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The Hits on Freeview - Heavy Compression

(September 2003)

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FU
fusionlad Founding member
Mark Boulton posted:
fusionlad posted:
chrisb posted:
I agree, I think since about two weeks ago the compression is more noticable than ever. The colder weather perhaps, making our aerials shrink?


I know your probably not serious about your last comment, but it's nothing to do with reception. Bad reception does not cause these compression problems.


Sorry fusionlad, I think you've been misinformed there.

Yes, reception problems CAN increase compression artefacts.

MPEG and other compressed systems (and even uncompressed ones like CD Audio) have a certain amount of 'error correction' tolerance built-in. This means that when the signal gets corrupted, the machine will work harder to uncorrupt the higher (more significant) bits of the digital word rather than even attempt to decode the lower (least significant) bits. On dirty CDs this manifests itself as a slight 'clicky' or 'bristly' edge to the sound, whereas on MPEG video this will manifest itself as appearing to have been compressed more - simply because the more significant parts of the image (i.e. the larger blocks) are being decoded (with trouble) and the receiver's processor doesn't have the time/stable enough signal to fill in the details.


That's a different problem mate. When there are reception problems, you git picture freezing etc. I say again, bad reception does not give you the sort of artifacts we are talking about here.
MB
Mark Boulton
It does mate, I'm sorry.

Extra artefacts and freezing are merely different points on the same scale - i.e. symptoms of slightly less than optimum reception and considerably less than optimum reception, respectively.
NG
noggin Founding member
Mark Boulton posted:
It does mate, I'm sorry.

Extra artefacts and freezing are merely different points on the same scale - i.e. symptoms of slightly less than optimum reception and considerably less than optimum reception, respectively.


I think that you are both - in a sense - right.

Compression artefacts can be introduced by poor reception should error correction fail and error concealment kick in...

If error concealment is not working too hard then the reception problems may seem to appear as small amounts of additional compression artefacts in certain blocks (but not usually uniformly across the picture). However concealment will fall apart if reception becomes worse, and then chirruping sound and missing/frozen macroblocks become visible.

If the compression artefacts are indeed uniform across the picture then it is likely that the problem is not reception related. Easiest way to check is to look at the corrected and uncorrected error listing on your receiver if it has one. (Pace DTVAs do) If there are no uncorrected errors then the problem is not a reception one..
SN
snarfu
The heavier compression with The HIts is probably due to the two extra EMAP audio streams now being accomdated on that Mux (Mojo and Heat).
PE
Pete Founding member
snarfu posted:
The heavier compression with The HIts is probably due to the two extra EMAP audio streams now being accomdated on that Mux (Mojo and Heat).


er no.

For a start the radio stations are tiny in size for a second their are on SDN.
SN
snarfu
All the other EMAP radio stations are on the same mux as 'The Hits' so I assume the other two are also. Why would EMAP go to another Mux operator for just two radio stations.

Also I am hypothesising here but with the allocation of bandwidth, EMAP have been allocated 1/4 of the bandwidth of the mux for their broadcasts. They have chosen to utilise this for 1 video stream (The Hits) and countless audio streams - so any extra streams (inc audio + MHEG) would make a difference to the overall bandwidth allocation for 'The Hits'. I know stat-muxing is used on the BBC Muxes (except BBC1) but I am not sure if this is the case on the Crown Castle, SDN or ITV/C4 muxes as it would be more difficult to charge for the bandwidth used.
NG
noggin Founding member
snarfu posted:
All the other EMAP radio stations are on the same mux as 'The Hits' so I assume the other two are also. Why would EMAP go to another Mux operator for just two radio stations.

Also I am hypothesising here but with the allocation of bandwidth, EMAP have been allocated 1/4 of the bandwidth of the mux for their broadcasts. They have chosen to utilise this for 1 video stream (The Hits) and countless audio streams - so any extra streams (inc audio + MHEG) would make a difference to the overall bandwidth allocation for 'The Hits'. I know stat-muxing is used on the BBC Muxes (except BBC1) but I am not sure if this is the case on the Crown Castle, SDN or ITV/C4 muxes as it would be more difficult to charge for the bandwidth used.


I doubt that Crown Castle use statmuxing between different clients - however I gues it may be possible to offer it if clients have more than one video service per mux (i.e. Sky Travel, Sky News and Sky Sports News could potentially be statmuxed together?) However I don't believe this is happening - I don't have a PC DTT receiver currently installed to check.

Another reason that SDN may be carrying the new services is that teh Crown Castle muxes may be "full" of services. Quite a lot of muxing kit can only mux 16 (or is it 15?) separate MPEG services - and video, audio, data and subtitle streams all count as different services I believe. This may mean that the existing Crown Castle muxes are at the limit of the number of services they can carry - rather than a question of data capacity?

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