Channel Master CM7000 vs Tivax STB-T8 dissapointed in BOTH

The Channel master CM7000 and Tivax STB-T8 are reputed to be two of the top rated boxes.

Frankly I am amazed! The BEST? Really???

That's hard to believe, given the really annoying issues that plague both these boxes.

I think that most of these issues have been reported by others, but I was a little surprised that the manufactures have totally ignored these complaints and never bothered to fix them.

For example, despite the CM7000's supposedly 'best in class' video, I have found that on each and every one of the channels I am getting, the sharpness of Channel Master CM7000 on S-Video is very disappointing.

When downsampling 1080i HD to S-Video the result should be AT LEAST DVD QUALITY (and potentially much better). The CM7000 picture does have good color, and is very smooth and clean with low-artifacts, but the Tivax STB-T8 1080i HD composite conversion output is actually SHARPER than the CM7000 is on it's S-Video output!

Composite should never be better than S-Video, even if your set has Comb filters. Nor is this because my Sony Trinitron set has poor S-Video performance, it's great on S-Video. I have had several DVD players and a Satelite hookup on this set in the past, and ALL were MUCH sharper than the CM7000. It's so bad, some of the channels are actually sharper ON THE ANALOG OFF THE AIR TUNER than their ATSC DTV counterparts. Sad, really sad!

Second, I am also very disappointed in the CM7000 teeny, teeny, teeny, tiny, tiny, tiny, channel guide text. This ridiculous situation with the guide makes one of the boxes best features pretty much USELESS on small portable screens (or even on larger screens when using the tuner for input). You can't even read the damn thing! If you look at the screen shots of the channel guide of the CM7000 posted elsewhere on this forum, you will see that this is really, really, really, STUPID because they didn't even use anywhere near all the available space, and the guide screen is shifted way over to the left! The box supports several larger fonts for close captioning, but they were not used for the guide, EVEN THOUGH THERE WAS PLENTY OF SPACE. Whoever the MORON was at Channel Master who signed off on this should be fired!

Not that the Tivax STB-T8 is any better . . .

- The Tivax T8 channel guide is also pretty BAD because of it's poor layout (but at least the text is larger). Unfortunatly, you only get a lame now/next view - with only 4 frigin lines of text - then it chops off! When someone questioned this on the Tivax website FAQ, some idiot explained that "That's how it's designed to work" like that explained EVERYTHING. This is like picking up a new car, driving 4 miles, and having the engine fall out, only to be told tersely "you should have read your owners manual, the engine is SUPPOSED to fall out every 4 miles!"

- As mentioned, although the Tivax STB-T8 composite video is a bit sharper than the CM7000, it's still not DVD quality, epically on SD sub-channels (even SD channels encoded at full 704x480 DVD equivalent resolution!).

The T8 is also very poor at properly scaling the video. On HD it will NOT remember the correct zoom ratio, which is annoying, and on SD it's even worse, and can't even get it right after you try to set it manually!

Manufactures like Tivax like to say that their boxes aren't broken, they are just failing to offer an optional 'auto aspect' feature, but this is GARBAGE. The friggin FCC requires in the overall requirements, that the box –

A. Comply with the MPEG2/MPEG4 standards (which require that the decoders in the boxes deal properly with non-square pixel aspect ratios).

and

B. At a minimum, at least duplicate the basic functionality of the original NTSC system. I don't know about you, but I don't remember having to constantly fight with my old NTSC analog television set to keep the picture from alternately turning into a tiny postage stamp, or blowing completely off the screen (or showing a screwed up distorted picture with FAT or SKINNY people).

Not only does the Tivax T8 fail to automatically deal with the pixel aspect ratios correctly, but it also screws them up on 4:3 SD content EVEN AFTER YOU SET THE PROPER ASPECT RATIO MANUALLY (you get none square display with taller skinner people, or alternately fat stretched people; no setting gives the correct 1:1 aspect ratio).

The good news is that (at least for the Tivax STB-T8), I think that owners may be able to force a software upgrade, because the whole argument about whether ‘auto setting’ is required is moot when you can’t even set SD content to a correct 1:1 aspect ratio manually.

Part of the most basic requirements for any CECB is MPEG2/MPEG4 compliance, and those standards require proper handling of all the standardized non-square aspect ratios PERIOD - END OF STORY.

Tivax can try to B.S. about how so-called 'auto aspect' is not actually called out in the standard, but as I said above, that's ridiculous, the user should not have to punch an extra 'aspect' button up to three times just to keep the set from displaying a totally non-compliant picture with incorrect horizontal/vertical scaling! In any case, you can’t even get it perfect by setting it manually, so I’d like to see them weasel out of that!

If these boxes are two of the best, I'd hate to see some of the worst!

- Delphin

Thanks for the suggestion about the S-Video cable, I'll try another one, but I doubt it will make much difference, since I did my side by side tests by moveing the source end of the same cable back and forth between the DVD player and the CM7000 (so that the S-Video cable, and port on the TV remained exactly the same).

Sorry if my comments seemed a bit negative. I just don’t like to see a basically good design like the CM7000 hurt by unnecessary silly mistakes, like putting tiny fonts that need 400 or 500 lines of resolution to read, on a box required to support connections through an NTSC off-the-air tuner that can handle (at most) about 330 lines of resolution.

One of the things that bugs me most about the guide text being so small and the S-Video being sub-par on sharpness is that these problems interact in a very annoying way.

I can actually use my Trinitron's sharpness setting to punch-up the overall quality of the CM7000 image quite a bit, so the sharpness with actual on-the-air video looks much better (quite nice actually), but this causes some slight ringing artifacts, which I could normally live with, but because the channel guide text is so damn small, even minor ringing causes the guide to becomes totally illegible and useless (which I can't live with), so this work-around is not a practical solution.

Grrrrr!!!

On the plus side, the Channel Master CM7000's ST chipset did do significantly better in my side by side testing with the Tivax T8 on things like dynamic multipath and hiding blocky low-bit-rate MPEG artifacts and mosquito noise (deblocking and deringing seem to be better).

Multipath is almost always a concern with ATSC's 8VSB modulation, and the other artifacts mentioned can get pretty bad when broadcasters try to push their bit budget too far with sub-channels. So, I don't want to trash the CM7000 too much, because in a lot of ways it does deliver where it counts.

In fact, my friend was impressed enough by these factors when I showed him my CM7000, that he will probably also get a CM7000 dispite the other issues.

So, I will be able to do a side by side with his box when it arrives, to see if my CM7000 is sub-par.

The Tivax STB-T8 also does a pretty reasonable job considering what these boxes have to deal with when decoding 8VSB signals in a residential setting (though not quite as good as the CM7000).

As to down-sampling vs. down-scaling . . .

Not to nit pick back, but I was trying to be fairly precise. You see, from what I am seeing, dignifying what my Channel Master seems to be doing as 'down-scaling' may be, I think, perhaps a bit over generous.

A decent bicubic resize from an HD source would be enormously sharper, and even the simplest of respectable "down-scaling" algorithms (a simple bilinear resize) should be very sharp when you have a good quality 1920x1080 source to start with.

But I don't think that's what is going on . . .

The cheap dirty approach is to just down-sample the video to the lower resolution using a resizing algorithm called 'nearest neighbor', which as you probably already know, just down-samples (or down-scales if you prefer) by grabbing some pixels and skipping others.

As a test I took a good sharp HD capture, then used Virtual Dub to downsize it to 704x480 using Virtual Dub's built in cheap dirty 'nearest neighbor' resize.

The result from the nearest neighbor type resize was a nice sharp image, but one with very noticeable 'jaggies' from the non-integer ratio down-sampling.

No problem though; just apply a mild gaussian or box blur function (for our S-Video analog video, just use a simple low pass filter on the converted signal) and BINGO - no more jaggies (but now, the video sharpness is nowhere near as sharp as it should be).

Virtual dub also has bilinear, bicubic, and lanczos resize, so, just for the sake of comparison, I tried downsizing the HD video with these more sophisticated true re-scaling algorithms. Every one gave very clean resizes, without jaggies or other artifacts, while maintaining very sharp output.

As I said above, the ST chipset in the Channel Master CM7000 does seem to do an above average job in dealing with dynamic multipath, and MPEG decoding artifacts are suppressed better than most of the other CECB boxes, but from what I have seen so far, it looks like the 'down-scaling' you refer to may be nothing but simple nearest neighbor down-sampling, followed by mild lowpass filtering

I could be wrong though – Maybe the CM7000 does use a more sophisticated rescaling algorithm. If so, it is either very poorly implemented, or, as has been suggested, my CM7000 is just not quite up to par.

As I said above, I will recheck with another sample of the CM7000 when my friends box arrives.

To Rammitinski, who commented that my expectations were probably too high to start with, I would only say that I agree 100%. As an engineer who was involved with the RF engineering of some of the very first high capacity digital radio links in this country, including some providing broadcast quality STL (studio to transmitter) video links, I have little patients for half baked hardware designs, and even less for bone headed software.

The manufactures are not allowed by the CECB rules to output an HDMI, DVI, or Split Component signal, but anything you can squirt out of a composit or S-Video jack is FINE, so it would have been nice if they had done a little better job in maximizing this option.

I just wish they had sweated some of the details on these boxes a little more before pushing them out the door.

- Delphin.

Funny you should mention that, this font was EXACTLY my choice ('Raised' Edge will also make the font BOLD), but did I miss something, or does that option only effect the CC (Close Caption) text?

Is there some way to use this font for the CM7000 Program Guide? (that would make me very happy)

I was not really very concerned the small CC text on my Sony, because I can get the CC text larger by just activating it on the set itself (all CECB boxes are required to pass through the main CC text channel to the set for decoding). Of course, if they ever start to use the other CC channels, then you have to use the CM7000 CC feature, because so far as I know, only the first CC channel is passed through to the Television for decoding.

So keep this in mind if you find that, even with the larger 'font 4', the CM7000 CC font is smaller than you would like, you can still use your sets built in CC decoder if it works better.

On smaller screen sets, the built in CC decoder text may be much larger and more legible, but none of this does anything to fix the small CM7000 "Channel Guide" text, so far as I can tell.


The Sony does have Comb filters for composite, but these are almost universal on large screen sets.

I also see little difference between Composite and S-Video with the CM7000, both seem to have only about 300 lines of horizontal resolution (barely what you get from a off-air analog broadcast).

I am now running my DVD player on the #2 composite input (this set only has one S-Video input), and with the CM7000 on S-Video and the DVD player on the composite input, the DVD player is still noticeably sharper.

However, this is NOT because the Trinitron has better processing on composite. Reversing the hookup, with the DVD player on S-Video and the CM7000 on Composite, or putting BOTH on composite, gives identical results. (400-500 lines of resolution for the DVD player, 300 or so for the CM7000)


I think we had crossing posts here, because if you look at my previous post, you will see that I noted exactly this same point, and was bemoaning the fact that what looked like optimum sharpening for the decoded MPEG video was not usable on the CM7000 (because then ringing artifacts TOTALLY TRASH the channel guide text).

Thanks for the info about the hidden Diag Menu. This is a nice tool, and it handily confirms that I had set the sharpening about as high as possible without ringing, and that my earlier Trinitron service mode adjustments for screen geometry were right on the mark.

It also confirms that the Channel Master folks knew right were the screen overscan boundaries should be. So why they chose to scrunch up the channel guide over to the left side of the screen with a ridiculously tiny font, when there was plenty of space available for a larger font (and they knew it), is beyond me.

I know some small screen sets are poorly adjusted and have excessive overscan, so apperently Channel Master was paranoid about keeping everything on screen, but the CM7000 text is sooooo damn tiny that it will be impossible to read on these sets when interfaced through the tuner anyway, so the cure is much worse than the disease.

Having used a dozen or more other devices on this set (VCR's, DVD Players, Satelite and Cable Boxes), I have never seen an on screen menu display with font legibility as poor as the CM7000 Channel Guide text. Never.

I hope there is some way to make this font larger (as I have already done with the CC text).

- Delphin

I am just estimating horizontal resolution by eye, based on comparisons to known resolution sources, like VHS (200-250 lines), sharp off air video (300-330 lines) and DVD (500 plus lines). I agree that some on air sources look quite a bit better than others (though none so far are as close to DVD quality S-Video as I would like)

- Delphin