Can anyone explain QAM (the for dummies version)?

[remmy925]

It seems most of the HD tuners coming out support over the air HDTV as well as QAM. From what I gather, QAM is an alteration to the cable signal which allows for more channels to be sent in the same bandwidth. Any experts out there able to explain for the Cable TV technology dummies like myself? Does QAM allow you to tune HD channels over cable TV even though you might not be paying for the HDTV package? Does QAM expand your channel lineup or does it just replace the current ones that you get over the cable lines? What exactly would a tuner with QAM support do for me?

[dvd_maniac]

I just started using QAM but from what I gather it is the same signal you would get through your cable's STB but without using the cable box.
Out of the many channels my cable company provides I can only recieve 10 of these channels through QAM, and a few of those are stupid pay per veiw preview channels. The rest are encrypted and can only be used through the cable box. QAM tuners do not record or encode the video but just copy the stream, bit for bit, onto your hard drive.

[Brent]

I'll try.

QAM tuners give you the ability to tune digital cable channels without a set top box. In the US, HD programming and even much of the SD programming is done via digital instead of analog - which requires a QAM tuner or a cable box. The catch with QAM is that many cable companies encrypt some (or even most) of those channels so that you can't tune them without a cable box or a CableCard tuner - a simple QAM tuner does not allow you to tune encrypted QAM programming. One other note: QAM tuners are built into many newer HD TVs so you can view these unencrypted QAM cable channels.

With Kansas City Time Warner, I get all of the local HD channels and some additional ones via my HDHR QAM tuner. Some cable providers leave many channels unencrypted while many others encrpt most of the non-local HD channels.

To get the encrypted QAM channels through your HTPC, you'll need a HD-PVR (requires a cable or satellite box) or a VMC computer (which would mean you'd need to use the Vista Media Center which is less desirable than SageTV imho.)

[dvd_maniac]

I have the HDHomerun and I am waiting for my recently purchased HD-PVR. My Homerun gets the unencrypted channels fine but I have heard horror stories about some other QAM cards not correctly scanning the QAM channels. If the OP is indeed interested in getting into QAM could somebody give him (AND ME) some advice in what card to get?
I really want the Hauppauge HVR-2250 but it has been out of stock for quite a while now.

[stanger89]

Well, we should probably start at the beginning. To transmit information from one place to another you need to have/create a signal. Think of the signal that goes from you amplifier to your speaker, the voltage goes up and down according to the music information. This up/down voltage caries the musical information from the amplifier to your speaker, call this the "message".

This idea works very well and can carry a high quality signal. You can run it over a wire, or with enough power you can send it through the air. However it is limited in that you can only send one piece of information at a time over the wire (the "channel"). What we have here is "baseband" signal.

Now as we probably all know there is a finite amount of information in that audio signal, ie it has a small range of frequencies, from 0-20kHz. What you can do, is combine that message, with another signal, a pure single frequency called a "carrier". The carrier signal can move the "message" to a higher range of frequencies, so that it will not overlap with the baseband signal. The simplest way to combine two signals is Amplitude Modulation (AM), basically the strength of the carrier is "modulated" according to the "message". A more complex method is Frequency Modulation (FM), where the frequency is modulated according to the "message".

So to transmit many messages at a time, each one is assigned a specific carrier frequency and bandwidth.

Now, that's audio of course, but the exact same is true of video. When you're talking about analog TV, be it broadcast or cable, it is transmitted with a slightly modified Amplitude Modulation, defined by NTSC, the particular modulation used is called "Vestigial Sideband Modulation" or VSB.

Then, there's digital broadcast. It works exactly the same way, except the data is digital so instead of an analog waveform like in the audio signal above, a series of "symbols" are used to modulate the carrier. For digital broadcast TV, ATSC defined 8 different symbols to be used, so digital broadcast uses so called "8VSB" modulation. The same fundamental technique, amplitude modulation but with each of the 8 symbols representing a different amplitude so that each symbol can represent 3 bits of data from the message.

Now comes cable, as mentioned above, analog cable works exactly like analog broadcast, NTSC-defined VSB modulation for each channel. However digital works a bit different. Cable could use 8VSB modulation for digital channels, and it would work fine. However that's not the most efficient way of transmitting data. Cable is a much more robust "channel" than broadcast, so you can use a much more complex (but fragile) modulation.

Cable has opted to use Quadrature Amplitude Modulation or QAM. In it's most basic form, QAM uses Amplitude Modulation to send it's message, however it can actually send two messages on a single carrier by altering the phase. When we get to digital QAM, what we're basically looking at is a QAM "symbol" represents a specific combination of amplitude and phase.

Most cable systems have gone to 256QAM, which as you've probably guessed, means that they use 256 different symbols, which means each symbol sent down the channel represents 8 bits of message data.

So, now you're asking what that all means, well to bring it back to every day life, there are basically 3 ways of getting TV from the source to your house:

• NTSC VSB - analog video
• ATSC 8VSB - digital OTA video
• Cable 256QAM - digital cable video

In theory, the cable company could use any of these modulations for any channel they wish, but in practice they use either NTSC VSB (standard analog) or 256QAM (a few places use 64QAM). So you need to get a tuner that understands the modulation you're trying to receive. A "QAM tuner" will allow you to receive 256QAM modulated, ie digital cable, channels.

So, to directly answer your questions:

Does QAM allow you to tune HD channels over cable TV even though you might not be paying for the HDTV package?

A QAM Tuner will allow you to receive all the QAM modulated channels your cable provider provides. Note I said receive and not view, cable providers also use encryption to prevent access to channels to which you don't subscribe, those that aren't encrypted are called "clear". So, a QAM tuner will allow you to receive and view all the clear channels your provider provides, this may include channels that you can't get with your analog cable tuner, like HD locals.

A QAM tuner doesn't (usually) get you access to HD premium channels, it's not a "way around" signing up for the HD package if you want channels like Discover HD, HD Theater, etc.

Does QAM expand your channel lineup or does it just replace the current ones that you get over the cable lines?

Cable is basically limited to 125 channels, and by that I mean analog frequency slots, or carriers. Using digital video on a QAM modulation, the cable company can send many "programs" down a single analog channel, for example I've seen upwards of 8-10 digital SD programs (TV channels) sent down a single carrier.

Between digital compression (MPEG-2), and QAM modulation, the cable provider can provide many more programs than they would without. In theory they could probably send around 1200 digital SD channels if they converted 100% over to digital.

What exactly would a tuner with QAM support do for me?

Basically, what it all boils down to, a QAM tuner will allow you to receive (and view) your local HD channels (and potentially some additional digital SD/HD channels) directly off your cable without the need for an antenna.

Silicondust has a nice database of the "clear" digital channels for most, if not all of the US, so punch in your zip code and you should get a good idea what you can get via a QAM tuner.

[evilpenguin]

Wow, great explanation, that post should be linked in the hardware FAQ post

[dvd_maniac]

Hey Stanger,

Isn't that exactly what I said?

OK I gave the Kindergarden version while you gave the MIT one....
Seriously though, Great Info!

[remmy925]

I appreciate the help from all of you. I am a very technical guy as far as computers go...but not so much with the transport of TV signal. (Funny thing...I used to work in the IT dept for a major cable provider.)

To put things into a nutshell...if I understand right????

QAM tuners pick up channels sent through a digital signal over the cable line so you can view them.

Cable companies encrypt some channels. You need their cable box to unencrypt them. The QAM tuner can't unencrypt those channels, so i can't watch them through the tuner. The channels I can view are really up to the generosity of the cable company.

I may or may not get more channels on my SageTV box. It is likely I can replace some local channels sent through an analog signal with the same channel sent over a digital signal. This may allow me to get them in HD, but again not a guarentee. I also may be able to get some channels that I get with my cable box providing the cable company was nice enough not to ecrypt them. (This means pay channels are definately not going to work, but I might find I can start recording some digital channels I couldnt before...again not a guarentee.)

Even though I may not be exactly correct in my technical description...I am pretty sure I understand now. If I am way off, please let me know!

Again, thanks for the replys. Your answers definatly are helping me make the decision of whether or not to purchase a new tuner!

[dvd_maniac]

Some poeple are able to pick up pay channels via QAM, but it is getting fewer and farther between.
As for the Local HD channels, the cable providers are not suppose to encrypt anything you should be able to receive OTA. Unless this has changed.

[evilpenguin]

Pop your zipcode into this tool to get an idea of what channels are going to be available in your area.

http://www.silicondust.com/hdhomerun/channels

[emveepee]

It is important to check out the silicon dust site for what is available in your area. Here is my location in Canada I get almost no clear QAM but I do find inn addition to what has been mentioned.

- I am also able to get 80 digital audio channels via clear QAM.
- Recording are "pure" signals as distributed by the cable, that it a an mpeg transport stream. I find the SD quality at 2.5 Mbs is better than analog captures at three times the bit rate
- AC3 digital surround sound.
- there is multi lingual programming

Some weaknesses.

- Most digital signals need a good cable feed with few splits. I had terrible error rates until the cableco replaced the RJ-59 with RJ-6
- Stations do change
- Cableco's compress HD signals on clear QAM. For example OTS ATSC HD is typically 20Mbs. A QAM - 256 channel is 40Mb. So there should be two HD broadcast in that spectrum, but cable co's are trying to put in 3. If the combined bandwidth exceeds 40Mbs there will be pixelization.
- Some cable co's are going optical, meaning clear QAM dies. Its actually worse than that because you need a second tuner but that is another thread.

Martin

[pjpjpjpj]

Remmy,

I can't add anything to what is above (technical-description-wise), but I will mention that apparently when premium channel packages have the "free preview weekend", apparently they often are just "turning off the encryption" for the weekend. So that means you would/could get those channels during that period, via QAM (if you were vigilant about when those deals were happening).

And apparently the channels that local providers send out encrypted change frequently. You may get something like Discovery HD or National Geographic HD or such, and a week later, it might change channel number, or go away entirely. You have to just accept the fact that, since you are getting it free, you have no one to complain to, and you can not do anything about any changes that occur. The biggest gripe I have seen with people around here who rely on QAM is that the channels switch places and then you have to manually remap them in Sage.

[heffneil]

I think Brent said earlier yuo would need an HD-PVR. I am curious because I don't think you need a PVR but an HD set top box. I wouldn't think anyone here who wants to setup SageTV and plans on tuning with a set top would want one that records? It seems redundant to a fault. Am I wrong in the terminology or are certain carriers not offering HD tuners only (i.e. without recording)?

Thanks,

Neil

[stanger89]

HD PVR

[jpwegas]

Originally Posted by pjpjpjpj

You have to just accept the fact that, since you are getting it free, you have no one to complain to, and you can not do anything about any changes that occur. The biggest gripe I have seen with people around here who rely on QAM is that the channels switch places and then you have to manually remap them in Sage.

Except in many cases you ARE paying for the clear QAM channels you are receiving - you aren't getting anything for "free". I like the fact that I can get some non-local channels via clear QAM without a STB.

I just wish that they would stop moving the stations around.

Note that anyone with a digital capable TV without a STB has the same problem - when the provider moves channels you have to rescan.

--John

[wayner]

Originally Posted by emveepee

It is important to check out the silicon dust site for what is available in your area. Here is my location in Canada I get almost no clear QAM but I do find inn addition to what has been mentioned.

Stanger89 provided an excellent description of QAM. Now for some Canadian specific info - Canada uses pretty much the same standards and even the same cable boxes as the US except that most (all?) cable companies in Canada offer few, if any, channels in the clear. This is likely because the CRTC does not require it - I believe that the FCC requires that local OTA channels be sent unencrypted - this is not the case in Canada. So, a QAM tuner has very little use in Canada.

[emveepee]

Originally Posted by wayner

So, a QAM tuner has very little use in Canada.

I agree for video it is awful, but I do listen to digital audio quite a bit. I had pretty much moved to Internet Radio (I dislike FM via a capture card) and it is nice to get the choice local channels and specialty channels that Rogers distributes on clear QAM in 192k bit AC3.

Martin

[brb84]

Originally Posted by dvd_maniac

I have the HDHomerun and I am waiting for my recently purchased HD-PVR. My Homerun gets the unencrypted channels fine but I have heard horror stories about some other QAM cards not correctly scanning the QAM channels. If the OP is indeed interested in getting into QAM could somebody give him (AND ME) some advice in what card to get?
I really want the Hauppauge HVR-2250 but it has been out of stock for quite a while now.

I first tried a couple HVR 1600 tuners, and they were nothing but frustrating for me. I used them in both Sage and BeyondTV. I live too far out for an antenna to pick up any of the HD stuff well. QAM worked ok some of the time, but very often the signal would drop out too low, and the tuners just couldn't hack it. I'd get garbled images through TV shows or they wouldn't pick up certain HD channels at all.

Fast forward to a couple months ago when I switched to a couple HDHR boxes - AMAZING. Yes, they are more expensive, but incredibly worth it for me. The scanning process is so much faster than the Hauppauge cards. The built-in channel mapping process is just awesome - you can set it within the HDHR software and Sage just instantly picks it up without having to do its own scan.

A lot of people have been very satisfied with the Hauppauage cards using QAM. I may have only had problems because of my particular neighborhood, who knows. Maybe the 2250 has been improved.

Either way, based on my experience I strongly recommend the HDHomerun if you want something really reliable and one that won't take up a lot of your time messing with channel scans and mappings.

[kevin.sn]

stanger89: Thanks for the awesome explanation! But a another question: Why do you say cable is basically limited to 125 channels? Microsoft's listing shows 158:

http://msdn.microsoft.com/en-us/libr...33(VS.85).aspx

What's more, my TV (Samsung 4061) appears to support at least up to 135, and finds channels in the 132-* range on my cable (Astound Digital San Francisco/San Mateo).

I'm happy to get these extra channels on my HDTV. I'm sad that the 2250 I just bought doesn't seem to... And of course all the good stuff (the clear HD channels) are up there...

[kevin.sn]

Any thoughts on this? Thanks! -- Kevin

Originally Posted by kevin.sn

stanger89: Thanks for the awesome explanation! But a another question: Why do you say cable is basically limited to 125 channels? Microsoft's listing shows 158:

http://msdn.microsoft.com/en-us/libr...33(VS.85).aspx

What's more, my TV (Samsung 4061) appears to support at least up to 135, and finds channels in the 132-* range on my cable (Astound Digital San Francisco/San Mateo).

I'm happy to get these extra channels on my HDTV. I'm sad that the 2250 I just bought doesn't seem to... And of course all the good stuff (the clear HD channels) are up there...