MPEG is decoded into what?

Discussion in 'Blu-ray & DVD Players & Recorders' started by Jeroen1000, Feb 12, 2009.

  1. Jeroen1000

    Jeroen1000
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    I hope I've put this in the right section if not, feel free to move:)

    I'm asking this question primarily out of interest (although it is allowed according to our cable company) and there is no need to flag this as illegal.
    If one capture's a stream from a setup box or another source, what exactely is being captured? If MPEG II is sent over an HDMI link, or MPEG 4, what is the difference? It has to be decoded before it is sent over the link (unless the TV has serveral decoders what it has not)and because it is digital it would have to be one's and zero's.

    So I'd assume capturing that stream would yield a very big decompressed file just full of one's and zero's? That file is no longer MPEG II of MPEG 4 but just bits? How can the TV tell what it is getting then? How can it tell or decide on the aspect ration or resolution? I've gotten a little lost here. Can anyone describe what happens?

    Cheers,

    Jeroen
     
    Last edited: Feb 12, 2009
  2. Trollslayer

    Trollslayer
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    It is decoded before being sent over HDMI, the data rate can be 4Gbit/sec or more.
     
  3. Gavtech

    Gavtech
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    It depends what and where you 'capture it'
    You mention cable. They may encode and deliver differently from the standard DVB broadcast model which [ currently] interleaves many channels into six digital pipes [ muxes ]
    I assume cable channels use multiplexing techniques also ... but I dont know the detail.

    A multiplexed stream would be absolutely no use to you ...assuming you are seeking to manipulate it.
    It isnt
    also not

    Digitised RGB is sent over HDMI
    It is not conceivably possible to 'capture' what goes over HDMI.
    For one it is a multichannel parallel system which is encoded in a very particular way and it will not send anything unless it get a security OK.
     
  4. Jeroen1000

    Jeroen1000
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    My wordings may be a little vague. I'll try my best to put it more clearly.

    The cable box allows to capture the 'bitstream' (so no HDCP issues here) that flows over the HDMI cable when its output is set @ 576i.

    If I understand you correctly Gavtech, the HDMI capture card will be capturing digitised RGB? I not quite sure I get what that implies. I thought it would be all 1's and 0's flowing over the cable and at some point they would be made analog (translated into volt levels) by the TV? When captured they would be left in digital form.
    I am aware both MPEG II and MPEG 4 need to be decoded before being sent over the HDMI cable. I'm just very in the dark how to make it MPEG II again without srewing up the PQ.

    @ trollslayer, I assume the datarate for 576i will be less demanding on disk space:)
     
    Last edited: Feb 12, 2009
  5. Alphathon

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    In that case it is not capturing the video from the HDMI, simply dumping the video and audio data to the hard drive as well as sending it over HDMI.

    I think you misunderstand what is meant by it is 0s and 1s. EVERY digital signal is 0s and 1s. It is how all data is encoded digitally (please note encoded does not meen compressed). RGB is basically 3 different "bitstreams" (streams of "bits" - which are made up of 0s and 1s) one for the Red colour channel, one for Green and one for Blue, hence RGB. I am assuming here that you know at least a little about how light behaves and how pictures on a TV screen are made up. If not I can explain further. And yes, you are right that it would remain digital in some form or other until it gets to the TV, as there is no reason to convert it to analogue until a particular device (in this case the screen and speakers) requires it.

    This would have to be done BEFORE being sent over HDMI for two reasons. 1 is that there are very few HDMI capture cards, and those which exist are very expensive.

    2nd is that when you encode an MPEG stream (another lot of 0s and 1s, only this time the video is compressed and not just encoded) you inherantly loose quality due to the compression. Both DVB-T (freeview) and DVB-S (sky and freesat) use MPEG so I assume DVB-C (cable) does as well (that's assuming you are in Europe of course, if you are American it will use a different standard, but is likelly MPEG encoded as well).

    Anyway, when you receive the signal, you box decodes it into raw video data (still a bitstream), then sends it over HDMI (or other connector) to your TV (it would be converted to analogue as well if using an analogue connector such as composite, component, s-vidoe or VGA). If you then re-encoded it as MPEG you are basically loosing a little data from the raw stream in order to compress it. However the stream you are compressing has already been compressed at least once, so has less image and sound data that it should already, and re-encoding it makes this worse. However, most PVRs (I don't know if your box is a PVR or not) will simply dump the MPEG stream to the hard drive before it is even decoded unless you tell it to compress it further (to save space).

    Than what? The higher the resolution the larger the file size. 576i is the basic standard for European TV (US uses 480i I believe) so that is likelly what will be dumped if you are using a PVR, but if you choose a lower resolution or higher compression setting it will be smaller. Video captured from HDMI will likelly be upscaled (and a card capable of capturing it would be extortionately expensive), so would either be 720p, 1080i or 1080p. If it is 1080p it will be roughly 12-13x the size if the audio is the same as the original, or even larger if that is matrixed up to full surround.

    I hope I haven't been too confusing here :rolleyes:
     
    Last edited: Feb 12, 2009
  6. Gavtech

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    Your cable box does all the necessary tuning/discriminating, decoding and processing and produces an analogue picture in RGB form.

    In other words it is already decompressed and has no 'Mpeg- ness'

    HDMI is a secure digital transmission system. It intakes the analogue signal, converts into digital and encodes it into the HDMI transmission standard.
    It is received at the far end and decoded from the HDMI standard into the discrete Video/ Audio/ Auxiliary and Data channels.
    You have not said ... but reading between the lines I gather you are trying to directly capture the cable output via HDMI... and it is obviously being limited to a degraded SD form to allow you to do so.

    So I do not understand the problem here... because your capture card will do the job of re-compressing into an MPEG form.
     
  7. Jeroen1000

    Jeroen1000
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    This is going to get complicated, I'm sure. Don't take offence to my arguing though. Just making good discussion:smashin:


    I'm not sure what you are getting at here but allow me to take a long winded guess:)(sorry for that but I must be overseeing something that appears simple to others). When a film or television show is recorded on the hard drive, the data stream is either encoded (read: saved) as an MPEG II file (used with SD and HD) or MPEG 4 file (for HD channels). Both are also forms of lossy compression. The way it was encoded by the cable company (the codec + efficiency of that codec and its bitrate) will determine the quality of the recorded content and the size of the resulting file(s) it is saved to (the cable box need not encode it as it simply has to save the MPEG stream). That would be why MPEG 4 is capable of delivering the same amount or more of video info at a lower bitrate than MPEG 2 is able to.
    But when the data (the cable coax signal) is directly routed to the HDMI out of the cable box, it's neither MPEG II nor MPEG 4 but 'simply' digitised RGB: the cable box's decoder has decoded the 'datastream on-the-fly'.
    Since I'm not allowed to open the cable box, I can only caputure what comes out of the HDMI port. In case of a recording, the cable box decodes the MPEG 2 file to digitised RGB? And in case of capturing a live broadcast, the cable box decodes the MPEG stream 'on-the-fly'? The foremost reason we do not and cannot save digitised RGB is because the file size would become very very large + there is nowhere enough bandwidth for the cable company to send this over the coax. That's what I make of it:D.

    Yes, I sort of know what RGB is, 8bits (or 10) for each primary colour. RGB can be both analogue and digital. Modern TV's also understand another color space too being YCbCr so I'm assuming there could be digitised YCbCr too?


    Hold the horses here:), The reason why MPEG II needs to be decoded is because the TV can't unless it knows how to (i.e. it does not have an MPEG decoder on board). You can't send MPEG II to a capture card directly -> that would simply be a file copy instead of decoding it. MPEG II is thus compressed/encoded RGB video data. I say compressed because it is a lossy codec. If it would have been lossless It would only be encoded EDIT: and also compressed but no information would be lost. Or am I horribly wrong?

    MPEG is a form of encoding/compression, be it lossy. Why would you MPEG encode an MPEG stream? Perhaps you meant save the stream to disk?

    it = MPEG4 or MPEG II stream?


    Agreed, if sent out of an analogue conector there will be an D/A conversion and if then captured to a PC again an A/D conversion.

    So to sum it up: The coax carries a encoded signal stream, MPEG II of MPEG 4 (which makes it a compressed signal too) which is then decoded to DIGITAL RGB and sent over the HDMI cable where the TV then makes it ANALOG RGB.



    My brain is cooking:rotfl:. But I really appreciate the effort! I know bits (hehe) and pieces but I'm trying to fill in the blanks.
     
    Last edited: Feb 12, 2009
  8. Jeroen1000

    Jeroen1000
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    I'm not really having a problem to get this done technically but rather I am trying to understand. I just like to know what happens under the hood. I hope I'm not bothering you too much:D

    HDMI is a digital link. Wouldn't it be digital RGB then with a resolution of 576i (576i because 1080i content would be HDCP protected and would never reach the HDMI capture card. The cable box should allow digital copies at SD resolution)?
    Agreeing here. The digital stream would have to be recompressed either after capturing or on-the-fly.
    Yes, and its encoding standard is either MPEG 4 or MPEG 2. Note I am not talking about the signaling (how it travels through the cable).
    Yes! With the BIG exception that the 576i SD content flowing over an HDMI link will remain digital MPEG 2 (edit oops no, just digital (RGB), I meant!). If I were to use component out on the cable box it would become analog and I would have to digitise and encode/compress it again. This saves 2 a/d conversions.
     
    Last edited by a moderator: Feb 12, 2009
  9. Gavtech

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    MPEG is a more efficient compressor. However it is important to understand that efficiency in compressor terms simply means ' better at throwing data away' so it is arguably actually worse because it throws more away [ ... in simplistic terms]
    Yes . HDMI can handle that too: [YCbCr 4:4:4 or YCbCr 4:4:2]
     
  10. Gavtech

    Gavtech
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    Yes - I said that earlier.
    But note it is in digital form whilst crossing the link ... and it is not in a digital form that has any relation whatsoever to any of the earlier digital forms it may have been in. HDMI has its own completely independent standard of transmission...but it should behave transparently - What goes in one end , comes out the other.




    No- It has no encoding standard at all.
    When travelling across the HDMI link it is uncompressed. i.e. not MPEG'ed in any form. It has been previously decoded by the cable box.


    You are misunderstanding. You ARE using analogue. The data coming over the HDMI link is NOT in any MPEG form. It is NOT raw transmitted data. It is decoded analogue video in RGB form. [ being transfered digitally ]
    You cannot achieve anything better via this method than you could achieve via an ordinary DVDR.

    That is why HDMI exists... to make sure you cannot.
     
    Last edited: Feb 12, 2009
  11. Jeroen1000

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    I think we're (mostly me:D) are missing a important point here.

    I agree no MPEG is going over the HDMI link. It is being decompressed/decoded first. I corrected it above in my post.

    I am however confused by your statements:

    1. "But note it is in digital form whilst crossing the link" (the HDMI link)

    2. "You ARE using analogue. The data coming over the HDMI link is NOT in any MPEG form. It is NOT raw transmitted data. It is decoded analogue video in RGB form."

    If the box outputs 576i over the HDMI link I can capture it digitally.
    If it outputs this over the component (or scart) link I have to make it digital.

    In both cases it's RGB, but whether is digital or not depends on the link.
     
    Last edited: Feb 12, 2009
  12. Alphathon

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    I think you've got it!

    Right again, but it is not as widely supported as RGB I don't think, in the same way as pretty much every scart socket supports composite video, but not necesarily RGB

    Yes...
    I don't really see what you're trying to say here, although what you are saying is correct (I don't see why you're saying it).

    What I said was basically meening it is possible to get a capture card to take a HDMI signal and write it to a file, it's just prohibitivelly expensive for home users (said cards are intended for video editing professionals). I wasn't suggesting it was MPEG video being sent via HDMI if that's what you thought.


    You misunderstand. I ment when an MPEG stream is created from raw video data. The point was when it was originally encoded, some data was lost. That data is still missing when it is converted to raw video. Then, when this video is converted back into MPEG, additional data is lost.


    yes.

    yes, although it is spelled analogue not analog (unless you're American...and your profile says you're Belgian) /nitpick
     
  13. Gavtech

    Gavtech
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    What you need to understand is that in course from acquisition to the display at your end the video signal undergoes many digital to analogue conversions... and vice versa.

    All broadcast television starts out as analogue.

    The HDMI link converts an analogue input signal and digitises it using its own encoding standards ...purely for sending it over the metre or so of cable that forms the HDMI link. That digital form has no connection with, nor resembles digital TV signals in any way. So yes it is digital ... but has no significance.

    The same signal that feeds the cable box HDMI output [ i.e. the RGB signal ] is also feeds to the component output [ after going through a matrix to modify its form]

    What happens next depends entirely how you connect to the next system.

    You can capture it digitally because your capture device is digitising it... in just exactly the same way as you would have to with the analogue component form.

    You are not simply digitally copying a raw stream. If that were possible, you could do that via USB.

    When you capture via HDMI you are digitising an analogue originated signal.
     
  14. Jeroen1000

    Jeroen1000
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    @Alphathon: Blame that on our education. We were allowed to spell it both ways (color, colour, centre, center) and I do try to keep track but in the heat of the argument...:)

    I do get most of it now. I'll have to let it cool down a bit to let it settle in.

    My point is that a 576i signal over the digital HDMI link will yield better quality than a 576i signal over an analogue:rotfl: link when capturing. Information will be lost using both ways, but I believe a/d conversions will do additional harm.
     
  15. Alphathon

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    I think you're right there as well. HDMI is purelly digital so the signal it sends is digital regardless of the resolution. I think Gavtech got confused by your post whichalso contained reference to Component video, which would indeed be digital(MPEG) > digital(uncompressed) > Analogue(RGB/YCbCr) > Digital(MPEG)
     
  16. Gavtech

    Gavtech
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    There is no confusion in my posts.
    You are quoting a post that was edited after it was posted.
     
    Last edited: Feb 12, 2009
  17. Jeroen1000

    Jeroen1000
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    That would be my fault. It takes a while for me to type my replies in somewhat good enough English. While rereading I often correct mistakes. Use the preview button Jeroen:oops:
     
  18. Alphathon

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    Oh I don't blame you, just pointing it out (It's not your fault I'm British, pedantic and hate American English ;)...mainly due to Americans trying to correct us on how to speak the language they took from us and then changed. There have been many times I have seen Americans on forums etc. claming that one of us spelled colour wrong and we should learn the freaking language. Damn american arrogance*).

    Anyway, yeah, the Digital-analogue conversion would likelly cause more harm, but if you think about it, all of the rest of the steps are there as well. It still has to decode the MPEG stream (which converts it into uncompressed digital video) and you still have to convert the digital stream into MPEG again. It's just that there are 2 extra steps (digital>analogue and analogue>digital), so yeah, if you can capture 576i from HDMI it will be better quality than from component, especially if using poor quality component cabls, which add the problem of noise from interferance.

    *Disclaimer: not all Americans are like that, and in fact the vast majority aren't. It's just that there are enough that are to justify being angry about it, and then some. Heck it's these same Americans who think that Scotland is at war with the UK (it makes me LOL), and that think Europe is a country. I feel sorry for the educated ones having to put up with that crap from thier neighbours
     
  19. Alphathon

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    Lol, I have a similar problem (although I am a native (British*) English speaker - I have an odd learning difficulty that makes me take absolutely ages to read and write stuff).

    Also, I would say your english is pretty good really. The only misunderstandings have been from interpretation...or constant post editing on all our parts...rather than the translation to english.

    I think maybe we should all use the preview button.

    *:D
     
    Last edited: Feb 12, 2009
  20. Jeroen1000

    Jeroen1000
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    Thank you:thumbsup: Don't reread any of my posts then lol. I did and usually I do take a little more care then what I've done over the course of this thread.
    Now, I'm showing some compulsiveness to correct them.
    Our discussion really kept me on the edge of my seat. Thank you both for the very educational (video related then:D) session, not to mention your patience.

    I just wanted to say the Blackmagic Intensity HDMI capture card isn't all that expensive. Building a RAID array to be able to store all the 1080i data quickly enough isn't either. I've invested a lot in my video gear so for the time being I'm going to pass on building a new computer (for now anyway). I've got a lot of experience in that area so it's really within reach.
     
  21. Alphathon

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    I suppose for what it is it really is quite good value, but still pretty pricey in my eyes. The only way I'd pay that much is if it suppoerted 1080p and was capable of all the surround formats (or at the very least LPCM up to 5.1 or 7.1. The compressed HD ones can be deocoded by my PS3 anyway [all I'd use it for]). Don't really see that happening though as it would lead to blatant piracy (although it'd probably be easier to just rip it from a Blu-ray). As for the RAID array, yeah, HDDs can be got for next to nothing thses days.
     
  22. Jeroen1000

    Jeroen1000
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    I feel no urge for ripping HDCP protected movies as I'm mostly interested in a proof of concept for myself. I hate being told by those film producers (does that awaken more "hate" towards Americans lol*)what I can and can't do with my bought blu-ray discs! I'm also not a fan of my cable company restricting us in every way they can think of.

    I have also found a series of articles demonstrating its (HDCP) weakness and am quite sure HDCP's days are almost numbered. I'll refrain from posting which devices can be used to breach HDCP but it is already possible today. I'm sure you're in the loop also:)


    * not intended offensively!
     
    Last edited: Feb 12, 2009
  23. Alphathon

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    Oh I know, and I wasn't suggesting you were. I agree that it is infuriating to not be able to copy a movie you own to your computer for easier access or whatever. Actually, in the UK it is technically legal to do that. Unfortunatelly it is illigal to break any encryption (be it the copy protection on a CD or HDCP). I think this makes it legal to download ripped movies and music as long as you own a copy already at the same or higher quality (like you couldn't DL it in HD if you only own the DVD, but you could DL a DVD rip if you own it on Blu-ray). Don't know how it would go down in a court though.

    As for the protection itself I think it is both neccessary and pointless. As you said HDCP has been broken, and I think Blu-ray copy protection may have been as well. Sky (satellite TV) has definitelly been broken, although they keep updating their protection. That is why it is pointless - it only slows down the uptake of piracy. A very good example of this is the restrictive limited activations of recent EA games. One of the first of these to be released was Spore. It's internet activation system and limited installs was hacked BEFORE IT WAS EVEN RELEASED. As a result, it harmed paying customers by limiting the number of installs (and installing SecuRom), but actually gave pirates a better experience by not restricing them in either way.

    I know companies need to protect thier assets, but the relative ease of piracy meens that it is futile to do so. For software there are always cracks. For copy-protection systems there are always ways to fool the system to let them in for copying. As a result, I think that copyright, in it's current form anyway, has a limited future. Our current model for copyright was developed back when in order to pirate something, you actually had to make a physical copy. The problem is that it just doesn't work the same way for DATA. If you want to copy a book, you no longer have to reprint it - you can just store it as text or images on a computer. The same is true for all media - movies, music, artwork, literature. Honestly I don't know of a better model for copyright, but unless one comes along, the entire entertainment industry will either have to become like the creative commons or sease to exist.
     

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