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What is the difference between scanning for stations and going directly to one?

What is the difference between scanning for chhannles and going directly to one?

I thought that letting a VCR or DVD recorder or TV scan for channels/ stations was only to compile a list in advance of channels a device could receive, by checking out every station and noting which had signals.

And that pushing 1 3 on the remote would go to channel 13 whether one had scanned for stations or not, whether digital station frequencies had changed since the last time one scanned or not. As effectively as if one scanned the whole spectrum, and then channeled up or down to get to 13.

Am I right about the paragraph just above?

And that for timed recording, when the dvd recorder goes to channel 13 directly, it looks for it if necessary, just like scanning does. And if it gives some reception, though bad reception, even though the transmitter is only 10 miles away, it's not because it's off frequency?

Or are digital tuners different from analog, in that scanning first is essential?

Thanks.

Reply to
mm
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Well, I don't know about over there in the U.S. but here in the UK, if you pressed "1" and "3" on the remote control, it would just cause the TV / DVDR / PVR to go to the thirteenth channel storage position, in which could be stored the frequency information for any channel, anywhere in the band. So, that could be any valid channel that you had chosen to store in that location, or just as easily, some arbitrary channel frequency that you can't receive at your location. Scanning for channels causes the tuner to move up from the bottom of the band to the top, stopping each time that it finds a channel that it can receive, and then storing the frequency information for that channel in the next available 'slot'.

I think that you are perhaps getting confused between a channel's name, and the actual physical channel that it's broadcast on ? Even over there, I think that VHF has long gone, hasn't it ? We have channels here called "Channel Four" and "Channel Five", but they have never been broadcast on VHF channels 4 and 5. They have always been up in the UHF band on various physical channel numbers between 21 and 68, in different parts of the country.

Digital is way way different from analogue. Take everything that you ever knew about how analogue TV was transmitted, and just forget it. Digital transmissions are 'lumped together' into blocks of frequencies called multiplexes. Within a single multiplex, there may be many channels, and no single one is independently identifiable, because various techniques are used to 'mix' the data from individual stations to produce a form of spread spectrum transmission containing encoded interleaved data. In order to resolve this into individual stations, a channel scan has to take place initially, and as each multiplex is found, the information about what is in that multiplex, and how to get it out, has to be decoded and stored and used to build the EPG that is a fundamental part of digital TV

Arfa

Reply to
Arfa Daily

It is.

Correct. If you select a channel that hasn't been stored in the set's scanning memory, you'll get that channel, whether or not a signal is present.

The "go to" is instantaneous. The set does not scan from 2 through 12 before hitting 13. The basic principle (I assume) is the same as for any digitally tuned receiver -- the LO is directly set to the frequency needed to receive channel 13.

Reply to
William Sommerwerck

On analog tuners, the frequency is a direct function of the channel number as seen by the user.

On ATSC digital, that is no longer true. The "channel number" seen by the user is no longer a 1:1 function of the transmission frequency.

There are actually two channel numbers involved... the transmission- frequency channel number (which corresponds to the old NTSC analog channel number) and the station-identification channel number. They may very well be different.

This architecture was put in place so that stations which were previously down in the VHF range, could move their transmitters up to UHF, without losing their commercially-valuable "channel number" identity.

So, when you tell a digital TV or DVR or set-top box to do a "channel scan", it's doing several things:

- It scans the frequencies, looking for NTSC and ATSC signals, and "remembering" which channels are in use.

- When it sees an ATSC digital signal, it pauses briefly, decodes and parses the signal, and "remembers" which "channel numbers" and sub-channels are being multiplexed/transmitted on that frequency. Subsequently, when you tell it to "tune to channel 13", it will look ints memory. If it previously saw that "channel 13" was NTSC, it just tunes to the traditional Channel 13 frequency slot. If, however, it detected a "Channel 13" ID in an ATSC digital stream on *any* frequency (even if not the traditional "channel 13" frequency) it will tune to that frequency, start decoding, and begin extracting that program.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

Aren't you and Arfa disagreeing? Is it different from the UK to the US?

Reply to
mm

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