, etc.).
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  2. #12
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    Re: HD-PVR has poor image quality

    I am using Mythbuntu 12.04 with the standard kernel. (I wouldn't know how to customise the kernel)

    I have made two 30 second recordings as requested. When I play them both back in VLC side by side it's much harder to see the difference, but you can see it a bit on the screen menus.

    https://www.dropbox.com/sh/iwh86wrj4w3ahzo/TSjmdAgXaa

  3. #13
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    Re: HD-PVR has poor image quality

    Hmmm, I guess mythbuntu uses a new kernel than ubuntu, or perhaps there has been a significant update to the kernel that I haven't installed (I rarely install updates on solidly working system)? I'm running Ubuntu 12.04.2 on most of my machines the kernel version is 3.2.0-39. I'm not at my home to view those files right now, but I'll try and take a look at them in the next day or so and see if I can be of any help...
    Linux & MythTV 4 Life
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  4. #14
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    Re: HD-PVR has poor image quality

    Okay, because I like to tinker more than I like my job right now, I downloaded these at work and looked at them. They do look very similar, but I can tell what you are talking about, the Linux one does appear a little "softier" or not quite as sharp. Now comparing it to TV, that's passthrough with no processing, so I would expect that to look better. I suspect the TV looks better than the windows or Linux recordings. I looked at the file attributes and for video they look about the same (very similar bitrate, same resolution, etc). Audio is encoded differently, but not an issue for you. Interestingly when I play the videos back they show up as the same window size when using VLC, but not when using Windows Media Player. The linux recording by default starts off smaller, even though files show as having the same resolution. Not sure what that's about, but I don't use Windows Media Player often enough to know if that means anything...

    The only thing I can think of right now is that Windows and Linux are using different picture settings (contrast, sharpness, saturation, etc.) when encoding. You might want to check what the settings are for those in Windows (I assume somewhere in the windows software you can see and change these settings) and compare them to what they are in Linux
    Code:
    v4l2-ctl --list-ctrls
    .
    Linux & MythTV 4 Life
    Running ubuntu on an Atom 330, Atom D525, Turion 64 X2 TL-56, X2 3800, X2 4200, X2 4600, E2180, Core i5 450M, and a Core i7-2600
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  5. #15
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    Re: HD-PVR has poor image quality

    v4l2-ctl --list-ctrls
    brightness (int) : min=0 max=255 step=1 default=128 value=128 flags=slider
    contrast (int) : min=0 max=255 step=1 default=64 value=64 flags=slider
    saturation (int) : min=0 max=255 step=1 default=64 value=64 flags=slider
    hue (int) : min=0 max=30 step=1 default=15 value=15 flags=slider
    sharpness (int) : min=0 max=255 step=1 default=128 value=128 flags=slider
    audio_encoding (menu) : min=3 max=4 default=3 value=3 flags=update
    video_encoding (menu) : min=2 max=2 default=2 value=2
    video_bitrate_mode (menu) : min=0 max=1 default=1 value=1 flags=update
    video_bitrate (int) : min=1000000 max=13500000 step=100000 default=6500000 value=6500000
    video_peak_bitrate (int) : min=1100000 max=20200000 step=100000 default=9000000 value=9000000 flags=inactive


    All the picture settings are the same in Windows except Hue which is 0, but I doubt that difference is causing my issue.

  6. #16
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    Re: HD-PVR has poor image quality

    Well, I'm mostly out of ideas... The only other thing I can think of is to play with the controls in Linux (specifically sharpness and maybe contrast) to see if you can generate a better video. The wiki indicates the settings have to be set before each recording... After you adjust settings you may want to verify via the command above that the settings have changed right before the recording starts.
    Linux & MythTV 4 Life
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  7. #17
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    Re: HD-PVR has poor image quality

    What are you trying to record here? Most people buy an HD-PVR to record HD. You seem to be recording at SD resolution, although you don't say whether the source is HD or SD.

    Encoding an analogue signal is inherently lossy and the picture won't be as good as, for example, recording a digital stream directly.

    Also what are you using to record in windows?

  8. #18
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    Re: HD-PVR has poor image quality

    The image of OSD gui shows the difference very well.
    The actual program source material is far too blurred/soft to be useful for easy comparison.
    That program is edited/mastered blurred/soft.

    IMO The OSD image shows the linux recording to be far worse than windows.
    The linux recording looks to have one one field repeated twice for each frame. i.e. half vertical resolution.
    Mediainfo shows both are interlaced video.
    HD-PVR does not transcode, only encodes in the same video mode as input.

    Playing the linux sample recording with field order swapped did not look any worse or better but then the OSD gui image is static position so this should be anticipated.

    The HD-PVR recordings (that I have) at SD480i60 & HD720p60 look good. The HD720p60 recordings are very good.
    It is possible the 50Hz support in the HD-PVR driver never had any testing.

    There was a collection of HD-PVR patches to the kernel in last couple months, including mention of 576i50 & other video modes.

    Can you provide a sample of high quality program with movement ? MotoGP or tennis?

  9. #19
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    Re: HD-PVR has poor image quality

    Ok it's been a while but I'm finally getting back onto this.

    I have been playing with the bitrates and sharpness at the command line and realised that the bitrates I set were being overridden by the ones set in the backend recording profiles. So I went there and started experimenting with different values and saw some minor improvement in the picture quality.

    I also thought I saw some improvement by increasing the sharpness, but that gets reset to the default value after reboot.
    Then for some reason I decided to set the sharpness to 0 and to my surprise the picture is near perfect. The picture looks good and all the channel overlays are crisp without the previous fuzziness.

    My problem now is I dont know how tto set the sharpness to 0 so that it sticks after a reboot.
    I see the wiki has a startup script to set the various values, but it doesnt say what to do with it or how to execute it.

    Can anyone tell me how to set the sharpness value permanently?

  10. #20
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    Re: HD-PVR has poor image quality

    startup scripts can usefully be placed in /etc/rc.local - which is run as root on bootup.

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