I recently had a question from a viewer who was receiving our digital signal and was quite happy except for the fact that occasionally he had what he described as “digitization” in the picture. Probably anyone that has watched any television for some period of time has noticed this artifact. It is sometimes called “pixelization” or “mosaic tiling” or “blockiness.” The professional term for it is macro-blocking and it is what happens to the picture when the digital decoder in the receiver starts to run out of data. Is it the edge of the abyss we call the digital cliff. The digital cliff is the reason why I stress taking all the steps possible to deliver as clean a signal to the receiver as possible. Those steps increase the margin or distance between the received signal and the cliff and the greater the margin, the better.
The viewer lives in a three story home with an outdoor antenna and uses splitters and cable runs to distribute the signal to five different television receivers. He had done some research regarding splitters and sent me a couple of links to websites that he had looked at and was wondering if there was any merit in the claims made about these splitters and if installing them would improve the reliability of his signal. I visited both websites and one was quite accurate in describing the splitter’s capabilities and function, although I am not sure why a splitter needs a “precision die-cast 24k gold-plated chassis.” Gold-plating the connector contact improves connectivity which gives some improvement in performance but I don’t know how gold-plating the outside of the splitter helps.
The other site was much more worrisome since it was full of misinformation. It started off with this statement “These 1080p rated 2, 3 and 4 Way, 2 GHz RF high definition splitters are solely designed for high definition systems.” Where do I start? An RF splitter’s function is to take an RF signal from an antenna or cable system and divide it equally. 2, 3, and 4 way splitters divide the single input into 2, 3 and 4 outputs respectively. They work with the radio frequency (RF) signal and therefore do not care if the content on the signal is digital or analog, high definition or standard definition. RF splitters have been employed since the very early days of broadcast so it is doubtful that this splitter was solely designs for high definition systems and 1080p is a high definition video display format and has nothing to do with the performance of this or any other splitter. The advertising copy went on to make numerous claims of performance of this splitter and inaccurately relating it directly to high definition television.
I have said it before, but what makes digital television digital is not the RF signal, but the content that is on the RF signal. So until the content is removed (demodulated and decoded), the DTV signal works pretty much just like the analog signal. In the RF domain, there is no such thing as a digital antenna, a digital splitter, or digital wire. These passives components are agnostic to the content and only care about the RF signal. To be sure, having good components will improve reception but the same good components will improve analog or digital reception. Don’t fall for the marketing hype.
Bill
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8 comments:
hi Bill
i'm from england and we have the same problem
so your letter has answered a few quieres we had .
mac
Mac,
Thanks for the feedback. Many of the issues that impact digital reception are universal because the problem is not really reception but decoding the data on the signal.
Bill
hi bill
Youv'e said why, but how do you fix it?
Thanks Doris
Hi Doris,
I am not sure what we're talking about "fix"ing. If it is pixelization issue, that is typically a receive/decode issue and not a transmit/encode issue so the "fix" is in the hands of the consumer. Although it doesn't require a degree in electronics, it does involve a basic understanding of the components used in reception like the antenna, cable, splitters, preamplifiers and such and some planning on how to install them.
The antenna is the most important part on receiving an over the air signal since it is the first device that comes into contact with the signal. This means that the antenna gets the signal in as good a condition as it will ever be and all other devices in the system add some amount of degradation to the signal as they bring down from the antenna to the receiver. This is why I always tell people that putting the antenna outside and up high is the best choice.
Even the best cable coming down from the antenna has loss so that a portion of the signal that is collected by the antenna never makes it to the receiver. So we select cable to add as little loss to the path as possible.
A two way splitter not only means that the signal coming into the antenna is divide in half but because of loss through the splitter, each of the two outputs will have slightly less than half of the original signal coming into the splitter. It is even worse on 4-way or 8-way splitters.
If the losses are too great for reliable service at the receiver I often recommend a preamplifier be installed at the antenna. The reason it should be installed at the antenna is that even with a preamplifier, I am not improving the quality of the signal that the antenna received, I am merely overcoming the losses associated with the cable and splitters and the preamplifier, like any active component in a system adds a little noise to the signal. If the signal getting too the antenna is very noisy, the small amount of noise added by the preamplifier may reduce the reliability of the signal at the receiver.
So if that is what you're looking to "fix," then these suggestions should help. If you're trying to "fix" the HD hype issue, I don't know what the solution there is. I have seen a few areas where the hype is just wrong and when the originating party is made aware of it they correct the error but there is an awful lot of it that isn't wrong, it's just may be unnecessary. Does gold plating a splitter that is going to be used indoor add any improvement to a system? I cannot say with absolute certainty that it doesn't but I have never seen a case where it did.
Bill
Bill,
We live 45 miles east of Des Moines. We have a roof mount antenna with coax cable. We have 4 TVs on this antenna. Between the antenna and first splitter we have a Winegard HDA-200 mounted in the attic. We still experience a fair amount of macro-blocking, and the TVs further from the antenna are worse as you would expect. I have an older Channel Master amplifier that I replaced with the Winegard, but I notice the specs are very similar. Would it be safe and/or effective to put this second amplifier ahead of the second splitter?
Hi Jeff,
Since you are seeing the macro blocking on all of the sets the problem appears to be too little total signal. There are a couple of things to check. I noticed on the specification sheet for the HDA-200 it states that there is a variable gain control. You might want to check and make sure that this gain control is turned up.
If the gain control is turned up then the problem may be signal loss between the antenna and the HDA-200. How long is the cable run from the antenna to the amplifier and what kind of shape is the cable and the connnection to the antenna in?
The HDA has a maximum gain of 24 dB if you come out of that into a 2-way splitter each output of the splitter will have a signal with a little less than 12 dB of gain from the amplifier. if you are then taking the outputs of that splitter and running through two more splitters to get outputs for four televisions, each one of those outputs only will have approximately 5 db of gain from the original 24 dB available at the output of the HDA-200.
You can try adding the other amplifier that you have, you may need to back down the gain on the HDA-200 so that it doesn't overdrive the second amplifier.
Bill
Thanks Bill!
The first set is actually quite good, the 3rd and 4th sets are really the problem. I would estimate the cable runs 25 ft from antenna to the amp, but I do have about 10 ft of excess coax coiled before the amp; is that enough to make a difference?
Hi Jeff,
Cable length does have an impact which varies with the quality of the cable, the length of the cable and with the frequency of the signal. Coaxial cables all have a specification for attenuation (signal loss) and it is usually quoted in decibels (dB)/100 feet. The term dB is a ratio of the amount of signal in versus the amount of signal out and we use it here because the numbers we work with can be so small or large that expressing things like loss or gain in dB make the results easier to understand. Also the relationships are not linear so referencing dB makes the concept easier to grasp. For instance if I say that a cable run is 100 feet long and the cable has a loss of 3dB/100 feet on the frequency (channel) that I am want to receive, then only one half of the signal on that channel will make it from the input of the cable to the output. If the cable run is 200 feet, that causes an additional 3dB of loss so that that the now only a quarter of the signal makes it to the output.
For your example let say you are using really good coaxial cable from your antenna to the amplifier. Good coaxial cable will have attenuation of 1.6db-6.6db/100 feet on frequencies between 50 - 700 megahertz (MHz) and those frequencies encompass the television broadcast channels from 2 (54-60 MHz) to 51 (692-698 MHz). The lower frequencies have less attenuation then the higher frequencies. For a 25 foot run the attenuation would be range from approximately 0.4 dB up to 1.8 dB. The decibel scale isn't a linear relationship so what the loss here tells you is that on your 25 foot long run between the antenna and the amplifier you are losing approximately 10% of the signal at the low channels up to about 34% loss of the signal at the higher channels. Whether that is significant or not depends on how strong the signal coming off the antenna is but since you are seeing a little bit of the trouble even on the sets closest to the amplifier, I would say you are flirting with digital cliff at the input of your current amplifier.
A mast mounted preamplifier, right at the antenna would overcome the loss associated with the cable down to the first splitter and might provide enough gain to drive all of the television sets. If more gain is needed, you could put in another amplifier at the first splitter or buy a multi-output distribution amplifier to over come the losses in the cable runs to each of the individual televisions.
The assumptions that I have made here is that your outdoor antenna is in good shape and receives both VHF and UHF service and that you are using good cable (RG-6). If the antenna is in bad shape, you may be losing signal at the collection point. If you are using lesser quality cable (RG-59) the losses per 100 feet are somewhat higher and that would also add to the problems.
I know this is a bit of science but for reliable digital reception it is more important to keep the signal above the cliff threshold because unlike analog reception where dropping below a certain threshold just means a noiser picture, in digital dropping below the threshold means no service.
Bill
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