DTV channel bandwith allocation - AVS Forum
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post #1 of 8 Old 07-26-2000, 03:01 PM - Thread Starter
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Does anyone know how DTV allocates bandwidth (resolution) to its channels? Are all channels allocated the same bandwidth or are some channels like PPV allocated higher bandwith to give a better looking picture.

I have noticed a difference in resolution between channels and wondered if it was due to the source or DTV bandwidth allocation.
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post #2 of 8 Old 07-26-2000, 03:18 PM
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S. Sanchez,

Are you refering to local, over-the-air DTV vs. analog? If that is your question, the answer is that analog and digital local OTA is the same bandwidth across North America and most of the Pacific rim: 6 MHz. In Europe it's 8 MHz. If you are refering to digital DBS satellite such as DirecTV, here is my understanding, but I would defer to an expert in the field: the allocated bits assigned varies depending on a number of things, such as how much "maximum" payload a given channel the operator has allocated for a given channel, based on whether it's a talking heads channel that doesn't stress MPEG-2 compression too much or a basketball game that would stress the compression system dramatically. The operator might allow more bits for the basketball game or, say a PPV movie, where picture quality might be considered to be more critical. The actual "bandwidth" RF-spectrumwise of each satellite transponder is fixed. However...each is carrying a bunch of data and all of a given "group" of transponders contributes to the RF spectrum needed to carry the digital data required for this big, bit-managed, multiplex system. Observe that on DirecTV CNN looks better than MSNBC, with fewer compression artifacts, even though each is a talking heads service. I have heard that this is because CNN pays DirecTV for more bits to achieve a better look and MSNBC just takes whatever DirecTV assigns them. I'd like to hear some corroboration on this story.

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[This message has been edited by Dave McRoy (edited July 26, 2000).]

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post #3 of 8 Old 07-26-2000, 04:00 PM
 
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post #4 of 8 Old 07-27-2000, 04:26 PM - Thread Starter
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Dave

I was refering to direct tv dbs service. I wonder if there is any way to find out how much bandwidth is allocated to each channel or do you think that is done on the fly by whoever is in the control room at any given time with a few basic guidelines?

Sandy
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post #5 of 8 Old 07-27-2000, 04:35 PM
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I believe that there is a maximum and a minimum allocation assigned to each 'service' within a transponder 'channel'. Barker/promo channels probably get something in the 2.5-3 MB range, typical cable channels probably around 3.5 MB, premiums and PPV probably around 3.5 to 4.5 MB. These are estimates based on what I observed on a data stream monitor at a trade show. In addition services are 'statistically multiplexed' together so that when one service does not require its entire allotment it can be used by another service the needs extra bandwidth at the time. Because of this the actual 'realtime' bandwidth can probably vary greatly.

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post #6 of 8 Old 07-27-2000, 11:41 PM
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While it is true that there are varying artifacts on video bandwidth allotted to different programs, most of you all are placing way too much importance on this aspect in determining quality differences. The biggest reason is that you all make a really bad habit of comparing apples and oranges and corn. In otherwords, you feel that the biggest differences between a channel like CNN and TNN is bandwidth allocation. The truth is that comparing these two channels is like comparing a live broadcast camera with a VHS tape dub from an SVHS master of a program. The range of quality between different channels on DirecTV and Dish network is more dependent on the program source than on bandwidth. However, the channel comparisons between CNN and MSNBC is a better comparison as both originate from a live camera feed on the studio shots. IF you can see the difference in quality between CNN and MSNBC studio quality and you also know that the cameras used for both are full component quality studio cameras then at this point you are justified in seeing a compression quality difference.
It is my opinion that the biggest differences in program quality is in order of:
1. Source media
2. Camera quality
3. component/composite
4. Compression artifacts.

This means that assuming the first three are identical then you can do a fair comparison to see the 4th (compression difference). The biggest differences you will see from channel to channel on the DBS services, and cable for that matter is source media between the channels. I have supplied TV shows to various networks on everything from VHS tape to D2 and even D1 per their spec on animation clips. I just received a spec today for a Philadelphia channel on cable and all they accept is VHS. I about fell over when that request came in. You people in Philadelphia probably wonder why your cable looks so bad. There's why!

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post #7 of 8 Old 07-28-2000, 01:01 AM
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In answer to S Sanchez's original question, I've heard of two actual working possibilites.

1) The channel allocation table from the DirecTV receiver can be displayed in some cases. I've heard that certain Sony models can be coaxed into doing this, though I don't personaly know which models or how to get them to spit this info out. However, this doesn't tell you how much bandwidth is allocated to a particular program. This only tells you which programs share the same transponder. It is possible that one program may get a lot of bandwidth while another only gets a little even though both share the same transponder.

2) I have seen a device that analyzes mpeg data streams and reports the actual data rate of non-redundant data in the stream. When connected to the mpeg audio and video data streams in to DirecTV receiver, this device reports the actual data rates of the particular channel turned and being displayed at the moment. The device currently isn't for sale, and if offered for sale for this purpose it would probably be too expensive to justify the use soley for the purpose of satisfying a curiousity of the channel data rates.

Commenting now on the loss of resolution, It is believed that DirecTV applies both vertical and horizontal bandwidth reduction to the signal. This is the technical way to describe the loss in detail or resolution you mentioned.

Depending entirely on how severly it is applied, bandwidth reduction can in fact outweigh all other factors in Don's list above even though it is a "compression artifact", i.e. a visible effect of compression with a loss that cannot be recovered after the signal is decompressed for display. Even though Don listed compression artifacts as #4, it is possible for VHS tape to wind up looking better than a HDTV source when sufficient bandwidth reduction is dialed in by DirecTV's selection in their plant.


[This message has been edited by Richard Adams (edited July 28, 2000).]

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post #8 of 8 Old 07-28-2000, 05:17 AM
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And, naturally, the more you roll-off high-frequency video content (detail) prior to compression, the more graceful the compression artifacts will appear, and so causing even fewer bits to be required. Meanwhile, S. Sanchez and I are going blind trying to watch MSNBC!

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[This message has been edited by Dave McRoy (edited July 28, 2000).]

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