WTVS (Detroit PBS SD) distortion

[Rick]

I have a strange artifact on this station (163). It is difficult to describe but is most obvious on faces -- it looks like I am seeing them through a piece of distorted glass that is moving. There are "waves" in the flesh! I never noticed this earlier on this station, but it is really annoying. I assume it is a result of over compression of the signal, but I really don't know. Does anyone else see this? It is on my 8000SD and my 8000HD whether live or recorded.

[57]

I recall a thread a while back about "jello heads". Do a search for "jello" and you'll find a few threads on the subject.

[Rick]

Thanks for that pointer -- I guess this is a common effect in some digital channels. I don't see it on all shows on WTVS, but it is clearly there on "As Time Goes By" (which I watch religiously wherever it appears!! :-) ). I also recall it was on the New York WB station when I used to watch Smallville there. Guess there is nothing to be done. Bummer -- I find it REALLY annoying.

(I like the description "jello heads" -- fits the effect quite well!)

[nfitz]

This channel has had an absolutely horrid picture ever since Rogers first started carrying it, with an unbelievable amount of digital compression ... and that's what I thought with an old 20" 4:3 television! Haven't really looked as the channel in a couple of years.

Looking at it today (with the same TV - HDTV is in the shop) ... picture looks okay until something moves ... and then it is just ... not right. I don't know how Rogers receives this channel compared to most of their channels, but it just doesn't look right, compared to WNED and KCTS.

Hmm, credits are running right now for a children's show ... the amount of compression artifacts around the text (when you pause it), are just unbelievable!!

Nick

[Rick]

OK, I have been ignoring this problem for a long while, but I would really like to complain to Rogers about it. I think there is a way to find out how much compression is being applied to this channel compared with others (eg KCTS which is right next to WTVS and is fine). I go into diagnostics, but which page is it on? Which parameter?

[57]

The only way to determine compression is to record a fairly long timeframe of SD programmes (say several hours) onto a DVR seeing how much HDD space is occupied and then backcalculate the bitrate. I see you have a DVR from the first post.

See the first post of the following thread under HDD Space available.

http://www.digitalhomecanada.com/for...ad.php?t=13405

Steps.

1. Record a long programme - say 10 hours.
2. Look at the number of "units" on the HDD (write down the value).
3. Delete the programme.
4. Look at the number of "units" on the HDD now. (write down the value).
5. Subtract one from the other to see the number of units used for that recording.
6. Based on the number of units, calculate the percentage of HDD used.
7. From the percentage, calculate the number of GB used.
8. Use the number of GB/10 hours to calculate Mb/sec (remember 8 bits/byte).

Your answer should be around 3-5 Mb/sec. See the following example which I did for 2 hours of HD. Remember, this will not be perfectly accurate since the HDD is not 100% utilized for programming, but it will allow you to compare several different channels.

http://www.digitalhomecanada.com/for...90&postcount=5

If you don't want to calculate the actual bitrate, you could just record two different channels for the same amount of time and see how many "units" are used for each to compare...

[Rick]

Bummer -- I thought there was a parameter. Oh well, I'll give that method a try.

Is the average bitrate used on a particular channel fairly constant? I mean, do they increase it at certain times of day maybe and then decrease it when they think no one would be watching anyway?

[57]

Rogers does use "rate shaping" software, so you'd need to compare two channels at the same time of day say. Shouldn't be too hard to set two 5 hour recordings simultaneously one on the "channel of concern" and one on a "good channel" to see the difference if there is one.

[Rick]

... which is exactly what I did. I recorded 5 hours of WTVS from 11:30pm to 4:30am and also 5 hours of WNED (PBS Buffalo) for the same time.

The diagnostics show I have a Maxtor 4R080L0 which is 80 GB. The AVFS "capacity" is 33165 units, so that works out to about 414.56 units per GB.

The 5 hour recordings used 2563 units (WTVS) and 3404 units (WNED). If I did my arithmetic properly, that works out to 2.7 Mb/sec for WTVS and 3.65 Mb/sec for WNED.

That doesn't seem like a huge difference, but the difference in picture quality on these two channels is definitely huge!! Could that difference in bandwidth account for it? WTVS is almost unwatchable because of the artifacts. Can I complain to Rogers that they are not really supplying this channel, given the quality?

[57]

You could complain and see if they do anything. The bandwidth they alocate to channels is usually based on several factors.

1. The number of viewers of that channel.
2. The "movement" on the channel - sports channels require more bandwidth.
3. The cost of the channel - premium channels like TMN get more bandwidth.