A down converting dilemma

[johnbasalla]

Full disclosure statement: I am not an engineer and have limited knowledge but I am always interested in learning.

Our school district has a local access cable TV station that, during the school day, works like this. The programming comes from the PBS broadcast affiliate. It comes from or through one of their sub-channels which they call "A-4". They send it (I think via fiber) to the head end of where all of the local access channels emanate from. Those folks put it over the cable access channel. PBS is going to show, on their "American Experience" series, a program about an internationally reknowned local legend. The local affiliate got permission for our school district to have the program on the local access station, for in school use. While they can run this, it appears that there is engineering work of some significance that has to be accomplished in advance to make it work. If it was only going to be shown once, they would do one thing, but if there are going to be multiple showings, they have to do something else. Down converting is one thing mentioned which I take to mean converting it from HD to standard definition. Anybody got an idea what's going on engineering-wise?

 

[w9wi]

Not entirely sure I understand what you're asking.

- This program is going to be broadcast on your local PBS station, and you want to re-run it on your local-access channel?
- This program is going to be available from PBS, but your local PBS station isn't going to broadcast it, but you want to run it on your access channel?

- Your school district sends any local programs you want to air to the PBS station, and they put them up on the local-access channel?
- You have some equipment at one of the school buildings, which plays programs that air on the local-access channel when it isn't running PBS programs?

I suppose the local-access channel is probably standard-definition; American Experience is HD.

If the program is going to be broadcast, I think I would arrange to record it on a DVD recorder. That process would automatically downconvert it to standard definition. And, most facilities that can play back TV programs can use a DVD as a program source. (either directly, or by dubbing it into a hard-disk playback system) You'd probably want to ask several people to record it, as at least one is probably going to make a mistake

If it's *not* going to be broadcast, then it seems to me the PBS station is going to have to record it for you, and how they handle that depends on what kind of equipment they have available. I would think they should be able to make a DVD for you. For that matter, if they're responsible for programming the community-access channel when you aren't, then I would think they already have the downconversion equipment (and the playback equipment) and they should be able to handle the entire process.

 

[nmoore6676]

There really needs to be more information to get a real answer. If I get it right there is a sub channel on the local PBS affiliate that the cable company is using as a program source for your local access channel which is available in the class rooms. Probably also it is available to home users unless there is some kind of encryption. If i have this correct then the conversions are done at the head end and although they likely have some kind of patching arrangement they are not probably set up for local insertion over this one particular channel.

I think the easiest answer is to record the PBS show on a DVD recorder and just playback the recording in each classroom. The cable provider is not likely to set up something like this for a one time only event. If this were going on regularly or has been done before and would be happening with some frequency they could install a video sensing switch at their headend and provide you with a feed through fiber (in the old days a reverse line through their system) or just a playback unit in their racks that would interrupt the normal programming only if video is coming from the alternate source.

 

[Lazy J]

Typically in smaller cable systems, the local stations (PBS, ABC, CBS, NBC) are picked up from the Over-The-Air transmitters with a bunch of antennas on a tower. The signals are then assigned a cable channel and sent down the line. Most likely PBS has the educational programming on one of their HD sub-carriers. I'm sure they have some permanent setup that receives the HD sub-channel and puts it on a cable channel for the school. Now, the school wants to take one show from the PBS main channel and pipe it down the schools educational cable channel. Am I right?

Depending on when the program airs on PBS, you could put in a simple A-B switch and click it over to PBS when it time to air the show. If you want to run the show at a different time from the main PBS station, then you will have to record it somehow. I would look into using a DVD player and an A-B switch to switch to the DVD player when it's time to run the show.

 

[johnbasalla]

The program is going to initially be broadcast on the local PBS affiliate in the evening. The school district asked for permission to have it "re-broadcast" during the school day on their local access cable channel. The PBS affiliate got the permission.
The PBS affiliate is recording it for use on the cable channel. Based on the responses here, it is my understanding that the issue is one of down conversion to S.D.

 

[w9wi]

I think we're still a bit short on information here.

- What kind of media is the PBS station recording it on? DVD? Blu-Ray disk? Computer file? Video tape?

- What kind of media can the public-access channel accept programs on? (see list above...)

If it's going to be broadcast, recording it at home on a DVD recorder is probably the most foolproof way of getting it converted.

 

[johnbasalla]

The PBS affiliate sends the programming to the local access channel facility...They send it via fiber (I believe). Unless I'm missing something, it doesn't matter what kind of media the public access channel uses to run programs since, in this case, the public access channel is not playing the program from their facility. Rather, they are getting a direct feed from the PBS affiliate. None of the school-day programming originates from the local access facility. The program we're talking about will be "re-broadcast" during the school-day programming. I have no idea what media the PBS affiliate is going to record the program on. Whatever works for them. I am just curious about the technical nature of preparing this HD program for re-use on the local access channel since it was shared that there is some special work involved.

 

[w9wi]

Ah, OK, it all plays out from the PBS facility.

The process really depends on how the facility is wired.

At our station, we used to run a second subchannel in SD. Both subchannels operated almost entirely in HD, with the last device in the chain a frame sync. The frame sync for the second subchannel was configured to output in SD -- the frame sync was the downconverter. It wasn't necessary to downconvert anything manually, nor to downconvert it before air. It just happened, without intervention.

A common layout involves "tie lines" in the system routing switcher. (think telephone switchboard) The router may have two "levels" of switching -- one containing all the HD sources & destinations, the other containing all the SD sources & destinations. You have a number of crossconverters connected. If you try to route a HD source to a SD destination, the router knows to send the source out to one of the crossconverters, then bring the converted signal back in & send it to the SD destination. As far as the user is concerned there is no difference from routing any other source to any other destination.

I would not be particularly surprised to hear of a cash-strapped PBS station having a much less flexible layout. It certainly could be a lot harder to accomplish. I would think that kind of thing would come up fairly often though -- if they have the ability to record the program, I would think they would have the ability to convert it to SD fairly easily.

 

[nmoore6676]

The PBS affiliate sends the programming to the local access channel facility...They send it via fiber (I believe). Unless I'm missing something, it doesn't matter what kind of media the public access channel uses to run programs since, in this case, the public access channel is not playing the program from their facility. Rather, they are getting a direct feed from the PBS affiliate. None of the school-day programming originates from the local access facility. The program we're talking about will be "re-broadcast" during the school-day programming. I have no idea what media the PBS affiliate is going to record the program on. Whatever works for them. I am just curious about the technical nature of preparing this HD program for re-use on the local access channel since it was shared that there is some special work involved.

The PBS station needs to output the program over whatever link they have to the cable headend. If it is over the air on a subchannel or fiber or the old school reverse feed it makes no difference. So they should have the capability as noted above to send it in standard definition.

Posted by: w9wi
Insert Quote
Ah, OK, it all plays out from the PBS facility.

The process really depends on how the facility is wired.

At our station, we used to run a second subchannel in SD. Both subchannels operated almost entirely in HD, with the last device in the chain a frame sync. The frame sync for the second subchannel was configured to output in SD -- the frame sync was the downconverter. It wasn't necessary to downconvert anything manually, nor to downconvert it before air. It just happened, without intervention.