Questions about bandwidth

[Indiana627]

This may sound like a stupid question, but can each station control how much bandwidth is allocated for each of their OTA channels (i.e.: channel x.1 gets 50% of bandwidth, channel x.2 gets 25% and channels x.3 gets 25%)? Or is the total bandwidth simply divided by the number of channels (i.e.: 3 channels = each get 33% of bandwidth)?

And how much bandwidth does a channel have? And how is that determined?

[evan_s]

If they are broadcasting multiple channels they are mixed together into a single stream by the broadcaster. That mixing is controlled by what ever equipment they have doing it and can definitely divide it up in a number of different ways. A HD sub-channel obviously needs a lot more bandwidth than a SD one. Exactly how they can tweak it and if it's a static or dynamic assignment depends on the equipment they have and how it's setup.

[davring]

I believe it can be divided into a maximum of five channels, naturally the more channels the less bandwidth each one has available. An HD with channel which has a couple of sub channels is sending a bit starved HD signal.

[Indiana627]

Do all stations get the same amount of bandwidth allocated to them? Does that come from the FCC? How much bandwidth are the stations given to start with?

[Stuart Sweet]

I could swear there was a local station in SoCal that had 5 or maybe 6 channels total. I think that with the right equipment you can carve it up any way you want.

[davring]

I don't remember the exact numbers, but I believe the allocation is the necessary amount needed to properly broadcast a true HD signal, 19.2 Mghz? or is it Giga hertz? (bps?)

[Indiana627]

Interesting article I found that's just over 1 year old.

"At the dawn of DTV broadcasting, a transmitter was capable of sending out data at the rate of approximately 19.4 million bits per second (Mbps). The data could be in the form of video, audio or digital files of various sorts—datacasting.

Today, a new DTV transmitter can broadcast at about 19.4 Mbps. That hasn’t changed.

What has changed are broadcasters’ common expectations about how much programming and data can be carried in that signal.

In the early days, some broadcasters argued that all 19-plus million bits were required to transmit a high-definition picture of acceptable quality. The purists maintained that dividing the capacity among more than one program stream would compromise HD picture quality. Many assumed a station would have to choose between broadcasting one high-definition (HD) program stream, or up to four SD program streams.

Though it seems that we signed on our DTV transmitters only yesterday, encoder technology advances in the meantime have already added significant program capacity for stations that invest less than $100,000 in new encoding equipment. Thanks to the use of best-in-class encoding equipment, for example, stations such as KQED-DT in San Francisco are broadcasting one HD plus three SD channels.

In northern Minnesota, where Lakeland Public Television is the only over-the-air broadcaster serving much of its viewing area, Bill Sanford, the g.m. and director of engineering, emphasizes programming choice over the best possible HD pictures. Lakeland PTV is offering viewers one HD plus five SDs.

Both San Francisco and Lakeland are using statistical multiplexing technology (the engineers call it “stat-muxing”), which automatically adjusts the number of bits assigned to individual multicast channels depending upon the needs of each channel’s program content.

For example, a relatively static public affairs talk show or an animated kids’ show require relatively few bits to maintain picture quality. In contrast, programs with a great deal of movement or detail in them — e.g., concerts or sporting events — need significantly more bits. The magic of stat-muxing is that it optimizes overall capacity by allocating, from one frame to the next, the number of bits given to each channel.

For stations that seek to accommodate datacasting along with multicast video services, the multiplexer can add data to the broadcast bitstream on an “opportunistic,” space-available basis."

http://www.current.org/dtv/dtv0702bitstream-liroff.shtml

[davring]

I'm not sure of how well that system works, our PPS station I can receive OTA and they usually have 4 channels, mixed. Some times the HD is superb and other times it is lousy. SD goes from bad to worse and it varies each evening. When they broadcast "The War' a couple of nights were unwatchable, I had to record a middle of the night replay which was excellent. Maybe they don't have the best equipment yet.

[Indiana627]

I agree about how good - or not good - this “stat-muxing” works. If that's what my local PBS is doing, then I say it doesn't work very well at all since Carrier this week has look horrendous. The War didn't look great for me, but it wasn't nearly as bad as Carrier is this week. And I'm recording Carrier when it's repeated at midnight due to scheduling conflicts during the 9PM slot when it's on here.

[davring]

One episode of Carrier looked great and the second I had to rerecord overnight because the evening broadcast was so cluttered with extra channels.

[Grampaw]

One episode of Carrier looked great and the second I had to rerecord overnight because the evening broadcast was so cluttered with extra channels.

Dave,if you can pick up WXEL 42.1 OTA, they turn off their sub-channels at night when broadcasting HD.

Walt

[steve053]

I believe it can be divided into a maximum of five channels, naturally the more channels the less bandwidth each one has available. An HD with channel which has a couple of sub channels is sending a bit starved HD signal.

My local PBS station channel 10 has 7 subchannels. Granted 3 of the subchannels have the same NOAH video, but each channel has different audio.

[Ray_Clum]

My local PBS station channel 10 has 7 subchannels. Granted 3 of the subchannels have the same NOAH video, but each channel has different audio.

7 sub-channels :eek2:

The most I have ever seen is 4... your local PBS must look like poorly filtered dog manure.

[n3ntj]

Theoretically, if a station is transmitting multiple channels (say, one HD channel and 2 SD channels), will be possibly be more difficult to lock on and watch the HD channel, compared to if it only had the 1 HD channel (assuming at least 14 Mbps)?

KYW-3.1 in Phily has no subchannels, whereas WPVI - 6.1, 6.2, 6.3 obviously has the 1 HD channel and 2 SD subchannels.

I know a low bandwidth can allow pixelation to occur on an HD signal, but just curious if this may also impact reception of the station.

[Jim5506]

Sub channels have no bearing on how easy the station is to lock on, because the signals are all multiplexed into the main carrier, which is what the tuner looks for.

Have you noticed that all channels and sub-channels have the same signal quality and signal strength. Thats because they are all part of the same signal like left and right audio channels on FM.