| Electronics > Beginners |
| Why does the average digital TV set take so long to "come on"? |
| (1/7) > >> |
| Chris Wilson:
It's probably a naieve question, but it's something I have long idly mused over. Why does a typical low end all semiconductor digital flat screen domestic TV take so long to emit a picture and sound after turn on, or after a channel change? Even the audio is slow to lock to a new channel, but my SDR radio receivers change instantly. Thanks, make me look an idiot.... ;) |
| bitwelder:
At least for slowness with channel changes, I remember rumours claiming it was to discourage 'zapping' channels (as in, changing channels during commercial breaks). It seems plausible but I don't know if it's true. |
| MosherIV:
I do not know exactly. My guess is that due to the streaming nature of digital tv, the decoder needs enough of the stream to reconstruct a whole frame or screen. I seem to remember that mpeg worked by encoding a picture every so many frames. In between these whole frames, only the difference is encoded. Maybe cheap decoders cannot give a picture until the whole frame. |
| T3sl4co1l:
Because all software is awful, and everything is a computer. Boot-up is literally booting up; it may not be a general purpose computer as such, but it's something programmable, at least at manufacture time (but, following the premise that all software is awful -- they may release with bugs and provide future firmware updates through some mechanism). You'd have to check and see what TVs have been hacked/jailbreak'd, if this is useful at all or whatever. As for operating delays, yeah, there's quite a long delay, a second or so of video sitting in the decode buffer. It also can't show a picture until a keyframe has been received, which is transmitted every second or so (I forget what exactly, I don't care too much about DTV). Until then, only deltas are transmitted -- which is how you get those wonderful image corruption warps where a keyframe is lost and faces melt into freakish gibberish. :) There may be additional delay due to video buffering. A lot of TVs do that, which makes them particularly unpleasant for real-time applications -- computers, video games, etc. Tim |
| Gyro:
When I was designing (the hardware of) DTVs and Set Top Boxes back around the turn of Millennium, the pressure from distributors was always to speed up the channel change. Every millisecond shaved off counted in terms of user experience. Channel zapping is essential, especially with Satellite receivers with several hundred FTA channels, even when favorites are set up, there's no attempt to hinder it (maybe there is on some broadcaster specific boxes, I don't know). Yes, a large element of the channel change time is being able to start with a clean image, which needs an I-frame rather than the intervening motion interpolation P-frame and B-frames, that can take varying times depending on the channel quality. The last thing you want to start with is a green-screen of corrupt, slowly forming, picture. The other time element is pulling the programme information from the stream to put up on the channel banner in the UI. The stream has multiple channels encoded, with audio, video, and data packets interleaved, not necessarily in presentation order (hence the buffering). In terms or turn-on time, apart from the general initialization and housekeeping functions, the major time elements are putting up the Splash screen, if enabled, and more time consuming, updating the EPG (programme guide) data. Edit: A lot of sets check for s/w updates too. Edit: These days I assume there is a fair amount of time taken up booting Linux, Android or whatever too. ::) |
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