Does anyone happen to know the following.
On an NTSC digital TV, what scan rates relate to the screen image that are controlled exclusively by the TV station transmitter?
In other words, are there any frequencies that all digital TV's within the same broadcast area would operate in synchronisation with?
Ken Morrow
AFAICR: at one time in the UK, the vertical sync was kept in step with the AC cycle of the mains, This was to prevent any beat effect due to a slight difference in frequency.
Also, AFAICR: all (most) frequencies in a TV are divisible into one master clock frequency.
With stable SMPSUs well established before HD arrived - there's no guarantee such constraints still apply.
Digital NTSC is called ATSC..
The only fixed frequency broadcast is the carrier and a naqrrow pilot "tone" at 309.440559441 kHz from the band edge... Everything else is compressed digital data..
The standard is condensed here:
Steve
In the US, the AC line is 60 Hz and the vertical scan rate was 60 Hz, but they weren't synchronized. A small offset isn't visible.
If a show came from New York, with analog distribution, it had New York timing, so could be a little out of sync with AC in various places across the USA. One could compare the video signal to the AC line, on an oscilloscope, and sometimes see phase jumps as the ultimate signal source changed. This was before scan converters were available.
Sometimes we would see a faint horizontal bar, from some AC line effect, that would slowly creep up or down the TV screen.
Digital is better. TV still sucks.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Can you tell us which aspects of the "compressed digital data that result in an operational frequency within the TV would be common to all TV's in the same reception area, _apart_ from the particulars of any tuned station?
What are the specific frequencies?
Ken Morrow
result in an operational frequency within the TV would be common to all TV's in the same reception area, _apart_ from the particulars of any tuned station?
I wouldn't think any. It's left open to the implementation. Not that NTSC needed to be locked, either (you could buffer whole screens and rescan it at any rate you like, if you could afford the equipment to do so back then!), but it was so many orders of magnitude better to do it mode-locked.
With digital, so much buffering and processing is needed anyway, I'd be surprised if any signals end up at the same frequency, or phase at least.
You also have mixed technologies: what few CRTs remain in use, LCDs, plasma, LED, and anything else. If you're thinking in terms of EM emissions, perhaps even ala TEMPEST, it'll be *much* more situation dependent. Likely, each manufacturer uses their own chipset, which will be similar enough, but too different to use in a similar fashion.
If it's more about, say, synchronized video, you probably want something as real-time as possible, like a computer monitor (or any TV that can be set up that way, without internal buffering). You wouldn't want, for example, a Jeopardy screen made up of disparate, unsynchronized displays, for playing real time video. More than a frame's worth of delay (i.e., <
17ms) between displays would be noticeably poor.Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Take a look at
ATSC supports a whole bunch of different frame rates - some interlaced, some progressive. I don't think there's any good reason to believe that all of the stations in any given geographic area would be using the same (single) frame rate at any particular time. I'd tend to bet against this being the case.
ATSC stations can transmit multiple independent program streams in an interlaced fashion, within their signal RF carrier. I don't think there's necessarily any linkage between the frame rates used by these streams. A station might be transmitting a high-frame-rate primary program (e.g. for sports), and using a lower resolution and frame rate for secondary programs (infotainment, other paid programming, weather displays, talking-heads shows, etc.).
And, if 3:2 pulldown is enabled, the frame rate you see on the display may not be the same rate as what the station is transmitting.
You can probably assume that all of the programs being transmitted for viewing, do use one of the (numerous) standard frame rates, since there's no assurance that peoples' TVs could display programs that don't comply with the ATSC/MPEG/H.264 standards.
I don't think there are any left. Different decoders take variable time to handle the digital signal so TV sets show slightly different frames and sound output. It is very annoying with digital audio if you have more than one set on the same station with audible flanging effects when you move between rooms. And really bad for doing simulcasts of broadcast quality studio sound on the radio with the picture on TV.
Presentation of the image on screen used to be phase locked to local mains to avoid hum bars but that restriction is lifted by modern LCD screens. The digital signal looks pretty much like spread spectrum noise and individual frames and differences get transmitted as needed.
One side effect of the rise of digital TV is that our non-thermal radio signature is turning back into something that looks like noise. So the Drake equation will need tweaking - civilisations are only radio bright in an obviously AM/FM modulated sort of way for about 50 years.
-- Regards, Martin Brown
Perhaps at the phy layer, all over the air TVs will get the same 8vsb symbol rate and data rate. You can look them up. Catv will be different. Mark
Make it 100 years. But of course a civilization will spend a limited time wasting power on inefficient radio transmissions, and when Drake has not included that into his equation that should be indeed modified.
I think factors like the time span of existence of a civilization was included as a factor.
We do not yet know if the time spent sending AM/FM signals by our civilization is short relative to the time sending digital noise.
The ATSC pilot is required to be on its designated spot +/- 4 Hertz, other then that, its a compressed video,spread spectrum free for all.
Steve
MPEG is just data, buffered wherever and however. It includes the information to frequency lock your receiver to the source so that your tv outputs precisely the correct number of frames per second but for consumer kit it can wander fairly freely.
Colin
Yesterday someone posted about thyristors and digital TV today. That only leaves COTs computing modules and my whole 25+ year career is fully defined.
Years ago I had a mental neighbour in the flat above shouting obscenities and banging so loud the bloke in the flat below was blaming me for it.
My solution; rig up a transmitter at the UK sound intercarrier frequency and inject the RF into the mains. Someone on the other side of the road who never shut their curtains, always got up and tried twiddling their TV tuning when I fired up the transmitter - so I know it travelled that far.
Probably "don't work like that" with digital encoding, the landlord has thoughtfully provided me with a replacement antisocial neighbour - but he has nothing like the stamina of the mental case, so its not pressingly urgent.
A small offset isn't visible - but undulation from a low frequency beat effect is very annoying.
Why on earth should there be such synchronization ?
Most of the current video compression methods are based on MPEG2/4 compression, in which the signal is sent as full I-pictures (about twice a second) and P/B pictures containing the differences from the previous I-pictures.
With the motion vectors at say 83.33 Hz display rate and 50/60 Hz signal rate, there is no much problem in upconverting to 83.33 Hz or whatever arbitrary display rate.
It is a long time in which 50/60 Hz frame rate conversion caused a problem.
On Wednesday, November 25, 2015 at 1:10:09 AM UTC-8, Martin Brown wrote: ....
...
Locking to the mains stopped when colour was introduced in the 60's. The required accuracy on the subcarrier and the tight relation to scan rate stopped that.
Luckily by that time ripple rejection was good enough for it not to be a (significant) problem.
kevin
On Wednesday, November 25, 2015 at 7:20:56 PM UTC, snipped-for-privacy@downunder.com w rote:
Because the P/B pictures can also contain differences wrt the next I frame. You have to display frames at the correct frequency or your buffer empties or overflows and you can't just repeat or skip the odd frame. Well of cour se you can, but you would get artifacts lasting several frames.
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Early B/W transmissions were 60Hz but color was never 60 Hz. It was 59.94 H z (3579545/227.5/262.5) Since all the broadcasters have to be able to run so me Standard Def video, the 59.94 stays with us whether we like it or not.
ATSC? There's no global synchronization. There's no scan either - just a frame rate.
Cable TV AVC so it's pretty much anything. You tell the cable box what your TV supports over HDMI and it resamples everything else.
LCD TVs can handle just about any frame rate. Plasma TVs need some fraction of their 600/second dither rate or they'll flicker/stutter.
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In the rest of the world, in which DVB-T/T2 is commonly used supports the SFN (Single Frequency Network) transport, in which all main and auxiliary transmitter work on the same RF channel, each of the 6800+ subcarriers in an area must be synchronized within microseconds.
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