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Old 2009-03-30, 04:09 PM   #1
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Default Rogers is now compressing some HD channels.

We had previous discussions of this topic in the following thread, however, at that time Rogers did not begin compressing. I encourage those who are new to this topic to please read a few of the pages in the following thread before commenting in this thread.

http://www.digitalhome.ca/forum/showthread.php?t=84134

Up until now, I believe that Rogers was the only major service provider who was not compressing some of the channels. I'm a bit surprised that they have begun compressing since they have rolled out out SDV (Switched Digital Video) which saves on bandwidth. Please do not discuss SDV here - there are already several other threads if you wish to discuss SDV.

I noticed this a few weeks ago and have since been checking the bitrates on various channels.

My Results:

During the few weeks, I have recorded the following channels. In each instance I recorded 5 hours to get a good handle on the bitrate. 5 hours means I should have less than 1 Mbps error in my summary.


I would suspect that unless the bitrate is below 15, there is no compression by Rogers. Edit - it is possible that even with a bitrate below 15 for some OTA channels, there is no compression by Rogers. One needs to compare the source bitrate with the Rogers bitrate - discussed later in this thread - See:

http://www.digitalhome.ca/forum/show...042#post906042


Channel (GTA) - Affiliate - GB used for 5 hours - Average Mbps Calculated - (OTA*)

(Channel numbers are from before the realignment)

Ch 502 - SportsNet - 39 GB - 17 Mbps
Ch 503 - TSN - 39-40 GB – 17-18 Mbps (Checked twice)
Ch 504 - Speed - 28 GB - 12 Mbps
Ch 505 - Discovery - 39 GB - 17 Mbps
Ch 506 - Raptors - 36 GB - 16 Mbps
Ch 507 - Score - 39 GB - 17 Mbps
Ch 508 - OMNI1 - 31 GB - 14 Mbps
Ch 509 - OMNI2 - 31 GB - 14 Mbps
Ch 512 - Sun - 26 GB - 12 Mbps
Ch 513 - CBC - 38 GB - 17 Mbps
Ch 514 - SRC - 31 GB - 14 Mbps
Ch 515 - Global - 40 GB - 18 Mbps
Ch 517 - CTV - 39 GB - 17 Mbps
Ch 519 - City - 39 GB - 17 Mpbs
Ch 521 - E! - 32 GB - 14 Mbps
Ch 522 - ABCW - 30-36 GB - 14-16 Mbps, (checked twice) (18)
Ch 523 - CBSW - 23-24 GB - 10-11 Mbps (checked twice) (14)
Ch 524 - NBCW - 23 GB - 10 Mbps (checked twice) (16)
Ch 525 - PBS - 22 GB - 10 Mbps (checked twice) (13)
Ch 526 - FoxW - 29 GB - 13 Mbps (15)
Ch 527 - ABCE - 26 GB - 12 Mbps (11)
Ch 528 - CBSE - 33-39 GB - 15-17 Mbps (18)
Ch 529 - NBCE - 35 GB - 16 Mbps (17)
Ch 531 - FoxE - 28 GB - 13 Mbps (16)
Ch 532 - WGN - 25 GB - 11 Mbps
Ch 533 - A&E - 40 GB - 18 Mbps
Ch 534 - Showcase - 24 GB - 11 Mbps
Ch 535 - Nat Geo - 25 GB - 11 Mbps
Ch 536 - HBO - 32 GB - 14 Mbps
Ch 537 - TMN - 24GB - 11 Mbps (checked three times)
Ch 540 - MPix - 27 GB - 12 Mbps
Ch 541 - HDNet - 39 GB - 17 Mbps
Ch 542 - TLC - 23 GB - 10 Mbps
Ch 544 - NW - 28 GB - 13 Mbps
Ch 545 - CNN - 31 GB - 14 Mbps
Ch 546 - Golf - 31 GB - 14 Mbps
Ch 548 - Treasure - 31 GB - 14 Mbps
Rogers Community - 31 GB - 14 Mbps

* OTA information taken from "Rabbit Ears" link, post 16 of this thread.
Compressed
Uncompressed
Unconfirmed

The above list is not complete and as I get more time, I may add more information, but I encourage others to perform and post their own test results. Note that due to statistical muxing (discussed in post 3 below), the results may vary.

Just as an FYI, this has been going on for quite some time (perhaps months), but there has been little/no discussion here, so I guess whatever Rogers is doing to the affected channels hasn't had a huge impact on PQ.
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Old 2009-03-30, 04:10 PM   #2
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Default How to calculate bitrate?

How to check this?

1. I recorded 5 hours of various channels to get an accurate number (shorter times would be slightly less accurate, but still give you an idea).
2. I checked the diagnostics on the SA8300HD for how much space on the Hard Disk Drive (HDD) was used.
3. I deleted the programme and again took a reading of the HDD space.
4. I subtracted the difference, which gives me the GB used for that particular programme. (Remember 8 bits per byte - b=bit, B=byte).

For example:

25 GB x 1000 for MB x 8 for bits / 5 hours / 3600 seconds = 11 Mbps (roughly)

If you record 5 hours, you can multiply the resulting GB by 0.44 to get the bitrate in Mbps.

The diagnostics on the SA8300HD can be accessed via the following link:

http://www.digitalhome.ca/forum/showthread.php?t=17719

You press/hold "select" until the "envelope" comes up on the front of the SA8300HD (takes a few seconds), then press "info". The diagnostics will come up and you can page through them.

The HDD info is available on pages 34 and 35 of the diagnostics. Page 35 is for the external HDD if you have one.

People with PCs and firewire connections are also able to do similar calculations using the filesize of the downloaded programme (note that sometimes a bitrate is given as part of the file, but this is usually not accurate).

Another crude way to do the experiment is to record a long programme and see the percentage of the HDD utilized in the "list" under "preferences". Then when you delete the programme, the percentage used drops. Typically on standard PVRs with 160 GB HDD, each hour of HD takes about 5% of the drive space, or about 8 GB/hour - this is not as accurate because some of the drive is set aside for IPG and other things.

If the programme is at a lower bitrate, the drive percentage drops to 3% per hour. Typically a 2-hour HD programme would take up about 10% of the drive. If it's at a lower bitrate then it may only take up 6-7% of the drive.

Those who don't have the ability to record the HD stream will not be able to check the bitrate.

Edit - Note, be sure to check if there were any simsubs in your recordings since that would provide incorrect data. I recorded all my programming off prime-time to avoid simsubs.

Edit - Following a firmware update in late 2010, the diagnostics no longer show the data for the HDD, so the calculation must now be done by using the % used on the HDD and recording for a longer time to obtain acceptable accuracy
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Old 2009-03-30, 04:10 PM   #3
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Notes on QAM & Stat Muxing:

A single QAM frequency (Modulation used by Cable Service providers) can handle roughly 38.8 Mbps, so with 2 channels per QAM you'd have a theoretical maximum per channel of about 19.4 Mbps when you have 2 channels per QAM. At 3 channels per QAM, the maximum (average) bitrate would be 12.9. Typically the channels on Rogers have been between 15 and 18 Mbps up until recently. The channel frequency can be determined in the diagnostics.

Note that if Rogers is using Statistical Muxing (multiplexing), the instantaneous bitrate can vary to handle movement, etc, however, the average bitrate will not reflect this variability. With Stat Muxing Rogers can assign more bandwidth to programmes, or portions of programmes that require it – like sports or action movies (I believe this is controlled by software and operator input is not required. This is why you may have one channel with 15 Mbps, another with 13 Mbps and another with 11 Mbps on the same QAM. The instantaneous variability may not always be apparent…

FYI - 19.4 Mbps is the maximum OTA broadcast allowed by ATSC. As stated above, most affiliates transmit at about 15-18 Mbps. The original HD stream is over 1 Gbps, so by using MPEG, the affiliate does a compression of more than 50:1. It's amazing that HD looks as good as it does with a compression factor of 100:1 in some cases.

Article on Stat Muxing:

http://broadcastengineering.com/infr...digital_cable/
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Old 2009-03-30, 04:36 PM   #4
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So although we can measure a drop in the bitrates for some channels, has there been a noticeable drop in the picture quality? Are they doing a good job with the additional compression?
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Old 2009-03-30, 05:29 PM   #5
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I suspected Rogers was using compression. This because I noticed an improvement in PQ on digital OTA channels compared to the same channels on Rogers.
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Old 2009-03-30, 05:38 PM   #6
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And which OTA stations would that be, since most (GTA receivable) OTA is not further compressed? The difference you mention may simply be the difference that many people mention when they view OTA vs Rogers, compressed or not. Please remember OTA is not the subject of this thread.
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Old 2009-03-30, 05:49 PM   #7
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@57: I realize this thread isn't about OTA, I just mention it out of comparison. And you could be right: OTA may just appear better than Rogers in general....
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Old 2009-03-30, 06:59 PM   #8
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I don't believe Rogers has ever offered the exact same digital signal they receive from channel providers to their customers in any sort of 'pass-through' network topology. Everything is re-compressed to some extent.

My understanding is that it's the MPEG-2 encoder inside the setup box (PVR) which will automatically set the bit-rate based on the nature of the video. And therefore if this encoder is a variable based one, the final file size of the recorded video will differ depending on the nature of the video (static shots versus action shots).
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Old 2009-03-30, 07:06 PM   #9
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Civuck, you are mistaken regarding digital channels (both HD and SD). There is no encoding taking place in the SA8300HD. It's a bit for bit recording of the incoming digital signal (for SD and HD). Also, the bitrate that Rogers previously supplied was identical to the bitrate from the (OTA) affiliate. Any compression is done at the head end, not at the STB, as this reduces the bandwidth necessary for transmission. If you think about it, Rogers wouldn't save any bandwidth if the compression were done by the STB/PVR.

I cannot comment for sure on what happened to bitrates from non-OTA (say TMN, TSN, etc) since they can sometimes supply say 50 Mbps streams to Rogers (and other service providers), which are then compressed to fit 2 channels per QAM (but were always close to the 19 Mbps max on Rogers). In most channel instances that was not the case (large streams sent to Rogers) and a stream of less than 19.4 Mbps was supplied by the "channel" and there was no further compression by Rogers. That has now changed on some HD channels.

History: For SD, there used to be STB encoding when Roger had analogue channels on the PVR, but that's not been the case for several years - as Rogers has all channels digital on the STB/PVR. I'm not familiar with what Rogers does at the head end regarding analogue channels, other than the fact that there is certainly compression there with roughly 8-12 digital SD channels per QAM, typically depending on the popularity of the channel. Note this has nothing to do with this recent HD development and is not the topic of this thread.
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Old 2009-03-30, 11:46 PM   #10
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Thanks for the info, 57.

Maybe I'll try some testing on my own, too, but even without doing it, I have been noticing for months a lot of macro blocking on a few of these channels. NBC West (Ch. 524) has had a lot of macro blocking in the last 5-6 months, at least. Same thing goes for HDWGN (Ch. 532). I have always thought that this was the result of additional compression by Rogers. I may have been right, now that I see 10 Mbps for NBC West.

I also remember reading in other threads of people complaining about the PQ of HD TMN. Your result for channel 537 (11 Mbps) confirms that, too.

It looks like we'll have lower picture quality at an increased subscription price. Way to go, Rogers!

I, too, understand that this thread is not about OTA, but more and more, it looks like an option worth considering.
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Old 2009-04-01, 09:41 AM   #11
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57 one tip I will offer is to note the QAM channel so that you can add up the constituents to see if you approach the max bitrate (38.8Mbps) per channel.
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Old 2009-04-01, 09:45 AM   #12
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The conclusion here that Rogers is further compressing these channels is far from the only possibility.

As we know, OTA channels are often multiplexed, including one main HD channel and one or more SD channels. In the past, Rogers has transmitted this entire multiplex, when only the primary channel is tunable by the end user.

To free up wasted bandwidth, Rogers could simply be stripping the primary channel out of the mux.

As for the Astral channels, do we know what bit-rate they are being supplied to Rogers at? If not, how can you conclude that Rogers is further compressing?
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Old 2009-04-01, 10:04 AM   #13
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Quote:
As we know, OTA channels are often multiplexed, including one main HD channel and one or more SD channels. In the past, Rogers has transmitted this entire multiplex, when only the primary channel is tunable by the end user.
Good point JohnnyG. On a minor note shouldn't that be "U.S. Channels are often multiplexed"

Having said that, this should be something that can be tested by comparing the bitrates of U.S Channel OTA vs. Rogers.

Quote:
As for the Astral channels, do we know what bit-rate they are being supplied to Rogers at?
I don't think we do, but perhaps 57 does. I can say however, in its letter last years, Rogers engineering did say that compression would be performed on TMN and Mpix. In addition, I can confirm that a Rogers technical person told me when they launched TMN HD 24/7 that it was being broadcast at the maximum bitrate.


FWIW, when I tried to discuss these issues with Rogers last year, they essentially told me to get lost. In addition, I am told that Ted himself was very pissed the information was made public on Digital Home. A few weeks later, I was told that all future ad campaigns on Digital Home were canceled and that this site was blacklisted from any being bought on any Rogers buys. Despite them canceling all ads, they never denied the existence of the networking engineering letter nor the fact that they were going to begin compressing HD signals.
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Old 2009-04-01, 10:05 AM   #14
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I bet you won't see the Buffalo and local TO OTA stations compressed since they can be recieved off-air.
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Old 2009-04-01, 10:07 AM   #15
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I have been in contact with both Rogers and Astral (TMN). The compression is real and bitrates on some channels are being reduced. As more (non-proprietary) information becomes available, I will advise.

If you recall, a few years ago the bitrate on Astral channels was reduced for a very short time by squeezing 3 channels into a single QAM. We contacted Astral and this was reversed by Rogers at the time.

If you note the bitrates for OTA channels, some of them are quite low and even when OTA channels are multicast, their bitrates are not that low. Previous bitrate analysis of Rogers indicated that channels were typically in the 15-18 Mbps range (previous discussions on this matter and as mentioned in post 1). This is no longer the case.

I would encourage anyone who has the capability to record OTA HD, to please post their results, especially for channels that are shown with very low numbers in my table. Edit - (Post by 99gecko below seems to have provided this information, thanks)
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