Questions about the Series 3

Discussion in 'TiVo Coffee House - TiVo Discussion' started by Justin Thyme, Jan 6, 2006.

  1. Justin Thyme

    Justin Thyme Contra sceleris

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    If you have a star type wired system, I don't see that you would have to isolate the Tivos in some way. Simply because they send fewer packets on the 100Mbit/sec conveyor doesn't mean the conveyer belt is going slower, or that congestion on their belt has any effect on anyone elses belt. And if you are imagining something like a "FedEx sorting room" problem at the switch congestion at the switch, most of that sort of thing doesn't happen because of the smarts in the switch that do things like effectively wire two conveyors together when they are sending traffic (no "collissions- no further management needed).

    That is a metaphoric representation of why I don't think you will have a problem even though the T3's might pump data out at slower rates.

    The way you are thinking about the internals of the T3 is mistaken, but your feeling that there will be network constraints there is true. Dan pointed out that because it is 100BaseT then the connection out from the T3 has an actual max aroung 70Mbits. So whatever they do inside the box, they are stuck with that because they didn't do 1000BaseT. So you aren't going to initiate a transfer and Boom- it's there on your local T3. Maybe you can jump ahead quickly to points by precaching, so maybe having the Boom transfer gives you nothing.

    The problem with the way you are thinking about it is that realtime embedded systems designed for particular tasks are not so centralized as general purpose CPUs. If the CPU can stay out of something, it will. Things are very much going on in parallel. The CPU is not involved with Mpeg2 encoding in an S2 for example. The CPU is pretty much twiddling its thumbs while the encoder shovels bits onto the hard drive. The CPU just steps in to initiate the transfers, configure them, and fire them off. I don't know the particulars, but it is wrong to assume the CPU being the crucial bottleneck. In a properly balanced and most efficient system (both from production cost and from user percieved performance)- the perfect system is where all components are right up very close to being overwhelmed by the task. The complication of this effort is why new custom engineering designs for realtime systems take so long. It is truly and art form that has to be understood to be appreciated.

    The guys that do such real time systems are amazing.
     
  2. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    The CPU doesn't have much to do with MRV transfers, but it plays a big part in TTG transfers. You see when a program is stored on a TiVo is it stored as seperate audio and video streams. When it is transfered via MRV those streams are simply transfered as seperate streams to the destination TiVo. However when it's transfered via TTG the system must multiplex the streams into a single file. This process can be pretty processor intensive and is the reason why TTG transfers are so much slower then MRV transfers on a S2 unit.

    My hope is that the new S3 unit has a CPU which is capable of multiplexing the audio video streams at a rate which keeps pace with the full throughput of the Ethernet port. That way the CPU will no longer be the bottle neck and TTG transfers will happen as fast as the network will allow.

    Another bottle neck in the system, with regard to transfering HD streams, is the hard drive. Just for standard operation the hard drive has to be capable of recording two HD streams while simultaneously playing back a 3rd. That's a lot of bandwidth. Throw in a transfer to the unit and another one from the unit and you're probably going to get pretty close to maximum throughput of the hard drive.

    Dan
     
  3. ZeoTiVo

    ZeoTiVo I can't explain

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    that is about the most beautiful thing I have seen stated in this forum for a while. "Just for standard operation" Wonder if I can ask my doctor to make me hibernate for 6 months :)
     
  4. Justin Thyme

    Justin Thyme Contra sceleris

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    No reason they couldn't use two standard IDE drive(s). One data bus for each drive.

    Right. Sometimes it is in way over its head, othertimes it is idling. ITV was attempting to extrapolate from the encoding task, thinking that since the input in the S3 was digital, the "CPU" would no longer have to do encoding and so there would be a benefit from being "freed up". But in fact it never was burdenned by encoding. Still the overall picture that there would be performance bottlenecks is correct. We just have no way of making much more than external guesses what they would be.
     
  5. nhaigh

    nhaigh Member

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    Lawrencevill...

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    It has been said here that an S2 won't be able to MRV a program in HD on the S3. If thye are both MPEG2 data why can't the S2 MRV the HD file? I did think this was down to the resolution being different but then I though a regular S2 can MRV a program recorded on the TiVo DVD Recorders and they are diferent resolution as well. Why can it do one and not the other - isn't it simply a case of degree?
     
  6. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    It's due to both resolution and bitrate. A S2 can output a maximum resolution of 720x480. A 720p HDTV signal is 1280x720. Also the maximum bitrate of DVD material, which the S2 can output, is 9Mbps. HDTV has an average bitrate of 17-20Mbps. Unfortunately both the resolution cap and bitrate cap are dictated by hardware limitations, so there is nothing that can be done to make a S2 TiVo play back HDTV content even downsampled to SD resolution.

    Dan
     
  7. dr_mal

    dr_mal It's CWS time

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    Actually, it's 1280x720. 1080i HDTV signals are 1920x1080.
     
  8. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    That would require a complete redesign of the system so that it was smart enough to record each of the streams to a different drive. Plus what would happen if your TiVo got to the point where it only had enough space on each drive to record 1 hour of HD and you wanted to record a 2 hour HD movie? Would it simply say that it couldn't be done because there is not enough room?

    Raid 0 might help, but it's always best to minimize the points of failure and having two hard drives doesn't fit well into that strategy.

    I missed that. You are correct the CPU has almost nothing to do with recording a program. (there is some minor buffer management code that is always running)

    Dan
     
  9. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    Oops! :eek:

    Dan
     
  10. Justin Thyme

    Justin Thyme Contra sceleris

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    Case of degree. The max resolution of the S2 is around 720x480. Also the inbound data rate to an S2 would not be up to the task of HDTv's 20Mbits/second.

    Outside of what Mega related in his trip report and TivoPony related, the behavior with S2 is all crackpot speculation, with myself being one of the worst offenders. You probably recall the speculation that you could MRV through the server to downres the file. Well, alternately, the T3 could perform a hardware downres conversion of the HD Mpeg. Nicer that way because the S2 gets it in real time, and gets it at a data rate that it can handle. Then there is the "Do it in a USB peripheral to extend the life of the S2 architecture" trick.

    But these scenarios are all shear fantasy at this point. A lot of fun, until TivoPony butts in and makes a definitive announcement about some mind blowing feature and proves us all wrong.
     
  11. interactiveTV

    interactiveTV New Member

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    Actually, I was thinking of the TTG multiplexing as the hardware codec should handle the encoding. My confusion. My assumption was that that multiplexing would no longer be needed.

    Am I right to assume the communication tasks are not a burdon to the CPU but are handled in the 100baseT adapter?

    802.11n ... hmmmm

    _ITV
     
  12. nhaigh

    nhaigh Member

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    Lawrencevill...

    So what you are saying is that natively it won't work but if TiVo really wanted to make it compatable in some way there there are ways it could be done. We just really need to wait and see if TiVo are going to be prepared to give us the tools.
     
  13. Justin Thyme

    Justin Thyme Contra sceleris

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    I thought we were talking about theoretical S3 system. How do we know how the system was designed? I understand that highly parallel systems introduce complexity. Cripesake- the bugga's got 6 tuners in it.[edit- actually only 2, according to TivoPony in the CES video] Anyhow parallel buses is a way to go, and HD storage balancing is not rocket science.

    With that said, I am think I am wrong because they said 250GB hard drive singular, and IDE. Now, if they said 280GB of storage, I would have some room to make a case. If they are 100Mbit transfer rate, the bandwidth is there, but there is going to be a whole lot of head banging going on with the high stress scenario mentioned.
     
  14. Justin Thyme

    Justin Thyme Contra sceleris

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    Gee. I didn't even notice that Dan already answered you. Sorry for butting in. Anyhow, if you are talking about MRV, then yeah, that's the picture and we are dependent on Tivo to develop it. If you simply want to watch an HD show on an S2 Tivo or for that matter on a DVD, then there will be tons of different ways for folks to do it- similar to the sorts of things you can do with TTG video on a PC. For that you wouldn't be dependent on Tivo at all, but such transfers would take on the order of hours to convert. Think lots and lots of hours for a typical HD movie, on a typical home PC.
     
  15. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    There is a very minute possibility that TiVo could some how use the advanced hardware in the S3 unit to downres HD content before sending it over to a S2 unit, but it's highly unlikely. They can't just use the MPEG encoder chip to do this, since it could possibly be in use recording an actual analog cable channel. Which means they would have to do the downres in the CPU. In order to do that in real time it would require a CPU equivalent to a 2GHz Pentium 4. It's highly unlikely the S3 will have that kind of power since for all other purposes it's completely unnecessary.

    I'm not a betting man, but I'd be willing to bet that once this is released you will NOT be able to transfer HD content to a S2 unit.

    Dan
     
  16. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    Just because the hardware is new does not mean the software is. TiVo has had the backbone for dualtuner HD recording built into their software arcitecture since the HD DirecTiVo came out a couple years ago. It's safe to assume they will reuse as much of that code as possible for this new system.

    Dan
     
  17. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    It's possible the CPU could be responsible for Ethernet operations, but even if it is the burdon would be small. Although it's more likely the Broadcom chipset they're using has dedicated Ethernet support which is probably why they're including it in the first place.

    Dan
     
  18. Justin Thyme

    Justin Thyme Contra sceleris

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    I wouldn't bet on seeing downresing as feature bundled in a box, either. I just mention it as a technical possibility. Because downresing/ transcoding is a VERY popular application though- less for this, than for generating portable video, I'd not be surprized if you saw a Downresing peripheral. Transcoding has some interesting optimizations you can do in parrallel if you have dedicated hardware.

    But in this case we are talking mpeg2 to Mpeg2. I don't know how they assemble the 8x8 DCT blocks, but if it is anything like JPEG, you could simply drop every other DCT and you'd have a downres'd picture without even having to transfor the DCT block into an actual 8x8 bitmap.

    Maybe I'm full of crud about that, but I am sure that a cheap external box could be built. Maybe people would like that as a docking station for a portable video device, but it could be unipurpose- it could also offload the Muxing process so TTG would go faster, and it could handle HDTV downresing.
     
  19. Justin Thyme

    Justin Thyme Contra sceleris

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    Excellent point.
     
  20. nhaigh

    nhaigh Member

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    Lawrencevill...
    Justin and Dan. Thanks for these answers and I apologize if the questions seem a bit dumb. Needless to say I have one more. If the S3 cannot "downrez" then how does it generate the SD output needed for non-HD television sets, i.e. the non-HDMI outputs? Is there anyway that mechanism could be employed?
     

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